PART V.--GELATINE AND GLUE.
SECTION I.--PROPERTIES OF GELATINE AND GLUE
Many of the chemical properties of gelatine, especially those which distinguish it from other proteins, have been described in the Introduction to this volume, and need no further comment. In this section its colloid nature and behaviour will chiefly be considered, for these points have greatest importance from the standpoint of industrial chemistry.
It is hoped, moreover, that this section will be of interest not only to the chemist concerned in the manufacture of gelatine and glue, but that it will be of value also to those concerned in leather manufacture. The difference between the "collagen" which composes the hide fibre and the high-grade gelatines is so small that for many practical purposes it may be considered negligible. Thus the description of the behaviour of a gelatine gel is very largely applicable to a hide gel also.
Gelatine has been crystallized by von Weimarn by evaporating a dilute solution in aqueous alcohol whilst in a desiccator containing potassium carbonate, the temperature being maintained at 60°-70° C. The carbonate takes up water only, and the concentration of the alcohol therefore slowly increases until the gelatine is no longer soluble. Gelatine is usually found and known in the colloid state, however, and its behaviour in this state only is of practical importance.
The fundamental idea of modern colloid chemistry is that colloids are heterogeneous systems, usually two-phased, in which one phase is liquid and the other phase either liquid or solid. The latter phase, which is divided into small separate volumes, is known as the "disperse phase," whilst the other is the "continuous phase" or "dispersion medium." The "dispersity" is the degree to which the reduction of the dimensions of the disperse phase has been carried, and is best expressed numerically in terms of "specific surface," _i.e._ surface area divided by volume, but it is also often expressed as the thickness or diameter of a film or particle. When the dispersity is not high, we have ordinary "suspensions" and "emulsions," which with increasing dispersity merge into the typical colloids. By analogy, colloids have been divided into "suspensoids" and "emulsoids," when the disperse phase is solid and liquid respectively. The classification, however, has not been found satisfactory, for some systems in which the disperse phase is undoubtedly liquid, exhibit characteristic properties of suspensoids, and _vice versâ_. A more satisfactory division, therefore, is found in the presence or absence of affinity between the two phases, the systems being termed "lyophile" and "lyophobe" respectively. If water be the continuous phase the terms "hydrophile" and "hydrophobe" are often used. Broadly speaking, the lyophile colloids correspond to the emulsoids, and the lyophobe colloids to the suspensoids. Gelatine is a typical hydrophile colloid.
Another fundamental idea of colloid chemistry is that the great extension of surface involved in a high dispersity causes the surface energy to be no longer a negligible fraction of the total energy of the system, and that the recent advances in knowledge respecting surface phenomena may be called in to assist in the explanation of the special properties of the colloid state. Particles which exhibit the Brownian movement, about 10^(-5) cm. diameter, down to the limit of microscopic visibility (10^(-3) cm.) are termed _microns_. Particles less than this, but just visible in the ultra-microscope (5×10^(-7) cm.) are termed _submicrons_. Particles still less, approximately 10^(-7) cm., have been shown to exist, and are termed _amicrons_. The dimensions of molecules such as may exist in true solutions are of the order of 10^(-8) cm. A colloid sol may contain particles of various sizes. Thus a gelatine sol (like other lyophile systems) contains chiefly amicrons, but submicrons are also observable.
1. THE CONTINUOUS PHASE
Owing to the contractile force of surface tension, it is concluded that the surface layer of a liquid is under very great pressure, much greater than the bulk of the liquid. Any extension of the surface of the liquid naturally causes a corresponding extension of the proportion of liquid which is thus compressed. If in a beaker of water there be placed a porous substance, such as animal charcoal, there is a great extension of the surface of the water, and a corresponding increase in the amount of compressed water. If instead there be substituted a large number of very small particles of a substance, a still further increase in the amount of compressed water is involved. As the specific surface of the substance inserted is increased, and its amount, the proportion of compressed and denser water increases also, until it is a practically appreciable percentage of the total volume. It is clear also that the extent of the zone of compression will be determined also by the nature of the substance with which the water is in contact at its surface, _i.e._ by the extent to which it is hydrophile, and this indeed may be the more important factor.
Now in a gelatine sol we have the necessary conditions for a system in which the compressed water bears an unusually large ratio to the total, owing to the enormous surface developed by the minute particles of the disperse phase (amicrons) and to the unusually wide zone of compression surrounding each particle caused by the strongly hydrophile nature of gelatine. It should be pointed out that these zones of compression do not involve any abrupt transition from the zone of non-compression, the layer nearest the particle is under the greatest pressure, and the concentric layers under less and less pressures, the actual compression being thus an inverse function of the distance from the particle. Now if there be a gradual increase in the concentration of the sol, the time will come when these zones of compression begin to come in contact, and the system will then show a considerably increased viscosity. With further increase in concentration the zones of compression will overlap throughout the system, and when the layers under considerable pressure are thus continuous, the whole system will acquire a rigidity much greater than water and approaching that of a solid body. This is a gelatine gel, or "jelly." With increasing concentration the jelly becomes increasingly rigid, and if it be eventually dried out under suitable conditions it forms what is practically solid body--gelatine--which, however, still contains from 12 to 18 per cent. of water.
It will be clear that, in the case of gelatine jellies (_e.g._ of 3-10 per cent. strength), an increase in temperature will cause an increase in the kinetic energy of the particles and effectively reduce the zones of compression. Indeed, they may be reduced to such an extent that they are no longer in contact, and the rigidity due to the continuous contact of the layers of great compression will then disappear; as we say usually, the jelly melts. On cooling, the decreased kinetic energy of the water molecules results in the return of the state of compression, with rapidly increasing viscosity and eventual gelation; as we say usually, the jelly sets. Neither of these changes takes place at a definite temperature (like a melting-point), and in "melting" (solation) or in "setting" (gelation) the temperature-viscosity curve is quite continuous. By various arbitrary devices, however, approximate melting and setting points may approximately be determined. The results also vary somewhat with the concentration of the gel or sol. Gels between 5 and 15 per cent. strong melt about 26°-30° C. and set at 18°-26° C.
On this view, we must regard a gelatine gel as a continuous network of water under great compression, and in this network are zones of still greater compression, which surround the particles of the disperse phase--the gelatine itself, and zones of less compression which in a weak gel, at any rate, have a compression equal to or much the same as the normal state of compression in water.
One consequence of this system is, that when a piece of gelatine swells, there is a considerable enlargement in the zones of compression; in other words, some, at least, of the imbibed water is compressed. Now the compression of water means that work is done, and when gelatine swells, therefore, we expect--and actually find--that heat is liberated (5.7 cal per g. gel). Hence also by the Le Chatelier theorem, we expect--and find--that gelatine swells best in _cold_ water. Further, the compression of water involves a decrease in volume, and we therefore expect--and actually find--that the volume of the swollen jelly is appreciably less than the volume of gelatine plus the volume of water imbibed.
Another consequence of such a compressed system is that a gelatine jelly, even in water, will have a surface tension towards water just as the water itself has such a tension to the water vapour above the liquid. This interfacial tension of the jelly will of course have a contractile effect, and will tend to resist swelling and to limit it as far as it possibly can. This force, tending to contract the jelly and resist imbibition is therefore one of the main influences at work in the swelling of gelatine, and is one of the two principal factors which determine the extent of the maximum swelling when equilibrium is established. The force tending to resist swelling is, in the ultimate, just surface tension. Its actual magnitude depends, of course, mainly upon the extent of compression in the dispersion medium of the gel, and will be a resultant which is a function of this compression. The magnitude will thus vary with the average compression in the continuous network of compressed water. It will be obvious that as the jelly swells the power of resisting the swelling will decrease, and the interfacial tension with the external water will tend to disappear. If the force tending to swell were great enough the swelling would continue until the zones of compression were no longer in contact and the gel would become sol.
As suggested above, it is probable that the extent of the zones of compression is determined by another factor in addition to the great development of surface. That factor is connected if not identical with that power which makes the system lyophile, and is evidently connected also with the solubility of the disperse phase, and may indeed be electrochemical forces tending to form a series of hydrates, or at least to cause an orientation or definite arrangements of the water molecules in the zone of compression. This idea receives some support from the hydrate theory of solution, and the zones of compression and orientation are the colloid analogue of the hydrates supposed to exist in solutions of electrolytes. The extension of such zones on cooling are then analogous with the series of hydrates formed, for instance, by manganese chloride with 2, 4, 6, 11, or 12 molecules of water when crystallized at temperatures of 20°, 15°, -21°, -30°, and -48° C. respectively, the idea being that the salts most hydrated in solution crystallize with most water.
As the compression is the result of two factors, one of which depends upon the nature of the disperse phase, we expect--and find--in other lyophile systems a considerable variation in their power of gelation. Some indeed, though very viscous, _e.g._ egg albumin, never quite set like gelatine, and others (_e.g._ agar-agar) set to a stiff gel from a much weaker sol than gelatine. When the zones of compression are large, as in gelatine, the magnitude of the compressing force on the outermost part of the zone is relatively small, and it is not surprising that time is necessary for the victory of this force over the kinetic energy of the water molecules. Hence we find a 5 per cent. jelly sets readily on cooling, but its elasticity increases steadily for many hours after it has set. This phenomenon, known as hysteresis, we should expect--and find--to be much more marked in a case where the zone of compression is unusually large (_e.g._ an agar gel). We should also expect--and find--that hysteresis is more marked in a high-grade gelatine than in a low-grade gelatine where both eventually form gels of equal elasticity. We should expect too--and we find--that hysteresis is more prominent in weak gels than in strong. These points are of obvious importance in testing gelatine by its elasticity, _e.g._ the well-known "finger test."
There are also other facts and considerations which have an important bearing upon the point under discussion. It is necessary ultimately to regard true solutions of electrolytes and other bodies as heterogeneous, though perhaps of a rather different order. From this point of view molecules and ions existing in an aqueous solution will present a surface and have associated zones of compression analogous with those suggested for the minute particles of gelatine.
Now recent investigations have shown that the essential physical properties of water are affected by dissolved substances in a definite manner and to a fixed extent, and that these substances exhibit a sequence in order of their effect. This sequence is also exhibited in the essential properties of water as solvent and as dispersion medium for colloid sols. The sequence is known as the "lyotrope series." Thus the numerical value of the compressibility of aqueous solutions is reduced below that of water by salts which, with the same kation, exhibit an effect in the following order:--
CO{3} > SO{4} > Cl > Br > NO{3} > I
This same order is observed, in the effect on the increased values for the surface tension, density and viscosity of these solutions. On the other hand, the kations have a similar sequence of effects,
Mg < NH{4} < Li < K < Na < Rb < Cs
which appears when salts of the same anion are chosen. It is not surprising to find that this lyotrope series exhibit an analogous influence on the chemical reactions of water, _e.g._ the hydrolysis of esters. In the hydrolysis by acids SO{4} retards the action, the other anions and the kations accelerate it, in the lyotrope order. In the hydrolysis by bases the series is reversed. Similarly the lyotrope series exert the same order of effect upon the inversion of cane sugar and other reactions.
This lyotrope influence has also been shown to exert considerable effect in the behaviour of lyophile sols. With the lyophobe sols the addition of foreign substances apparently affects the disperse phase only, but with the lyophile sols the effect on the continuous phase is also important, and may overshadow the other. Now, in gelatine and in hide gels and tanning sols we are dealing with lyophile systems, and there are many points of behaviour in which lyotrope influences become prominent. Similar effects are observed upon other lyophile sols (_e.g._ albumin, agar-agar, etc.) which differ widely in chemical nature. Thus the salting out of albumin (reversible precipitation) is influenced by sodium salts in lyotropic sequence as follows. The anions hinder precipitation; in order of precipitating power they are:
citrate > tartrate > SO{4} > acetate > Cl > NO{3} > ClO{3} > I > CNS
The sulphates illustrate the kation effect, which is independent and which favours precipitation:
Li > K > Na > NH{4} > Mg
If the experiments be carried out in faintly acid solution this order of effect is exactly reversed, iodide and thiocyanate having the greatest effect and citrates the least. The coagulation temperature of albumin and the coagulation by other organic substances are similarly influenced by the lyotrope series.
Lyotrope influence also exerts a powerful effect on the behaviour of gelatine sols and gels. The gelation temperature is influenced thus:--
raised by SO{4} > citrate > tartrate > acetate
lowered by Cl < ClO{3} < NO{3} < Br < I
The kation effect (small) is Na > K > NH{4} > Mg
Other lyotrope substances raise or lower the temperature thus:--
glucose > glycerol--(H{2}O)--alcohol < urea
The effect on gelation is also illustrated by the change of viscosity of the sol with time. The same lyotrope order is found.
In the salting out or precipitating of gelatine with salts, the order of anions is lyotrope:
SO{4} > citrate > tartrate > acetate > Cl
Also the osmotic pressure of gelatine sols is markedly lowered by neutral electrolytes in lyotrope sequence:
Cl > SO{4} > NO{3} > Br > I > CNS
Similarly lyotrope influences are shown in the modulus of elasticity: substances which favour gelation increase elasticity, whilst substances which favour solation decrease elasticity. The order is again lyotrope.
The permeability of the gel is affected by lyotrope influences; alcohol and glycerol reduce diffusion through gelatine (or agar); and urea, chloride and iodide increase it. (Similarly the diffusion of sols through "semipermeable" membranes is affected by lyotrope influence.) The lyotrope series also influence the optical activity of gelatine sols and the double refraction of strained gels.
The swelling of gelatine (and other gels) is very strongly influenced by the lyotrope substances and merits more attention than it has received. Hence this lyotrope influence exerts a profound effect in the manufacture of gelatin, and perhaps even greater in the manufacture of leather. This is only to be expected. If a gel comprise a continuous network of compressed water, as suggested above, the presence of other substances in the gel which cause increases or decreases in the compression must modify accordingly the properties which depend upon this state of compression, such as the viscosity of the melted gel, the rate of gelation, the elasticity of the gel, and the rate and extent of its imbibition. This indeed we find to be the case. Now the substances which affect the compressibility, surface tension, etc., of water _least_, _i.e._ the substances producing little or no compression of water, are just those which reduce the compression of water in a gelatine jelly, and cause a decreased viscosity, elasticity, surface tension, etc., and which therefore naturally allow the gel to swell more than in pure water. Conversely, the substances which cause the greatest compression of water, the greatest increase in its surface tension and viscosity, are also the substances which increase the compression, viscosity, elasticity, and surface tension of gels, and which therefore hinder imbibition. The effect on swelling is as follows:--
Sodium sulphate > tartrate > citrate > acetate; > alcohol > glucose > cane sugar; (water) chlorides-potassium < sodium < ammonium; < sodium chlorate < nitrate < bromide < iodide < thiocyanate < urea.
As the amount of compression will depend upon the amount of substance, we expect--and find--that the effect is usually additive, and that suitable mixtures of substances having an effect in the opposite sense will produce no change.
The interpretation of lyotrope influence is of course somewhat speculative, but considered as a surface phenomenon, the surface specific of the molecules and ions of the lyotrope substance must be one of the factors involved. One naturally also connects the effect with solubility and the tendency to form hydrates in solution, the zones of compression being zones of orientation and of electrochemical attraction. The hydrate theory of solution again affords an instructive commentary. The fact that, broadly speaking, the polyvalent anions and the monovalent anions also group themselves together, suggests that electrical forces are at work, and the order of effect of monovalent anions almost suggests that what are called "residual valencies" are in operation. It is difficult to resist the conclusion that in the lyotrope influence, in the crystallizing of salts, and in the formation of a gel, we have zones of compression and orientation which are manifestations of the same forces--surface and electrical; the chief differences in the case of gelatine being that the zones are larger and that the electrical effect is perhaps of less definite magnitude.
However these things may be, the fact of water compression determines the rigidity of the gel, and the changes in this compression of the continuous phase determine the surface tension resultant which hinders swelling, and which is one of the two main factors fixing both the rate at which gelatine swells in water, and the final volume attained by the gel.
Before leaving this point, it is desirable to note the effect on the swelling of gelatine of the extremes of this lyotrope influence. Substances like iodides, thiocyanates and urea prevent a gelatine sol from setting to a gel at all, and a piece of gelatine in such solutions swells rapidly until it solates. On the other hand, sulphates, tartrates, etc., make a stiffer gel on account of the enhanced compression. Gelatine in such solutions may swell, but at a much slower rate than in water and with a decreased maximum extent. A gelatine gel may in such solutions not only fail to swell at all, but actually contract and in some cases, indeed, be practically dehydrated. If a gel be in a very concentrated solution of such a substance, it may be that the lyotrope compression in the external solution is greater than the compression in the dispersion medium of the gel; in which case the surface tension effect is reversed, and the external solution tends to increase in volume and the gel to contract. Hence we find that the saturated solutions of such substances as ammonium sulphate and potassium carbonate will dehydrate a gel almost completely, and will also, by a similar action on pelt, make a kind of white leather. It is important to remember this contractile effect of strong solutions of salts, because it is very easy to confuse this effect with a similar result produced in another manner, viz., by a reduction of the force tending to swell.
2. THE DISPERSE PHASE
A very important feature of the colloid state is that the particles of the disperse phase appear to possess an electric charge, and if this charge be removed a colloid sol no longer remains such, but precipitates, flocculates, coagulates, etc. As to the origin of this charge several theories have been advanced, but the most generally accepted is that it is a result of the adsorption of electrically charged ions by the particles of the disperse phase. The enormous specific surface possessed by this phase renders it particularly liable to such adsorption. This view harmonizes well also with the general behaviour, of colloid sols and gels, in endosmosis, kataphoresis, precipitation, etc. According to this point of view the particles of the disperse phase are surrounded by a surface layer in which these ions are in much greater concentration than in the volume concentration of the dispersion medium. The hydrion and hydroxyl ion are particularly liable to such adsorption. In the case of a lyophile colloid, like gelatine, the charge may be either positive or negative, according to the nature of the predominant ions in the dispersion medium, and the amount of adsorption is determined by the concentration of these ions in accordance with the adsorption law.
In effect, therefore, the particles of the disperse phase each carry an electric charge of the same nature, and as similarly charged bodies repel one another, the particles of the disperse phase will tend to separate and to occupy a bigger volume. It is the author's opinion that this repulsion of similarly charged particles is the cause of the swelling of gelatine. The amount of charge and force--tending to swell--is due possibly to several ionic adsorptions, which may be considered to operate independently, and the power of repulsion is determined by the nett charge, which in the case of a "positive colloid" is positive, and in the case of a "negative colloid" is negative. As ions possess different electric charges, the charge on the disperse phase is subject to the valency rule.
Now the repulsive force between two similar and similarly charged bodies is proportional to the amount of charge and is inversely proportional to the square of the distance between them. The amount of charge on a colloid particle will be determined by the dispersity--best signified by the specific surface (s)--and by the operation of the adsorption law
y = mac^(1/n)
The distance between the particles varies with the degree of swelling, and is determined by the cube root of the volume of the gel (_v_). Hence if F be the force tending to make the gelatine swell, we may write
F = Q/(d^2) = (sy)/v^(2/3)
Now with all electrolytes, even with water, we have both positively and negatively charged ions, and y is consequently determined by the difference in the amounts adsorbed. Hence in the case of an electrolyte with an equal number of oppositely charged ions y = ma{1}c^(1/n{1}) - ma{2}c^(1/n{2}), where a{1}, a{2}, and n{1}, n{2}, are the appropriate constants for the particular ions concerned. Hence at constant temperature, pressure, etc., we may write
F = [ sm( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) ] / v^(2/3)
The force tending to make a piece of gelatine swell is proportional to its mass, which is perhaps fairly obvious. The swelling force is also an inverse function of the volume of the gel, and as swelling proceeds therefore the force tending to swell further decreases. The force tending to swell is proportional to the specific surface of the disperse phase, other factors being constant. To illustrate this one has only to imagine that one particle of the disperse phase be split into two particles each carrying half the original charge. It is clear that a new repulsive force becomes operative, which did not before influence the swelling, and that the distance between the particles is halved. In the swelling of gelatine, however, we may consider the dispersity constant for constant temperature, and if we consider unit mass we see that the force causing swelling depends upon the operation of the adsorption law and upon the degree to which the gel is already swollen.
In the swelling of (say) one gram of gelatine to its maximum, both the contractile force of surface tension and the expanding force of electrical repulsion are in operation. At the commencement the latter is much the greater force--hence the rapid imbibition. Both these forces decrease in magnitude as the swelling proceeds, but the force tending to swell decreases at a more rapid rate, and the time comes when it has decreased to the precise value of the force tending to resist swelling. At this point equilibrium is established and the maximum swelling attained. Obviously this maximum will in many cases be determined largely by the value of a{1}c^(1/n{1}) - a{2}c^(1/n{2}). This factor, therefore, demands particular consideration.
Now, unfortunately, the adsorption law constants for the different ions have not yet been numerically determined, so that we are still somewhat in the dark as to the operation of ionic adsorptions. It is possible, however, to form conclusions of a qualitative or relative order, and these are such as to throw much light upon the question at issue. In the first place, we know that in general the various ions are not usually very widely different in the extent to which they are liable to be adsorbed. If this were otherwise, the valency rule would hardly operate so well in endosmosis, kataphoresis, and precipitation. In consequence we must expect the differences between the ions to appear in small rather than in large concentrations, the amounts adsorbed being under those conditions more affected by changes in the volume concentration. At the larger concentrations, therefore, the value of a{1}c^(1/n{1}) - a{2}c^(1/n{2}) is small, and the force causing swelling often tends to zero.
There are, however, noticeable differences at lower concentrations. Thus we know that if a substance be primarily a positive colloid, it will absorb kations more readily than anions. As gelatine falls into this class, we may therefore conclude that usually a{1} > a{2}. Further, it often happens that very adsorbable substances are less affected by concentration changes, and in the case under consideration, therefore, we should expect that n{1} > n{2}. Moreover, we know that the hydrion and hydroxyl ion are much more readily adsorbed than other ions, _i.e._ have a large value for _a_. Hence in the case of gelatine we expect that a{1}c^(1/n{1}) - a{2}c^(1/n{2}) will have a comparatively large value when one of the ions is H+ or OH-. Also we know that organic anions are usually much more strongly adsorbed than inorganic anions, and hence that in such cases a{1} is more nearly approached by the value of a{2}. It should be emphasized perhaps, at this point, that these various considerations are not based upon any facts relating to the phenomena of imbibition in gels, or in gelatine in particular, but are based upon the behaviour of colloids in endosmosis, kataphoresis, electrolytic precipitation, adsorption, etc.
Now if we select a few simple figures which are in accord with the above considerations, we can examine the value of the factor a{1}c^(1/n{1}) - a{2}c^(1/n{2}) in a purely illustrative and typical way, and at any rate form some idea as to the manner in which it is likely to vary. The figures might be:--
Ion. | _n_. | _a_. -------------------------+---------+--------- Hydrion _or_ hydroxylion | 20 | 10 Kation of a metal | 15 | 7 Organic anion | 10 | 8 Inorganic anion | 6 | 6
For the sake of simplicity we can assume that these ions are all monovalent. The ions adsorbed by unit mass will then be 10c^(1/20), etc. If these hypothetical adsorption isotherms be plotted as usual we get the fairly typical curves shown in Fig. 1.
Now in practice there are always two of these ions, each giving its own specific effect in opposite senses, and the difference ( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) represents the nett charge adsorbed. Hence we have the following combinations:--
Inorganic acid 10c^(1/20) - 6c^(1/6)
Organic acid 10c^(1/20) - 8c^(1/10)
Alkali 10c^(1/20) - 7c^(1/15)
Inorganic salt 7c^(1/15) - 6c^(1/6)
If we plot these values of nett adsorption against the concentration we obtain the curves shown in Fig. 2.
On the assumption that the nett charge adsorbed is the dominant factor in determining the maximum swelling at equilibrium, one must therefore regard the curves of Fig. 2 as representing the changes in volume of the swollen gel as the concentration is increased. Now in _type_ these curves correspond to those obtained by experiment from hydrochloric acid, acetic acid, caustic soda, and common salt. The maximum swelling with hydrochloric acid increases rapidly with the concentration at first and then rapidly decreases, though not at such a great rate. The swelling with acetic acid increases less rapidly and to a less maximum, but decreases more slowly. With common salt there is a slight swelling followed by contraction. Caustic soda gives a rapid increase in volume at first, afterwards much less so, and finally yields an exceedingly slow decrease. The correspondence of these facts with the type-curves inevitably suggests that the phenomenon of swelling might be accounted for, in part at least, along these lines.
Of course it is not likely that the simple figures selected for the illustration of the argument are either relatively or absolutely correct. Thus we know that the adsorption curve for hydrions and hydroxylions are not likely to be quite identical, as assumed above. As gelatin is primarily slightly positive, it is probable that the values of _a_ and of _n_ for hydrion adsorption will be relatively slightly greater. The relative values supposed, however, are near enough to illustrate the contention that the type of the maximum volume curve can be explained on this assumption of different adsorption isotherms for each of the ions.
If the remarks on the compression of the continuous phase be recalled, it will be obvious that in the present paragraphs we have been giving the question of equilibrium-volume a rather one-sided consideration. The volume of the gel when equilibrium is established may be determined in type by the nett charge adsorbed by the disperse phase, but it will be modified also by the lyotrope influence of the particular substance on the continuous phase. When gelatine swells in solutions the influences on both phases are always in operation, and either upon occasion may become predominant. In the case of neutral organic substances, such as cane-sugar, the lyotrope influence is the determining factor. In the case of neutral salts the predominant influence is decided by the place occupied by the salts in the lyotrope series. If at either end of the series the lyotrope influence is uppermost and the effect of ionic adsorptions is practically swamped. Thus sodium sulphate and sodium iodide hinder and promote imbibition respectively as could be expected from their strong lyotrope power. On the other hand, in the case of sodium chloride, which has comparatively feeble lyotrope influence, the relatively different adsorptions of its ions comes to the fore. With acids and alkalies the relatively large adsorption of the hydrion and hydroxylion causes this to be the predominant influence, but we must concede the possibility that purely lyotrope influences may be at work in some cases, and especially at the greater concentrations. Indeed, it is sometimes a difficult problem to decide whether an increase or decrease in swelling is due to lyotrope or adsorptive influence, but, broadly speaking, we can expect strong lyotrope effects at either end of the series and also at large concentrations, and we can expect strong adsorptive effects in dilute solutions, in the middle of the lyotrope series and in the case of alkalies and acids.
For much of the above explanation of the nature and behaviour of gelatine, the author must himself take responsibility, and in this section he has freely quoted from his own papers upon the subject (see References). He claims that his view of a gelatine gel as involving a network of compressed water, liable to modification by lyotrope influence upon the continuous phase and by ionic adsorptions of the disperse phase, is most in harmony with the recent advances in our knowledge of colloids; that much of the theory is a necessary corollary of those discoveries; and also that he has found this view to be a sound guide in practice, both in tanning and in gelatine manufacture.
Many other theories have been advanced, but most are generalizations over too limited a field, and from experiments with only a few substances, and show little or no correlation with the wider facts of colloid behaviour. That of Procter, for example, discards altogether the idea of a two-phased structure of the gel as an "unproved and rather gratuitous assumption," dismisses surface tension considerations as "more complicated and less verified," and adsorption as "wholly empirical," whilst it ignores lyotrope influence and the analogy with agar gels completely. Procter's theory applies mainly to the swelling of gelatine by acids, which swelling he considers to be due to the osmotic pressure of the anion of a highly ionizable salt formed by the chemical combination of the acid with gelatine. On this assumption, mathematical considerations show that the electric charge on the gelatine is given by the expression z = sqrt(4ex + e^2), where z = the amount of ion taken up, x the concentration of the surrounding solution, and e the excess concentration of diffusible ions in the jelly.
The property of gelatine and glue which is chiefly used in classifying them into grades of different commercial value, is the strength of the jelly obtained as compared with any arbitrary standard gelatine. An enormous number of other physical tests have been devised, but none are nearly so simple or so reliable. Gelatine is unfortunately very liable to hydrolysis even by water, and long before any amido-acids, etc., have appeared there is a change to a not greatly hydrolyzed product (sometimes called [beta] gelatine) which has lost the power of setting to an elastic gel. It is thus the lyophile nature which has been altered, and the fall in elasticity corresponds to the fall in power of compressing water, which is proportional to the concentration of [alpha] gelatine. Now the elasticity of a gelatine gel varies as the square of the concentration. Hence if one so arranges the concentrations of standard and unknown samples that gels of equal elasticity are obtained, the concentration of [alpha] gelatine is the same in both gels, and the _relative_ amounts of [alpha] gelatine in the original samples are inversely proportional to the weights used to give gels of equal elasticity. The "strength" of a gelatine or glue is therefore usually stated as the number of grams of a standard gelatine which will yield a gel with elasticity equal to that from 100 grams of the gelatine or glue being tested. Elasticity is matched by lightly pressing with the finger-tips.
It is also possible to grade samples of gelatine and glue by the estimation of "peptones," whose amount indicates the degree of hydrolysis. Nitrogen is estimated by Kjeldahl's method in the sample and in the precipitate obtained by saturating a solution with zinc sulphate. The difference is calculated as peptones by multiplying by 5.33. Trotman and Hackford say that the results are in the same sequence as those of the finger test. The method, however, is much more laborious than the "finger test."
Gelatine is also graded according to the results of bleaching and clarifying, but with quite arbitrary standards, largely determined by the fancy of the customer.
Chemical analyses, involving estimations of ash, lime, fat, acid, water, insoluble matter, and poisonous metals, _e.g._ arsenic, copper, zinc and lead, are of value for special cases according to the destiny of the goods. Special physical tests, such as "breaking strain" and "foam test," are also of some little value in special cases.
REFERENCES.
"The Chemistry of Colloids," W. W. Taylor. 1915.
"Handbook of Colloid Chemistry," W. Ostwald. 1919.
"Chemistry of Colloids," Zsigmondy and Spear. 1918.
"Introduction to the Chemistry and Physics of Colloids," E. Hatschek.
"Surface Tension and Surface Energy," Willows and Hatschek.
"Chemistry of Colloids," V. Pöschl.
"Grundzüge d. Dispersoid Chemie," von Weimarn.
"The Lyotrope Series and the Theory of Tanning," Bennett, J.S.L.T.C., 1917, p. 130.
"The Swelling of Gelatine," Bennett, J.S.L.T.C, 1918, p. 40.
"The Swelling of Gelatine," Procter, _J.C.S. Trans._, 1914, =105=, 313; and _Koll. Chem. Beihefts_, 1911, =2=, 234.
"The Swelling of Gelatinous Tissues," Procter, J.S.C.I., April 16, 1916.
"Summary of Procter's Views, and Bibliography," Collegium (London), p. 3, 1917.
"Lyotrope Influence and Adsorption in the theory of wet work," Bennett, J.S.T.C., 1920, p. 75.
For the "finger test," see--
"Glue and Glue Testing," Rideal, 2nd ed., p. 158.
"Leather Trades' Chemistry," Trotman, p. 241.
SECTION II.--RAW MATERIALS AND PRELIMINARY TREATMENT
The raw materials for the manufacture of gelatine and glue may be classified according to their origin. The preliminary treatment, which comprises chiefly purifying and cleansing operations, is varied according to type of manufacturing process for which it is a preparation.
In the case of hide or =skin gelatine=, the raw material is a bye-product of the leather industry. After the hides or skins have passed through the preparatory processes which convert them into "pelt" (see Part I., Section II.), they are so trimmed that all that is left will make a useful leather. These "trimmings" or "roundings" include ears and noses, the udders of cows and heifers, and also include parts from the butt, belly and shanks which are collectively termed "pieces." The operation of fleshing (Part I., Section II.), in which fat and flesh are cut from that side of the hides and skins which was next the flesh, also involves cutting into the collagen to some extent, and these "fleshings" comprise another very large class of raw material. The fleshings obtained by hand labour contain distinctly more hide substances than those obtained by machine work, and their commercial value to the gelatine manufacturer is of course proportionate to the collagen content. Some hides and skins are split in the pelt (Part I., Section IX.; Part II., Sections II., III. and IV.), and the "flesh split," though sometimes made into leather, is also used in making gelatine, a high quality being obtained from such material. Minor sources of material are tendons and cartilages, and also hides and skins which have been too much damaged by partial putrefaction or by accidents to make sound leather. Of course the material from the hides for heavy leathers form the greater bulk of raw material for skin gelatine which is thus derived principally from ox hides but sheep and goat skin pieces have also an important place. The skins of other animals, such as dogs, cats, hares and rabbits not usually made into leather can also be depilated and used for making skin gelatine and glue. Horse hide fleshings and pieces are sometimes used, but are notorious for the poor quality of their product. They seem to contain less [alpha] gelatin. All these materials are of course readily putrescible and must be put "into work" without much loss of time. When it is impossible to convey them from the tannery to the gelatine factory quickly enough, _e.g._ foreign material, the "glue stock" is dried out completely and sold in that condition. In the manufacture of pickers from limed pelt there is some superfluous material, and this is cut into shavings and dried. This "picker waste" also forms a useful source of raw material. Skin gelatine material is not very strong in gelatine-substance. The fleshings, pieces, etc., contain much water, even up to 80 per cent. This, however, is very variable, and only a practical test or a hide substance determination can indicate the commercial value of any particular material. This value, moreover, is determined not only by the yield and quality of the gelatine which can be obtained, but also by the yield of grease, the valuable bye-product.
The preliminary treatment of material for skin gelatine consists essentially of liming and of washing. The object of each process is to purify. Liming has much the same action on hide pieces, etc., as on hides, and indeed the liming treatment is somewhat superfluous on cuttings from well-limed hides. The material is plumped up and the partially hydrolyzed products are taken into solution. Lime also acts as mild antiseptic, stops any putrefaction and liberates ammonia formed by fermentation in transit to the factory. When plumping is particularly wanted (as in wetting in dry stock) caustic soda is sometimes used as an assistant (_cf._ dried hides). Sodium sulphide has also been used for this purpose. The liming is in brick pits, an excess of undissolved lime being always used. It is advantageous frequently to disturb or agitate the goods in the lime pits. Up to ten weeks liming has sometimes been given, but about three weeks is now generally considered sufficient, and the tendency is to shorten the time. The lime and soda have also a detergent action on soiled stock, and they probably assist in hydrolyzing the pigments of the hair roots and sheaths. They also saponify and emulsify the grease, and it is obvious, therefore, that liming can be carried too far. Slaked lime, of course, must always be used.
After liming the soaked, softened and plumped stock is washed as thoroughly as possible. To do this it is necessary to supply repeated batches of clean cold water. Some manufacturers, however, use the warm water from the evaporators. Wooden vats or brick pits with arrangements for agitation, for draining off and for inspection, are used for this purpose. The agitation may be carried out by means of revolving shafts or drums with projecting curved spokes or vanes. An American patent (Hoeveler's glue stock washer) involves the use of a paddle wheel. It is combined with a settling tank to gather particles of stock. In the washing the chalk, excess lime, dirt, etc., are quickly removed and a slow deliming process is commenced. The sediment from the washers and wash waters has some value in making fertilizers. Deliming cannot be carried on further than certain limits by water alone. Hence acid is often added to finish off the process. Hydrochloric acid has the advantage of forming soluble salts, but if they are not removed completely their lyotrope influence is to weaken the gelatine. Sulphuric and sulphurous acids are even cheaper, and the lyotrope influence of their salts is in the opposite sense. The latter also has the advantage of destroying sulphides, an important advantage for food gelatines. Whatever acid is used, however, it is evident that an abundance of pure cold water is the fundamental requirement of a pure product. It is a sound maxim in gelatine manufacture to avoid, if at all possible, the addition of any soluble substance, for it is always present in a more concentrated state in the finished article. Thus if its solubility be even moderate, one is likely to attain supersaturation in the "cake" and consequently a dull product. Further, lyotrope influences can never strengthen a gel very much, but may and often do weaken it very considerably. Hence the aim of most manufacturers in the preliminary treatment is so to delime that a nearly neutral and salt-free product is obtained. An exception is the case of skin gelatine in which excess of sulphurous acid is used. This process has for its object not only deliming and purifying, but also a bleaching action.
In the case of =bone gelatine=, the raw material is such that there are much longer and more elaborate preparatory processes. This arises from the fact that about half the bones of animals consists of mineral matter, chiefly calcium phosphate. Bones, of course, vary in composition to some extent, and those from younger animals contain distinctly less of the mineral constituents. Approximately speaking, bones have the following average composition:--
Gelatinous matter 21-1/2 per cent. Fat 12-1/2 " " Calcium phosphate 48 " " Calcium carbonate 3 " " Alkali salts, silica, etc. 2-1/2 " " Water 12-1/2 " " -------- 100 " "
It will be seen, therefore, that the manufacture of bone gelatine and of a comparatively large proportion of phosphate involves the recovery and purification of much fatty matter. The manufacturing processes are naturally subject to considerable variation. One respect in which they differ is the stage in which grease is removed. Sometimes this is simply done as the need and occasion arise, and it is skimmed out in the acid or water extractions, but it is now more usual to have a special "degreasing" process. There are, moreover, two quite distinct types of manufacture. In one of these (the boiling process) the routine bears some resemblance to that for skin gelatine. In this process the bones are washed and cleansed and then immediately subjected to extraction with water. This removes the gelatinous matter and leaves the phosphate and earthy matters behind. Grease may be removed before the water extraction, but is also sometimes removed by skimming off during the extraction, as is usual in the case of skin gelatine. This procedure is now not much favoured unless only a low-grade glue is required. In the other type of manufacture (the acid process) the material is first degreased, and then the mineral matter is extracted or dissolved by acids, leaving the gelatinous matter behind for subsequent refinement and solution. The acid process has long been preferred for high-class bone gelatine, and hence needs further discussion.
The degreasing operation was once brought about by steaming only, but is now accomplished with the assistance of fat solvents.
The object of cleansing is not only to remove dirt, but also fleshy matter which often adheres to the bones. This may contain a little gelatine, but consists mainly of other proteins and insoluble fibre, neither of which are wanted in the water extraction. The mill consists of a large cylinder of stout wire gauze. This revolves round the axis of the cylinder, and the bones are fed in at one end by a hopper and are discharged at the other. The revolution of the mill causes the friction which polishes off the fleshy matter. The dirt and flesh fall through the gauze and are sent to the fertilizer factory. The polishings are sometimes further separated by a similar machine. Raw bones may thus yield nearly 60 per cent. of degreased bones, and about 56 per cent. cleansed bones ready for extraction, and 3 or 4 per cent. "bone meal."
The next stage is the extraction of the mineral matters by acid, for which purpose hydrochloric acid has proved very suitable, as both phosphate and carbonate of lime are dissolved by it. The usual counter-current system of extraction is used [_cp._ Leaching and extract manufacture, Part I., Section III., p. 35]. The process is methodical and regular, the acid liquor passing successively through a battery of six vats in such a manner that the liquor richest in lime salts comes into contact with the bones most recently charged; the fresh acid thus acts upon the nearly extracted bones. The hydrochloric acid used is of 8 to 10 per cent. strength (5° to 7° Bé.). Stronger acid is apt to hydrolyze ("rot") the gelatine, whilst weaker acid takes longer time. The process takes 8 to 10 days, though up to 14 days is sometimes given, and, on the other hand, the process has been occasionally reduced to 4 days. The gelatinous matter undissolved has the shape of the original bone, but is much swollen. When the acid liquor is saturated with lime salt, the liquor is drawn off from below the vats and sent to the phosphate precipitation tanks. The phosphate is usually precipitated by adding just sufficient milk of lime to neutralize the hydrochloric acid. The precipitated phosphate is then well washed by decantation to remove calcium chloride. It is then drained, and dried at a low temperature. As a large bulk of phosphate is obtained it is often filter-pressed and dried quickly in long revolving chambers through which a current of air is passed. The phosphate is sometimes also precipitated by ammonia. It is then more easily washed and dried, and the ammonium chloride is recovered and may be used to regenerate ammonia, or be sold as a valuable bye-product. Sometimes the acid liquor is not used for making precipitated phosphate, but is evaporated with animal charcoal and silica and then distilled to make phosphorus.
The next stage is the purification by washing of the gelatinous matter which remains. The vat is filled up with pure cold water and the material allowed to steep for six or seven hours. The acid and salts remaining diffuse outwards into the water. This is drained off and replaced by fresh water, and the procedure repeated half a dozen times or as often as necessary. The end is said to be determined by the absence of a precipitate on adding silver nitrate to the wash water, or by the absence of any action on blue litmus paper. It will be seen, however, that there are two actions involved, one being the removal of calcium chloride and the other the removal of excess acid. The former is the easier, and is almost necessarily brought about by the latter. Hence in some factories the neutralization is brought about, therefore, by the addition of a certain quantity of soda, or more usually by lime, and the material is sometimes submitted to a veritable liming by which it remains in milk of lime for about three weeks, the lime liquor being renewed several times. The product is finally washed again to remove excess lime. This is carried out in a rotating vessel through which passes a continuous stream of water. If a slightly acid gelatine is required, however, the lime and liming are both superfluous, and the procedure is simply to wash as thoroughly as possible and then to immerse the material in a 1 per cent. sulphurous acid solution for 3 hours to bleach, and then to proceed with the water extraction or solution of the gelatine. The hydrochloric acid used for these processes should be as pure as possible, and the degreasing as thorough as possible, for, if not, a gelatine with a bad odour is liable to be obtained.
Instead of using hydrochloric acid for the solution of mineral matter, sulphurous acid is sometimes employed, and has the advantages that its bleaching effect is thereby obtained throughout the process, and that it is recoverable for subsequent use. The Bergmann process, most generally favoured, is described very concisely by Rideal thus: "A sulphurous acid solution is made to circulate over the bones in a series of closed tanks, the solution being continually enriched with sulphurous acid from a cylinder of the liquefied gas. The resulting liquor, containing an acid calcium phosphate and calcium bisulphite, is heated by steam in a leaden digestor, when the excess of sulphurous acid is liberated and passes back to the tanks, while neutral calcium phosphate and sulphite are precipitated. The latter is decomposed by an equivalent of hydrochloric acid, setting free the remaining sulphurous acid, which is returned to the tanks, leaving calcium chloride in solution, and neutral calcium phosphate in suspension." Not more than 5 per cent. of sulphurous acid is said to be lost in this process, and the gelatine is more thoroughly bleached. It is subsequently well washed before extraction.
=Recovery and Purification of Grease.=--The degreasing operation, which is applied usually to bones and to skin glue scutch, was once brought about by steaming only, but is now accomplished with the assistance of fat solvents, though in the latter case steaming together with mechanical centrifugal force has proved sufficiently successful. On the Continent carbon disulphide was once largely used as solvent, and in this country benzene has been employed, but their low volatility and high inflammability, as well as their expense, make both these substances somewhat unsuitable, and it is now usual to make use of petroleum oils, whether Scotch, American or Russian. A fraction which boils about the same temperature as water is usually employed, and all of it must be volatile under 280° F. Before the actual grease extraction the bones should be sorted over and unsuitable substances (horns, gravel, iron, etc.) removed. They are also usually put through a mill and roughly crushed or broken. The actual grease extraction plant consists of large copper vessels which will each take 5 tons of bones. These extractors are arranged in sets so that the degreasing is proceeding in some whilst the others are being emptied and recharged. The doors for charging and emptying must be securely fastened. When the extractor is charged the solvent is run in and heated by a steam coil which eventually causes it to distil. After some hours the remainder, which has dissolved much grease, is run off, and a fresh lot of solvent is added and heated up. After four such extractions only about 1/4 per cent. of grease remains in the bones. To remove the remainder of the solvent high-pressure steam (80 lbs.) is blown through the bones. The extractor is then opened and the degreased and somewhat dried bones are mechanically conveyed to the cleansing mill. The grease solutions obtained are subjected again to steam with a view to removing the solvent and obtaining it for repeated use in this sense. The efficient distillation and recovery of the solvent is indeed an essential element in the success of the process.
The greases obtained, whether by the use of fat solvents or by skimming off during extraction, or in any other way, are mixed together as is appropriate to their origin and purity, and subjected to further purification, the object of which is to remove gelatinous and albuminous matters, and to decompose lime or soda soaps. The precise methods of purification are, of course, dependent mainly upon the impurities known to be present, but the readiest method is to give the grease further steaming or boiling with water, and so effect by washing and by solvent action the elimination of non-fatty matters. In many cases it is found advantageous to employ mineral acids or oxidizing agents to assist the process. The process may be repeated as often as is desired.
The recovered and purified greases are often of a high standard of purity, and the best are quite fit for edible purposes. The large extension of the margarine industry in this country has indeed caused a larger proportion than ever of this bye-product to be so used. In some cases it is found commercially advantageous to submit the grease to action of the filter press, and so to separate it into solid and liquid portions, the former containing a much larger proportion of stearin, and the latter of olein. Much of the grease from the gelatine trade is also found suitable for soap manufacture, and is therefore a valuable source of glycerine.
=Other Raw Materials.=--Whilst hide pieces and fleshings, and animal bones, comprise the principal raw material for the manufacture of gelatine and glue, there are also minor sources of raw material which, though often not suitable for gelatine manufacture, will yield a satisfactory glue. Thus the skins, bladders and bones of fish form the source of "fish glue." Sole skins, indeed, when deodorized by chlorine and decolorized by animal charcoal, are made into gelatine. The bladders of some fish (_e.g._ the sturgeon) are washed, purified and dried with rolling to make "isinglass," a form of natural gelatine in which the original fibrous structure is retained. There is a limited demand for this material for clarifying purposes by brewers, wine merchants and cooks.
Leather waste may sometimes be used to make a low-grade glue. Vegetable-tanned leather offers much difficulty unless very lightly and recently tanned. The tannage must be stripped by drumming with weak alkalies, _e.g._ borax, sodium sulphite, or weak soda. Chrome leather may be stripped easily and completely by Rochelle salt and other salts of hydroxy acids (Procter and Wilson), and also by ammonia acetate, oxalate and similar salts (Bennett), also by certain organic acids (Lamb). Processes are patented by which chrome leather is digested with lime to make glue, the chromium hydrate being insolubilized. Viscous and tenacious substances are also obtained from some vegetable matters and are called "glue."
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.; Skin Gelatine and Glue, pp. 25-48; Bone Gelatine and Glue, pp. 59-66.
"Gelatine, Glue and their Allied Products," T. Lambert, pp. 11-52.
"Encyclopedie chimique," Fremy, tome x.
SECTION III.--EXTRACTION
The term "extraction" is applied to that essential process by which the gelatinous matter from whatever raw material is used, is actually dissolved in water and removed from the rest of the material. Extraction is often termed "boiling" or "cooking." Whether one is treating hide fleshings and pieces or whether one is dealing with raw or acidulated bones, the general principles of extraction are much the same, and most of this section is equally applicable to any class of material.
The chief principle of extraction is so to arrange the process that both the material and the extracted liquor are maintained at high temperatures for the shortest possible time. As we have observed, gelatine is readily hydrolyzed by hot water, and as hot water is needed for its extraction or solution, care must be taken to remove the solution as soon as possible from the source of heat. In practice this can only be done somewhat imperfectly, as it is necessary to obtain a gelatine sol of several per cent. strength before removing it from the extraction vessel. The stronger this sol is made before removal, the less the time, trouble and expense is incurred in evaporation subsequently, but the more is the exposure to heat with consequent weakening of the gelatine. Hence in practice it is necessary to compromise. The matter is complicated further by the necessity of obtaining a clear sol, for which it is desirable that the sol obtained in extraction should not be too concentrated, as impurities settle and filter much more readily from weaker and less viscous sols.
It will be understood, therefore, that whatever material is being extracted, the most favoured procedure is to extract in fractions. The first fraction, which is least exposed to hydrolytic decomposition, produces the highest quality products, and the subsequent fractions (nearly always two more, and sometimes several) yield products which gradually become of inferior quality owing to the number of times the raw material has been re-heated.
Within limits, the precise temperature of extraction does not have the importance one would expect. Lambert suggests the temperature of 185° F. as suitable for both skin and bone gelatine, and most manufacturers would, on the whole, endorse this. If, however, a higher temperature be preferred, the hydrolytic action is increased in intensity but decreased in its time of operation, whilst if a lower temperature be adopted the decomposition is retarded in speed, but is increased in totality because of the longer time needed to obtain a suitable strength of liquor. Thus, with care, much the same result is obtained by extraction at near boiling-point for a short time as by extraction at 160° F. for a long time. The higher temperatures have the definite advantage of speed, whilst the lower temperatures have the advantage that one may choose to be satisfied with a weaker extract, and so gain a little in the strength of the gel, by throwing more work on the evaporator. One other point should, however, be borne in mind in this connection, viz. that a gelatine sol kept at temperatures above 185° F. begins to deteriorate in colour. Whilst, therefore, much depends upon the precise class of material, it is broadly true to say that the higher temperatures are advantageous for glue, whilst the lower temperatures are preferable for the highest quality gelatine.
Extraction in open vats is used both for skin and bone gelatine. It is usually preferred when it is intended to extract at the lower temperatures, and it is usually adopted also when the material is such that the extraction is comparatively rapid, as for example in the case of skin gelatine and bones by the acid process. The vats themselves are often constructed of wood, in which case they are heated by a copper (or brass) steam coil. They may be constructed also of iron, cast or wrought, the former being cheaper, less liable to corrosion, but more liable to fracture. In the case of iron vessels the heating may also be done by a steam coil beneath a false bottom, but it is sometimes arranged that iron vats are heated by a steam jacket, and even by a hot-water jacket. Heating in either wood or iron vessels has been brought about by direct application of raw steam, but the results are both uncertain and unsatisfactory owing to local overheating. Whatever appliances are used agitation of the material or liquor is advantageous.
Extraction in closed vats is also used. This is generally associated with extraction at higher temperatures, and more often also with the manufacture of glue than of gelatine. It has been used on the Continent for skin glue, and in this country for bone gelatine and glue by the "boiling" process. In this system of working the vessels are usually made of 3/8-inch steel plates, and will take a charge of 3 to 5 tons of material. It is claimed for the system that there is a lessened steam consumption as well as lesser manipulation, that strong liquors are more easily and quickly obtained, and that the material may be more thoroughly exhausted. Extraction is sometimes made by steam and water playing alternately on the material, but many manufacturers prefer the use of direct steam, keeping the pressure at 15 lbs. for about 2 hours. The pressure is then reduced considerably and the process finished off by spraying the material with water. From such a procedure a 20 per cent. glue sol may be obtained. It is common to work such extractors in couples or in batteries of four to six. It will be readily understood that the process is suitable for making bone glue when the phosphate has not been dissolved. The high temperature is in this case almost necessary to ensure thorough extraction. It will be equally clear that the process is not so suitable in the manufacture of a strong gel.
As alternatives to the systems of fractional extraction, several processes have been devised in which the extraction is continuous.
Amongst these is the tower system, in which the material is placed upon a series of perforated shelves arranged inside a steam-tight cylinder or tower. Water is admitted from the top and trickles down over the material whilst steam is admitted from the bottom. Superheated steam is sometimes used. The material may thus be digested with a minimum amount of water, and the sol passes out of the apparatus and from the action of heat soon after it is formed. From bones the sol obtained is of good colour, but is somewhat dull. Several variants of this process have been patented.
Another continuous system of extraction is that involving the use of the Archimedean screw. The material is fed into one end of a cylinder carried along and discharged at the other end by the screw. The cylinder is of metal gauze and is steam jacketed. (Lehmann's patent, 1912.)
Continuous systems, involving a battery of digestors connected by pipes, have also been devised. Arrangements are made of course for admitting water and steam as required.
REFERENCES.
"Glue and Glue Testing," by S. Rideal, D.Sc., 2nd ed., pp. 47-56 and 61.
"Gelatine, Glue and their Allied Products," by T. Lambert, pp. 21-24, 40, 42-44, 49 and 51.
"Encyclopedie chimique," Fremy, tome x., p. 83.
PATENTS.
Edison: U.S.A. patent, 1902, 703204.
Bertram: English patent, 1892, 951.
Dorenburg: German patent, 1911, 239676.
Lehmann: French patent, 1912, 441548.
SECTION IV.--CLARIFICATION AND DECOLORIZATION
After the raw material has been appropriately prepared and an aqueous extract or gelatine sol obtained therefrom, there are certain refinements necessary before the weak sol is evaporated. These purifying processes include (1) clarification, (2) decolorization, and (3) bleaching. Whilst most manufacturers have more or less successfully solved the problems involved in these processes, the practical methods that are in common use have been evolved and elaborated in a purely empirical way, and the underlying principles have been very imperfectly recognized, and indeed often confused and misunderstood. Hence it is even yet not uncommon to find these terms rather loosely used, and it is one aim of this section to define and distinguish these various operations in principle as well as in practice.
Clarification consists essentially in the removal of suspended matters, with the consequent production of a sol or gel which is bright, clear, and apparently homogeneous. Bleaching consists essentially in destroying the colouring matters of the sol by chemical action, such as oxidation or reduction. Decolorization involves the removal rather than the destruction of colouring matters, and does not therefore imply a chemical action in the ordinary sense.
Clarification may be now considered more particularly. It is necessary in this connection to consider what is meant by "suspended matter." The modern view is that the difference between a true solution and a muddy liquor or an emulsion is one chiefly of degree. If the particles of matter in suspension or emulsion (the disperse phase) be reduced in size they eventually merge into colloidal sols which are sometimes analogously named "suspensoids" and "emulsoids," if further reduced in size into "suspensides" and "emulsides," and with further reduction into true solutions. On this view not only suspensions and emulsions, but also sols, solutides and solutions are all heterogeneous. Now in practice the clarifying of a gelatine sol involves only the removal of the particles which are evident to sight. What is needed is that the product should make a sol or gel which to the naked eye appears to be optically clear both to reflected and to transmitted light. If desired, the limit could be expressed in terms of dispersity or specific surface. Now it is a comparatively easy matter to remove the coarser substances which often pass into the sol, _e.g._ undissolved portions of raw material or the insoluble portions, such as the hair, the grain (hyaline layer), and the elastic fibres of skin gelatine material, and the fibres which even remain in extracting acidulated bones. A more difficult proposition is the removal of still finer particles which may be almost said to be in colloidal solution, but which at any rate are so large that they cause a visible opalescence or even a turbidity of the gelatine sol. A more difficult task also is the removal of minute particles of grease, which are an exceedingly common cause of turbidity and which are often very effectively emulsified in the sol.
Now at this stage it is necessary to point out that besides the difference in the size of the particles of the disperse phase, there is another important difference involved, viz. that the particles of a colloid sol carry an electric charge owing to the adsorption of electrically charged ions of the electrolytes (salts, acids or alkalies) present. If this charge be removed the colloid is precipitated (coagulated, flocculated) and is then filtered off with comparative ease. This precipitation can be brought about by a reduction or elimination of the potential difference between the disperse phase and the continuous phase. The electric charge given by the adsorbed ions may be reduced by dilution, for dilution causes a lessened adsorption of the charging ions. Hence the well-known practical fact that it is more satisfactory to filter a dilute gelatine sol. Further, the electric charge may be reduced also by causing the adsorption of an ion of opposite charge. This is the principle underlying the precipitation (of any colloid) by adding electrolytes. It is essential here to consider which ions are most likely to be adsorbed, and also to bear in mind what charge they carry. Now the hydrion (H+) of acids and the hydroxyl ion (OH-) of alkalies are most strongly adsorbed, so that to precipitate a negative sol, acid is very effective, whilst with a positive sol an alkali is an appropriate precipitant. Further, it is known that organic ions are usually more strongly adsorbed, hence when precipitating from an alkaline sol (negative sol), one should preferably select an inorganic or mineral acid rather than an organic acid. Thus in clarifying an alkaline gelatine sol, hydrochloric or sulphuric acid is to be preferred to acetic or lactic acid. Again, it is necessary to remember that a divalent ion carries twice the charge of a univalent ion, hence the precipitating power of an electrolyte depends upon the valency of the ion whose electric charge is opposite to that on the sol (Hardy's valency rule). Thus a negative sol is most easily precipitated by a monobasic acid. Thus hydrochloric acid is better than sulphuric, on account of the stabilizing effect of the divalent SO{4}-- ion on a negative sol. In such a sol, also, the valency rule indicates that the multivalent kations, _e.g._ iron, Fe+++; chromium, Cr+++; and aluminium, Al+++, should have great precipitating and clarifying effect. This of course is known to be the case, aluminium salts having long been used. The rule indicates, also, that aluminium chloride would be better than the sulphate or than potash alum. Another feature of precipitation worthy of mention is the phenomenon of "acclimatization." This describes the fact that when the precipitating reagent is added very slowly, or a little at a time, a larger amount must be used, and the slower the addition the greater the excess required. Hence in precipitating matters from an alkaline gelatine sol the acid, if practicable, should be added all at once. In any case it is clear that one should aim at filtering a gelatine sol when it is near the iso-electric point, which is stable enough for gelatine itself, but a point of instability for many undesired impurities. Yet another phenomenon of colloid chemistry is concerned, viz. "protection." The particles it is desired to precipitate not only adsorb ions of electrolytes, but also the gelatine sol itself, and the particles, thus covered by a layer of a stable emulsoid sol, attain much of the stability of this gelatine sol. Unfortunately for gelatine manufacturers, gelatine possesses very great powers as "protective colloid," and this no doubt greatly enhances the practical difficulty of obtaining a clear and bright sol or gel. Here again dilution of the sol reduces the adsorption and correspondingly reduces, to some extent, the difficulty.
With regard to the turbidity or opalescence in a gelatine sol due to minute globules of grease, the case presents some analogy to the coarser colloid solutions, but the analogy has its limits, for an emulsion of grease is not an emulsoid sol. Doubtless the grease globules exhibit adsorptive phenomena, in which case the valency rule comes into force; the gelatine, also, by lowering interfacial tension, assists in protecting the emulsion; but grease emulsions are certainly stabilized in alkaline media (hence the detergent effect of soap, soda, borax, etc.), and it is undoubtedly easier to separate the emulsion by making the medium acid. Hence the practical fact that an acid sol is more easily clarified from grease than an alkaline or even than a neutral one.
The next stage in clarification is the separation of precipitated matters and of the coalesced particles of grease. This may be attained by the two processes usual in such a problem of chemical engineering, viz. sedimentation and filtration. After precipitation, therefore, the sol should be allowed to stand for some hours, during which time the precipitate not only flocculates but also settles to the bottom, and the globules of grease coalesce further and rise to the top, from which they may be skimmed off. Sedimentation alone is both too slow and too incomplete to be sufficient for proper clarification, and in these days it is always supplemented by the use of the filter-press. This well-known appliance can easily be adapted to the local requirements of the manufacturer. As speed of working is an essential requirement it is necessary to have a large filtering surface, and this may be done either by increasing the number of plates in the press or by increasing the area of the plates used. The large plates, however, are often cumbrous and inconvenient, and if of metal are very heavy. The plates may be constructed of well-seasoned wood, or in the case of alkaline gelatine and glues, even of iron. The framework is in any case usually iron. Acid gelatines and glues may have wooden plates, but "acid-proof" alloys are sometimes used to make them. Where it is essential to filter quickly two presses may be arranged _in parallel_, thus doubling the active filtering surface. When it is essential to obtain the highest possible clarity, two presses may be worked _in series_, which, in effect, means that the sol is filtered twice. In using the filter press for gelatine and glue it is most necessary to observe the most scrupulous cleanliness, and the plates must be frequently washed and sterilized. Rideal recommends weak chlorine water or bleaching powder solution for this purpose.
The process of _decolorization_, by which colouring matters are removed without being chemically altered or destroyed, usually precedes or takes place concurrently with the filtration. The underlying principle of this operation is adsorption. The colouring matters are usually in colloidal solution and most frequently are emulsoids, hence they are substances which are known to be exceedingly susceptible to positive adsorption. It is probable, also, that in a gelatine sol are particles which cause turbidity, though not coloured, and which are capable of being adsorbed. Hence the adsorption of colouring matters not only makes the sol more colourless, but in all probability makes it brighter and clearer. Further, decolorization by adsorption probably also involves the removal of the last traces of emulsified grease. It will be clear, therefore, that in the improvement in brightness and colour of a gelatine sol, adsorption fulfils a triple usefulness. The ordinary processes of dyeing fabrics or leather are adsorption processes, and the decolorization of gelatine sols consists essentially of the same process, except that the concentration of the dyestuff is much less, and the liquor remaining, instead of the adsorbent, is the primary consideration.
Decolorization of gelatine sols may be effected by any substance with a large specific surface. Indeed, a great variety of adsorbents are actually used in practice, and each factory has its favourite material or mixture, and its favourite mode, place, and time of application, determined partly by the nature of the adsorbent and partly by the precise form of apparatus used. Amongst the adsorbents which have received special favour are sand, kieselguhr, asbestos, animal charcoal, wood pulp fibre, albumin and alumina. Sand is very effective, but a comparatively large weight is needed, and its cleansing for repeated use is troublesome. On the other hand, it may be completely renovated by ignition. Kieselguhr is a very powerful adsorbent, and only a little will do much good; it is, however, hardly sufficient alone. Animal charcoal has great specific surface, but its pores are very small for viscous liquors, and its use is less suitable in the case of gelatine than in the decolorization of liquors which may be boiled. Wood pulp fibre is a very popular decolorizing material, not only in gelatine but also in other trades. Its short, woolly fibres give a clarifying as well as a decolorizing effect. It may thus act as a mechanical filter for suspended matter and grease, as well as an adsorbent for colouring matters present as sols. Its two functions, however, are often confused. It may be regenerated for repeated use by careful washing, and special pulp-washing machines are manufactured and sold for the purpose. Detergents are usually employed in the wash waters. Asbestos is also a good adsorbent, and its long fibres make it much less liable to non-operating "channels" and "bursts." It also has the advantage that, if desired, it may be regenerated by ignition. It forms a very useful mixture with pulp fibre.
All the above decolorizing materials are insoluble and hydrophobe, and act in virtue of their finely divided conditions, which causes them to have a large specific surface; but there is another type or branch of substances, whose effect is due to surface action of rather a different type. These are the hydrophile gels. In a gelatine sol the colloid particles have largely adsorbed the colouring matters which it is desired to remove. This adsorption, which is after all only an equilibrium, is reduced by introducing another very strong adsorbent. This latter, by adsorption from the continuous phase, reduces the adsorption of colouring matters by the gelatine particles. In the case under discussion another lyophile colloid is introduced, and after bringing about such an action is removed by appropriate means. The use of albumin has long been known for such a purpose, its special advantage being that after its admixture and adsorptive action, it may easily be removed by raising the temperature above 70° C., when coagulation takes place, and by subsequent mechanical filtration. The coagulated albumin takes down the adsorbed colouring matters. Albumin has been used in this way not only for gelatine and glue liquors, but also for tanning extracts (Part I., Section III.) and other commercial preparations. Into this class of decolorizing agents fall the insoluble inorganic gels which have been advocated by W. Gordon Bennett, _e.g._ alumina cream. Freshly precipitated alumina hydrate is a colloid gel with very considerable adsorptive powers. It has also the advantage that it is quite insoluble, easily removed in filtration, and has a powerful adsorptive action upon other objectionable impurities, especially the poisonous metals, arsenic, copper, zinc and lead. Its use is an undoubted advantage when in addition to the other clarifying agents and adsorbents. It is conceivable, in some cases, that when alum is employed as clarifying agent in an alkaline gelatine liquor, some alumina may be formed, and as such contribute to the total effect.
SECTION V.--BLEACHING
The adsorption law indicates that however much colouring matter is removed from the volume concentration (continuous phase) there must always be some left. After all that the decolorization processes can do, there still remains much colour that can only be removed by a chemical action of the ordinary sense. The amount of colouring matter of this kind is not large, but it is a deep red-brown, and when the gelatine sol has been evaporated and dried out the final product, if untreated, possesses this typical colour, and is known as glue. If, before gelation, a chemical bleaching action is applied to destroy this pigment, the product may be then dried out in a nearly colourless condition and is known as gelatine. Gelatine, therefore, is simply bleached glue. Many other definitions have been given, and many elaborate distinctions drawn, but the fact of bleaching is the essential difference. In these days when gelatine is so valuable, the higher-grade products are nearly always bleached, and the term "glue" is consequently more often applied to a lower-grade product, and is sometimes used in a sense implying this fact.
If it be desired to manufacture gelatine, it is fairly obvious that the task is lightened by observing the axiom that prevention is better than cure. If steps are taken to prevent the presence or development of such colouring matter, a great advantage is attained, for not only is the problem of bleaching easier, but also quicker and less expensive in chemicals. The nature of the colouring matters is but imperfectly investigated, but in the case of skin gelatine the pigment of the hair roots and epidermis is doubtless one factor. A long liming is said to assist in its destruction, possibly because this completes the loosening of epithelial structures and possibly because the alkali causes some hydrolysis of the pigment. In both skin and bone gelatine sols, however, there is a considerable tendency to develop the brown colouring matter typical of glue. This tendency is enhanced by an increase in temperature and also by the presence of acid or alkali. These facts seem to indicate that its development is associated with a partial hydrolysis of the gelatine in some direction. Rideal says this colouring matter is allied to caramel. In harmony with this is the experience that its development is greatest in products which have been "burnt," _i.e._ subjected to unusually high temperature. The practical maxims which arise from these considerations are fairly obvious and widely known, viz. to conduct the extraction and evaporation at as low a temperature as possible and in as neutral a condition as practicable. The temperature is particularly important during evaporation (see Section VI.).
Fortunately for manufacturers of gelatine, the colouring matter to be attacked is very susceptible both to reduction and to oxidation, and both types of bleach are widely used in practice. It is somewhat curious that the same colouring matter should be destructible both by reduction and by oxidation, but there is no doubt that each type gives a perfectly satisfactory bleaching action and can result in a practically colourless gelatine. On the other hand, the reduction is the more unstable reaction, for the glue colour slowly develops again in the gelatine on keeping it, even in a dried condition. Gelatine bleached by oxidation, however, retains its colour quite well, and even tends to improve with keeping. It is quite possible that quite different reactions are involved in the two processes, but in the light of the above facts it is somewhat surprising to observe Rideal's statement that reduction followed by oxidation has been successful in practice.
Although there is a wide choice of reducing and of oxidizing agents, those which are suitable for application to gelatine cover a very limited field. This limitation arises not so much from the ineffectiveness of the bleach, as from the other effects of these substances upon the purity of the product and upon the elasticity of the gel which it can yield. Especially important is the lyotrope influence of the bleaching agent. Many reactive substances are ruled out simply because they either insolubilize the gelatine or weaken the gel it makes. Others are inadmissible on account of their poisonous nature. It must never be forgotten that whatever is used in bleaching is, like the gelatine itself, much concentrated during evaporation and drying. Its possible percentage in the finished product should be considered, and also the possibility that in these finishing operations what is present may not remain in solution, owing to supersaturation.
=Bleaching by Reduction.=--Of all the reducing agents suggested, sulphurous acid has proved to be much the most suitable and successful. It has been used with equal success both for bone and for skin gelatine, but on the whole has proved more suitable for the former.
Sulphurous acid can fulfil in this instance a double function, viz. that of acid solvent for the bone phosphate, and that of bleaching agent also. As it penetrates the bone material, dissolving the phosphate, it also exercises its bleaching influence on the gelatinous part of the material. Changes of liquor tend to complete both actions, so that a counter-current system is found most convenient. The "acid process" for the manufacture of bone gelatine has been previously described (Section II.), and the use of sulphurous acid in this connection is typified in the Bergmann process. In this process bleaching is in effect merely a continued treatment.
In the case of skin gelatine, also, sulphurous acid may fulfil a double function, viz. that of deliming agent as well as of bleaching agent. In such instance it is necessary to use excess of bleaching acid, some acting as deliming material and the remainder as bleaching agent. As it is desirable to get rid of the lime and soda salts, several changes of liquor are given to the goods, possibly with intermediate washing. Here again approximation to a counter-current system is of advantage, as the employment of used bleach liquors for deliming purposes effects considerable economy of sulphurous acid. Indeed, there need be no waste acid at all.
Whether the material be for bone or skin gelatine, however, it will be seen that the extraction is conducted in an acid condition and the resulting sol is also acid. Most usually the decolorization and filtration processes are also conducted with such an acid sol. From what has been said (Section IV.) of the value of dibasic inorganic acids as clarifying agents, it will be understood that the presence of sulphurous acid at this stage is of great advantage in the production of a clear and bright gelatine. Indeed, it is well known in trade circles that sulphurous acid gelatines are usually of exceptional clarity and brightness.
The disadvantage of sulphurous acid processes is also found in the same fact that both sol, gel and cake are in an acid condition. To complete the bleach it is sometimes necessary to add sulphurous acid to the sol after extraction, or even after evaporation, but this is to be avoided if possible. Usually the ideal attempted is that the bleaching action should be as much as possible before extraction; the excess of sulphurous acid is then washed off just before the extraction, as far as practicable, and the rest is boiled off during extraction. The ideal is practically never attained, for the acid is strongly adsorbed, and the result is that the finished article is always an acid gelatine, and sometimes indeed very decidedly such. The acid condition is objectionable in the case of some forms of filter press on account of the solvent action on the metals, and is objectionable in evaporation for similar reasons. Acid gelatines are also objectionable for many purposes for which gelatine is usually sold, and this limits the commercial possibilities of the product thus obtained.
Sulphurous acid is itself, of course, a gas, and whilst the gas itself has been used for treating the material (_e.g._ bones), it has been found not only more convenient but also more effective to use an aqueous solution. This is mainly because it is possible to attain a greater adsorption in a liquor. Unfortunately, however, sulphurous acid is not a very soluble gas, and although 8-10 per cent. solutions may be, with great care, obtained, they are really supersaturated and readily yield the gas, even with slight mechanical agitation. Solutions even of 2 to 3 per cent. strength are also liable to this, and the general experience is that 1 to 2 per cent. solutions are most economical and convenient for practical purposes. As the freight on weak solutions is prohibitive, the manufacturer using sulphurous acid is faced with the necessity either of purchasing cylinders of sulphur dioxide liquefied by pressure or making the gas and solution himself. The former is the most convenient course when only small amounts are required, but the latter preferable for a gelatine factory of any size. Sulphurous acid is easily manufactured by burning sulphur and leading the fumes by induced draught up a scrubber down which water slowly trickles. Forced draught may also be used, as in the Sachsenburg plant.
Of the other reducing agents which have been used, sodium hydrosulphite (Na{2}SO{2}) deserves mention. It is a very powerful reducing agent, and has been found most useful when employed as an assistant to sulphurous acid. This reagent is usually added to the sol, after evaporation and before gelation. It is sold as a white powder, usually under trade names. Sometimes a mixture of bisulphite and powdered zinc replaces it, but this is objectionable for pure food gelatines. Its use also involves an impurity in the finished article, and a greater amount of "inorganic ash."
=Bleaching by Oxidation.=--Many oxidizing agents have been suggested for bleaching gelatine, but most of them have some practical disadvantage. Most of them contradict the maxim (previously noted) that it is desirable to avoid adding any soluble substance, as this involves a permanent impurity, possibly concentrated to supersaturation in the finishing processes, and possibly involving a disadvantageous lyotrope influence. There is another objection to oxidizing agents also; whilst their bleaching action on the pigments is undoubted, some of them have also a special action upon the gelatine itself which is in reality akin to tanning, and may indeed involve an insolubilization of the gelatine. Thus, chlorine gas (which Meunier patented for tanning) has been used for bleaching gelatine, but the conditions of success have not yet been thoroughly elucidated, and it is problematical indeed whether the process is consistent with best results. Hypochlorites and bleaching powder have also a similar action, which has been utilized with some success in practice. Rideal suggests that a suitable concentration for these reagents is 1:2000, and emphasizes the care necessary. An advantage of all these chlorinations is the formation of the strongly antiseptic chloramines, which preserve the gelatine from putrefaction. Ozone has also been tried as an oxidation bleach for gelatine, but not successfully, partly on account of difficulties in controlling the quantity used. Peroxide of soda has also been used, but it is not only alkaline, but liable to contain sodium hydrate and carbonate as impurities, and this involves neutralization either before use or in the gelatine sol, and the consequent presence of sodium salts in the finished article. Peroxide of calcium is open to the same objections, except that calcium is more easily removed from the sol than sodium. Rideal's suggestion for removing this lime, viz. precipitation by a current of carbonic acid, merits attention in this and in other directions also. Rideal also states that in the case of an acid bone gelatine, a good peroxide of lime is almost an ideal reagent for bleaching, inasmuch as "the lime carries down phosphate, several impurities and colouring matters." It thus acts as bleach, as neutralizing agent, and as precipitant, and the precipitate itself is a strong adsorbent. On account of its freedom from bases, and because its residue is simply water, peroxide of hydrogen has been found of great service in practice, and in most factories it has shown itself superior not only to the other peroxides, but also to all other oxidizing agents. Its application is simple, a concentrated solution being added to the gelatine sol before or after evaporation. It is the most "fool-proof" of all the oxidizing agents used in bleaching, and it yields the purest product. Its bleaching action is perfectly satisfactory, but only in a non-acid sol. Hydrogen peroxide is moderately stable in acid solution, and its bleaching action is best in slightly alkaline solution. An acid sol bleaches too slowly, or not at all; an alkaline sol induces evolution of oxygen and consequent waste. The great disadvantage of peroxide of hydrogen is its great expense, which is enhanced by an increasing demand for it in other industries. A minor disadvantage is its instability, which leads to loss in transit and storage. It is sold usually in strengths indicated by the volume of oxygen obtained from unit volume of the solution, when treated with permanganate in a nitrometer (_e.g._ "15 vols. peroxide").
It is a fortunate feature of both the oxidizing and reducing agents usually employed in bleaching, that they have considerable antiseptic power. This assists materially in preserving the gelatine from putrefaction during the critical period between extraction and concentration.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed., pp. 61-66, 78-82.
"Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49, 51.
"Chemical Engineering," _J.R. San. Inst._, No. 2, 1910. S. Rideal.
On adsorption phenomena:
1. "Chemistry of Colloids," Dr. W. W. Taylor.
2. "Chemistry of Colloids," V. Pöschl.
3. "Chemistry of Colloids," Zsigmondy and Spear.
4. "Chemistry and Physics of Colloids," E. Halschek.
5. "Surface Tension and Surface Energy," Willows and Hatschek.
SECTION VI.--EVAPORATION
The evaporation of the weak gelatine sols (3-9 per cent.) obtained by the processes described in previous sections into sols of such concentration (20-55 per cent.) that they readily set to a stiff gel on cooling, is now an essential feature of gelatine manufacture, and is one of the most important processes.
In the early days of this industry, manufacturers aimed at obtaining a concentrated sol, as this saved time in drying, and so reduced the possibilities of putrefaction. The advent of evaporation has reduced these possibilities to a minimum, and has also enormously reduced the space required and the capital outlay needed in the drying sheds. It has, in addition, given the practical advantages involved in dealing up to the last minute with a much less viscous liquor. As the liquors extracted are weaker, the extraction is more complete and the decolorization more easily effected.
The earliest attempts at evaporation were not very successful, partly on account of the prolonged "stewing" which ruined the setting power, and partly because of the poor economy of heat. Thus in the open evaporators the sol was maintained at a high temperature for a long period, and this process only proved suitable for low-grade products.
A great stride forward was made by Howard's invention of the Vacuum Pan. This made it possible to undertake concentration at much lower temperatures, a most important improvement in the case of gelatine and other organic matters easily damaged by heat. The process, however, was still slow, and the sol exposed to heat for a long time, as must be the case when evaporation takes place in bulk. These disadvantages were still fatal to the production of the highest-grade gelatine. There were also the practical difficulties of entrainment ("blowing over"), in which parts of the sol were carried away by the escaping vapour, and also of "incrustation" which so rapidly reduces the heating efficiency and evaporative capacity of the machine. The vacuum pan, however, presented two decided advantages--evaporation at a low temperature, and, as a corollary, the possibility of utilizing exhaust steam to attain this temperature.
Whilst the vacuum pan was a satisfactory machine for many branches of chemical engineering, the problem of evaporation was still unsolved for gelatine liquor because of the "stewing" involved, until the advent of the "film evaporator," which dealt with the liquor not in bulk, but in a continuous stream. In this way the product was only exposed to heat for a comparatively short time. Many evaporators of this type came into being, and rapid improvement was made in the constructional details. The film evaporators retained usually the advantage of evaporation _in vacuo_, so that it was now possible to evaporate gelatine sols by exposure for a short time to a comparatively low temperature. Of this type of evaporator, the Lillie, Yaryan, Schwager, Claassen, Greiner, Blair Campbell, and the Kestner machines are well-known examples.
A further advance in solving this problem was the application of the principle of multiple-effect evaporation. The vapour driven off during evaporation possesses of course many heat units, and is of very considerable volume. In multiple-effect evaporators this vapour is used to work a similar evaporator, and the evaporated liquor passes immediately into what is practically a second machine, and is further evaporated by the heat from the vapour just driven from it. Such an arrangement would be termed a double-effect evaporator. The vapour from the second effect may of course be similarly used to operate a third effect, and the vapour from this to work a fourth effect, and so on. Thus, we may have triple effect, quadruple effect, etc., even up to octuple effect. The great advantage of multiple-effect evaporation is in the saving of costly steam. Reavell gives the following figures to illustrate the economy thus obtained:--
WATER EVAPORATED PER 100 UNITS STEAM.
-----------+-----------+-----------+-------------- Single. | Double. | Triple. | Quadruple. -----------+-----------+-----------+-------------- 95 | 150 | 220 | 300 -----------+-----------+-----------+--------------
There is naturally a limit beyond which the capital cost of the machine neutralizes the advantage of steam economy, and it is seldom that octuple effects are used. There are probably more triple effects in use than any other machine.
An essential and important part of the modern evaporator is the "condenser," in which the vapour from the last effect is conducted into water (jet condensers) or over cooled surfaces (surface condensers), with a view to producing and maintaining the vacuum.
A lasting vacuum cannot be maintained without an air-pump, as air is often introduced (1) with the steam, having entered the boiler dissolved in the feed water; (2) by leakage from the atmosphere into the condenser and the connected vacuous spaces; and (3) in jet condensers, in solution with the circulating condenser water. That from the first two sources may be reduced, but the third is beyond control: hence if high vacua are necessary, surface condensers are to be preferred. Dissolved air is usually 5-20 per cent. of the water volume, and is least for sea-water. It should be noted that water leaving a surface condenser is in a very air-free state, and therefore particularly suitable for boiler supply. Apart from the capital cost of a condenser the chief cost of maintaining a vacuum is in pumping the circulating water, of which up to 70 lbs. is usual per lb. of steam condensed.
If W = weight of steam condensed (lbs. per hour); Q = weight of cooling water circulated (lbs. per hour) T{i} = inlet temperature (° F.) of cooling water; T{o} = outlet temperature (° F.) of cooling water; then T{o} = T{i} + 1050(W/Q)
It will be understood that for high vacua, low temperature of cooling water (T{i}) is more important than copious supply (Q/W). It is advantageous, however, to choose a site yielding plenty of cold water, such as a river or canal side. Otherwise it is often necessary to use cooling towers or spray nozzles. The cooling is by evaporation (= 60 to 80 per cent. of W), cold water replacing that evaporated, and yielding water 75° to 80° F. If T{i} = 80° F. and Q/W = 70°, a vacuum of 28.34" is possible, but the 0.34" should be allowed for the partial pressure of the air, determined exactly by the air entering and by the displacement of the air-pump.
Another feature of the modern evaporator is the "heater" or "calorifier," by which the liquor to be evaporated is led in a continuous rapid stream through heated tubes immediately prior to its entry into the first effect. It is the aim of the heater to raise the temperature of the liquor to the temperature of evaporation, and so to avoid this being necessary in the first effect. The heater thus further avoids stewing, ensures steady running, and effectively increases the capacity of a machine.
It is noteworthy that superheated steam is not desirable for working an evaporator. The principle of evaporation by steam is not merely that the temperature of the liquor is raised to boiling point; it is that in the condensation of the heating steam its latent heat is yielded to the liquor being evaporated. To evaporate quickly, therefore, the heating steam must condense rapidly. Hence, as superheated steam has a rate of condensation 20-30 times slower than saturated steam, the latter is much to be preferred. A slight superheating, however, may be justifiable where the steam has any distance to travel before use. It is the fact that it is the latent heat of steam which is mainly utilized which gives steam its great practical advantage over hot non-condensable gases. Steam in condensing yields an enormously greater number of heat units per lb. than hot waste gases. Steam has also the advantage of more constant temperature.
The capacity and efficiency of an evaporator depends upon a good many factors, some of which are worthy of discussion at this point.
The transference of heat and the amount of evaporation are directly proportional to the mean temperature difference between the heating steam and the liquor being evaporated. These temperatures, however, both vary somewhat, the steam losing part of its pressure and temperature as it passes along the heating surface; the liquid generally increases in temperature. The mean difference in temperature, moreover, is not the arithmetic mean between the smallest and largest temperature differences, but is given by the following expressions, which yield results not wide apart:--
If [theta]{a} = temperature difference at commencement; [theta]{e} = " " " end; and [theta]{m} = mean temperature difference;
then
[theta]{m} = ([theta]{a} - [theta]{e}) / log([theta]{a} / [theta]{e})
or = ([theta]{a} - [theta]{e}) / [ n(1 - [nth root of]([theta]{e} / [theta]{a})) ]
This mean temperature difference is in practice usually spoken of as the "temperature head" or "heat drop." It will be clear that this temperature head is increased by using steam at higher pressure (temperature), and by evaporating under reduced pressure. Since most liquids have their boiling points reduced about 40° C. by operating _in vacuo_, the advantage of the vacuum is apparent. It should be remembered that the temperature head has not the same value in any part of the scale: it has more value higher up the scale, because the steam is denser and more heat units come in contact with a given area in a given time. It must also be remembered that whilst the pressure gauge is a most useful indicator of steam temperature, it is not necessarily accurate. The pressure in the hot space is the _sum_ of the pressures of air and steam, and since the temperature (the important condition) of the hot space depends upon the pressure of the _steam_, and not on the sum of the pressures, the temperature in a steam space is always rather lower than would be supposed from the pressure indicated by the gauge.
The transference of heat is influenced by the velocity of both the heating fluid and the fluid being heated over the heating surface. The more rapidly each fluid moves, the more rapid is the transference of heat, because a greater number of particles of both fluids are brought to the heating surface in any given time. This is popularly known as the effect of "circulation," and is illustrated by the advantage of stirring a liquid being heated in bulk. In the film evaporators the circulation is through tubes at high speed (up to 2 miles a minute), and the maximum effect in this sense is thus obtained. The increase in heat transference is not directly proportional to the increase in velocity, but in a lower ratio, sometimes approximately the square root of the velocity. In such a case, if either velocity be quadrupled, the heat transference is doubled. Other advantages of high velocity are that the heating steam more readily sweeps away condensed steam from the heating surface, and the high-speed film similarly "scours" away "incrustations" on the interior of the tubes.
The transference of heat is also proportional to the conductivity of the metal forming the heating surface. For gelatine liquors, copper tubes are almost invariably employed, the advantage being great even when price is taken into consideration. The following conductivity coefficients illustrate this point (calories per hour through 1 sq. metre of metal 1 metre thick, with a temperature difference of 1° C.):--
Copper...330
Iron.....56
Steel....22-40
Tin......54
Zinc.....105
Lead.....28
The coefficient of heat transmission decreases the more with increasing thickness of wall, the worse conductor is the metal. For copper tubes, however, this decrease is usually unimportant.
The transference of heat is also much influenced by the viscosity of the liquor being evaporated; the greater the viscosity, the lower the coefficient of heat transmission. Unfortunately for this process of evaporation, gelatine sols are exceedingly viscous, and thus the difficulty in obtaining a concentrated sol is thus greatly enhanced.
The transference of heat is often greatly hindered by incrustations of the tubes, which incrustations generally conduct heat very badly. Thus the relative heat conductivities of copper and chalk are as 1000:5.
The amount of heat transferred is of course determined also by the area of the heating surface. The amount of evaporation needed thus determines the number of tubes (of standard size) in the evaporator, and thus the capacity of the machine. An evaporator should have its heating surface area chosen with a view to the duty required of it.
In practice the working of an evaporator is often not a very difficult matter, and large numbers of machines are operated by unskilled labour. Troubles generally arise from inconstant steam pressure, incrustation, leakages of air, which reduce the vacuum, the temperature head, and hinder heat transmission. For the evaporation of gelatine liquors the Yaryan, the Kestner, and the Blair-Campbell film evaporators are the most widely used. The velocity of the liquor through some of these machines is so great that occasionally no vacuum is used. The temperature obtained is high (200° F.), but the time is very short, if rapid cooling of the evaporated liquor is arranged.
REFERENCES.
"Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand. Scott, Greenwood & Son (1916 Ed.).
"Evaporation," by E. Kappeschaar. Norman Rodger (1914).
"Evaporation in the Chemical Industry," by J.A. Reavell, M.I.Mech.E., _J.S.C.I._, 1918, April 11th.
"Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59.
"Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29.
"Notes on Condensing Plant," J.M. Newton, B.Sc., _J. Junior Inst._ Engineers, Aug., 1912.
SECTION VII.--COOLING AND DRYING
The conversion of a gelatine sol into cakes of gelatine has been much simplified by the advent of the evaporator. Before this machine was used much trouble was experienced with putrefaction, and in hot and thundery weather, especially on the Continent, it was often necessary to suspend operations. Evaporation has, however, materially contributed to the possibility of rapid and satisfactory cooling and drying.
From the time the weak sol is decolorized and bleached, the finishing processes consist essentially in the removal of water. This is now usually done partly by evaporation of the sol, and partly by the desiccation of the gel. There is an obvious elasticity in method, and factory practice does actually vary considerably in the relative proportions of these two alternatives. Some factories evaporate to a 20 per cent. sol, approximately, and rely upon drying sheds and lofts to complete the desiccation: other factories evaporate up to a 55 per cent. gelatine sol, and so can manage with less shed room. Something depends upon local conditions, but the main issue is between the cost of steam in evaporation and the cost of land and buildings required for sheds. On the whole the modern tendency is to evaporate more, for this course has the additional advantage of speed, involving both a quicker turnover and less liability of putrefaction. Lower-grade products need relatively greater evaporation to form a gel of equal rigidity.
After evaporation and bleaching, the concentrated sol is first cooled rapidly until it has set to a stiff gel, then cut up into cakes according to the size required, these being dried out on network frames arranged in tiers, through which a draught of air is usually forced or induced. This general description is of course applicable to many factories with innumerable variations in detail, most of which variations originate in local convenience and are unessential parts of the manufacture.
An essential principle is that the cooling or gelation should be done rapidly, not only to avoid putrefaction but also to avoid the action of heat on the elasticity of the gel. A hot sol or gel is liable to hydrolysis and loss of setting power, and should have its temperature quickly reduced, but a warm sol or gel (say 100° F.) is most liable to putrefaction, so that the cooling should be continued quickly. On the other hand, the gel should not be frozen. For cooling purposes a copious supply of cold water is most usually employed, but some factories have installed refrigerators. These plants operate by the rapid evaporation of liquefied gases such as carbon dioxide, sulphur dioxide, or ammonia, so arranged as to cool a solution of common salt, which forms the circulating liquor and is returned after use to the refrigerator. Where such plants are used, it is natural that their use should be extended to the drying sheds to cool the air entering in the height of summer. In some factories the cooling is attained neither by cold water nor cooled brine, but merely by cold air.
The kind of vessel in which gelation is induced varies widely in different factories. For lower-grade products metal boxes are used, heavily galvanized iron being the most common material. If the liquor be muddy, deep boxes are preferred, but if clear, rapid cooling is best attained by having them long and shallow, and so exposing a relatively greater area to the cooling action. In either case the boxes may contain up to 1/2 cwt. of jelly. Lambert mentions boxes 24" × 6", which are 5" deep; Cavalier suggests rectangular moulds holding 30 litres. In place of galvanized sheet iron, boxes of sheet zinc or of wood lined with zinc are sometimes used. In any case the most scrupulous cleanliness should be observed in all cooling-house work, and in some factories the most elaborate precautions are taken for cleansing vessels, tools, floors, etc., and even for their disinfection and sterilization. Iron, tinned iron, and copper cooling vessels are ruled out on account of their tendency to rust and tarnish, and the last is unjustifiably expensive. Many of these vessels are unsuitable for pure food gelatines in which traces of copper, zinc and arsenic are held to be very objectionable. For the best gelatines, therefore, a very shallow vessel (1/4" to 1/2" deep) with a sheet glass bottom is preferred, and the concentrated sol is run on to this for gelation.
Glue (or gelatine) which has set in this way is sometimes called "cast glue." That which sets in metal boxes in blocks is termed "cut glue," because the blocks of jelly need subsequently to be cut into slabs of the desired size and shape. Jelly blocks may be cut by hand with the "wire knife" which yields a characteristic wavy appearance to the finished product. This may also be done by machinery, the block of gel being placed on a series of correctly spaced wires and forced through the network by hydraulic pressure. A cutting machine (Schneible) has also been used to cut up blocks of jelly into slices of the required thickness, but these machines have not made great headway in this country. It will be clear that cast glue is cooled more rapidly than glue in blocks; it is therefore not surprising to note Lambert's statement that the former comprises the larger proportion on the market.
The cut or cast cakes are next placed upon network frames, and a series of such frames are placed on a bogey. The bogey is run along tram lines into the drying tunnel, through which air is forced or induced by a fan. Many such bogeys are, of course, passed into each tunnel, and as many tunnels as required may be constructed. Care is necessary to expose the cakes evenly to the action of the air. It is mostly necessary to warm the air at the inlet by means of steam pipes and so increase its drying power. This is especially necessary in winter or wet weather. In summer, however, it is often arranged that the air is cooled before entering the sheds. This is accomplished by passing the air through pipes from a refrigerator. When heated air is used, it is stated by Lambert that the maximum temperature should be 25.5° C. (78° F.); Rideal considers 21° C. (70° F.) should be the maximum. In all cases the drying power of the air is easily ascertained from a wet-and-dry bulb thermometer, and the amount of air passing along the shed from a wind gauge. Lambert states that drying normally occupies four to five days. The final product is still a gel, of course, and contains from 10 to 18 per cent. of water. It appears, however, very hard and solid. The dried cakes are removed from the frames and transferred to the warehouse, where they are sorted according to quality and packed in bags or tin-lined boxes. Some material is ground to powder.
The network of the drying frames has been made from many materials. Cotton or string netting is very common, but is liable to sag and to get dirty. It also has a short life. Ordinary galvanized iron soon loses its galvanizing cover, and the iron then is liable to rust. Attempts have been made to use sheet zinc and other alloys, which are cut or punched into nets with square or diamond-shaped holes. These were found to warp and break. Rideal's conclusion, which is confirmed by the general experience, is that the best material is a heavily galvanized iron wire netting. He suggests that it should have 15 to 25 per cent. of its weight of zinc, and that it should be strengthened by stiffer ribs arranged both longitudinally and transversely.
Many attempts have been made, and many patents taken out, with the object of making the cooling, cutting, and drying processes as continuous and as quick as possible, and with a view to saving labour, which is rather costly at this stage. These attempts, however, have only met with indifferent success. A common idea is that a continuous supply should fall upon a revolving appliance, and be instantly congealed in a thin state, which last lends itself to more rapid desiccation. Vacuum drying has also been attempted.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., pp. 68-74.
"Glue, Gelatine, and Allied Products," T. Lambert, pp. 30-35.
_Chem. Zeit._, 1911, 85, 17 (Cavalier).
PATENTS.
Eng. Patent (1894) 11,426 (Hewitt).
Eng. Patent (1898) 2,400 (Brauer).
Fr. Patent (1909) 398,598 (Lehmann), _J.S.C.I._, 1909, 897.
U.S. Patent (1912) 1,047,165 (American Glue Co.).
SECTION VIII.--USES OF GELATINE AND GLUE
Gelatine and glue have both been put to an immense variety of uses, and the list is constantly extending. Indeed, no one who considers the following account of their applications can doubt that gelatine and glue have become a necessary part of our civilization.
Gelatine for edible purposes certainly forms a very considerable part of the total used, and great pains are now taken to obtain a pure product. Thus, a gelatine with more than 1.4 parts per million of arsenic, or more than 30 parts per million of copper, is not considered good enough for "pure food." The food value of gelatine, compared with other proteids, is exceedingly low; its use in this connection has no connection with the "calories" of heat energy it will yield. It is used almost entirely because of its property of forming a gel. Table jellies form, of course, one popular use of gelatin, but the manufacture of sweets makes also a great demand upon the gelatine trade. Culinary operations often require a little gelatine, especially is it used in pies and soups. An extension of the same idea is found in its employment for many manufactured foods, _e.g._ tinned meats, meat extracts, and the concentrated foods. The use of gelatine in connection with the first of these received a big impetus during the war period. In gelatine for any of these purposes, the presence of excess of sulphurous acid is objectionable, as its taste is easily noticed.
Gelatine for medicinal purposes finds an ever-growing number of applications. Gelatine capsules for holding greasy liquids and solutions of nauseous drugs are increasingly popular, for the dose may be swallowed without unpleasantness. In making these capsules some sugar is also used, and the finished article is often protected from atmospheric moisture by treatment with a weak solution of alum. In a similar way pills are often coated with a 33 per cent. gelatine sol. Such pills are not only pleasanter to swallow, but are less liable, after being dried, to stick together in the box. Alcohol solutions of drugs (or essences, perfumes, etc.) may be suitably stored in gelatine instead of metal tubes. Medicated wines are detannated by gelatine before the addition of drugs which would have been precipitated by the tannin. The British Pharmacopoeia specifies four kinds of "Lamellæ," which are small discs of gelatin and glycerin, each containing a minute but definite dose of some powerful alkaloid. Glycerin jelly is a mixture of gelatin glycerin with some water. It is used for chapped and rough hands; the mixture is also used for glycerin suppositories, and for mounting microscopic sections. The mixture also forms the basis of gelato-glycerin, used in nasal bougies, and of glyco-gelatin for medicated lozenges. Gelatine insolubilised by formalin (formo-gelatin) has been used for making tabloids, wound dressings, and artificial silk.
Gelatine is in constant demand for bacteriological work, for which purpose a high-grade product is desired. Nutrient media for the culture of bacteria are solidified by 10-15 per cent. of gelatin, and the growth of colonies of bacteria often show typical formations. By inoculating into a melted and sterile quantity and setting quickly in a flat dish after mixing, the number of bacteria in the volume introduced can be judged from the number of colonies which develop. Bacteria are also distinguished often as "liquefying" or "non-liquefying" according to their type of culture on nutrient gelatine media. Gelatine for such work should be neutral and of high clarity.
The gelatine required for photographic purposes is also a high-class product. It should be neutral, colourless, and free from chlorides and other mineral salts. Grease also is objectionable. Gelatine is used in the numerous carbon processes, in which the principle is that gelatine is made insoluble in water by the action of potassium dichromate under the action of light. It is used also in Poiteoin process for copying engineering drawings, which is based upon the power of a ferric salt to render gelatine insoluble so long as it is not exposed to the actinic rays.
Gelatine is used in the manufacture of the "crystalline glass" used for decorative purposes. Advantage is taken of the immense contractile force it exerts on drying. When ground glass is coated with gelatine, and the latter dried, it tears away the surface of the glass itself, and leaves peculiar fern-like patterns. Inorganic salts dissolved in the sol influence the nature of the pattern obtained.
Gelatine is used also very largely in the textile trades, for finishing coloured yarns and threads, for sizing woollen and worsted warps, and for thickening the dyestuffs used in printing fabrics. It is also used for finishing white straw hats; as a size in the manufacture of high-class papers, and as a wax substitute for covering corks and bottle necks.
Glue is used instead of gelatine in all cases where colour is not a matter of much moment. The fact that it has not been bleached makes no difference to its suitability in such a case, and the cost is substantially reduced. Thus, for dark-coloured straw hats, textiles, sweets, papers, and in all suitable woolwork, glue is used in place of the more expensive article.
A very large quantity of glue is used in the manufacture of matches, where it functions as the material binding the "head" to the stem. A 15-50 per cent. sol is used, containing nitrate or chlorate of potash as oxidizing agent. The mixture is kept at 38° C. and the phosphorus cautiously added, and when this is emulsified, the friction ingredients (sand, glass, etc.) are also added. The glue acts also in preventing premature oxidation. Glue is also used in making the match-boxes, and similarly in making sand, emery, and glass papers and cloths.
There is a large consumption of glue by joiners, carpenters, cabinet-makers, and all kinds of woodwork and fancy work. It is used in the manufacture of furniture of all kinds, of pianos, organs, billiard tables, panels, picture frames, and of toys and brushes. Mixed with white lead, chalk, and sawdust, it forms a composition used for mirror frames, rosettes, etc. Glue is used for veneering, for mosaics, plaques, trays, fingerplates, leather wall coverings, and for staining floors.
There is also a considerable sale for glue in bookbinding, for which a sweet, light-coloured, and strong product is required. It has been found particularly suitable for leather bindings where the grain has been artificially printed or embossed, and in finishing and gilding.
The compositions used for printing rollers all contain gelatine or glue together with sugar or glycerin and possibly oil and soap. They are often hardened with formalin. Similar mixtures are used for the beds of hectographs.
Glue (together with waste leather) is used in the manufacture of imitation leather and leather substitutes. Cotton and wool fibres are often incorporated, and sometimes textile fabrics.
Much glue is converted into "size," which is a weak gel used as a filling rather than as an adhesive agent. A low-grade glue is often therefore preferred for such purposes, as having "body" rather than "strength." Size is often sold in cake, but sometimes in the form of the gel itself, in which case it may never have been evaporated. Indeed, size is often overboiled glue, made by crude and out-of-date methods. It is largely used in the paper trade, and for wallpapers, millboards, papier-mache, paper and cardboard boxes, etc. Mixed with logwood and iron, and possibly alum, it formed the "blue size" once largely used by bootmakers as a foundation for blacking, and is similarly used in currying. Size is also used in making oil paints and varnishes. Distemper is a size with which is incorporated whiting or gypsum and coloured pigments. In all applications of size, it is common to use antiseptics. Salicylic acid has been widely used in this sense. Low-grade glue is used for the manufacture of cheap brushes and for fly-papers.
Innumerable patents have been taken out and mixtures invented for the production of plastic materials, which frequently involve gelatine or glue. Thus, gelatine and glue are used in making plaster casts, and for imitation ivory, wood, stone, and rubber. Many of these inventions have been investigated by Rideal, who points out the features common to most of them. Usually a viscous sol is thickened by the addition of inert fibres and powders, and with the object of making the preparation more waterproof it is customary to incorporate oils, fats, waxes, tars, and resins before the gel is set. The surface is hardened by "tanning" with formalin or tannin solution, finally painted or varnished.
Equally innumerable are the inventions, recipes, and patents for making glues that shall remain liquid. The convenience of this ideal is obvious, but many of the suggestions are useless. It is quite easy to incorporate into a gel substances which keep it liquid--any soluble substances with a lyotrope influence of the iodide type will do this--but these also prevent the glue setting when used. Even in small quantity they will influence the tenacity of the joint. Other methods depend upon a partial hydrolysis of the protein. Amongst the most successful of these attempts are to dissolve 3 parts of glue either in 12-15 parts saccharate of lime, or in 9 parts of 33 per cent. acetic acid.
Many special glues and cements are made from commercial glue, according to the purpose required. "Marine glue" contains no glue; it is made from shellac and rubber mixed with benzene or naphtha. Its advantage is waterproofness.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.
"Uses of Glue," chap. iii. p. 83.
"Uses of Gelatine," chap. iv. p. 100.
"Special Glues," p. 108.
"Liquid Glues," p. 119.
"Gelatine, Glue, and their Allied Products," T. Lambert.
"Uses of Glue and Gelatine," chap. ix. p. 80.
"Liquid Glues and Cements," chap. viii. p. 69.
SECTION IX.--THE EVOLUTION OF THE GELATINE AND GLUE INDUSTRY
The manufacture of gelatine and allied products has received a great stimulus in this country from the circumstances arising from the European War. The large restriction of continental--especially French and Belgian--supplies of gelatine, led to greater demands for the British-made product, and resulted not merely in a period of greater prosperity, but in a period in which much greater efforts were made to supply a high-grade article in larger quantities. Most manufacturers strove to make high-class gelatine rather than low-grade glue, great extensions were made, and many new businesses were established. The development of the leather trades, more particularly in respect of greater production, caused a bigger supply of raw material for skin gelatine, and the slaughter of home animals for food caused a more plentiful supply of bones. At the same time it was realized that greater production not only reduced working costs, but also that a bigger turnover in any one factory involved a proportionately less capital outlay. These facts tend to counterbalance the heavy freight on the raw materials. Production is thus not only on a larger scale but more intensive.
One of the greatest difficulties of this industry is to produce a regular or standard article, for the raw material is so exceedingly variable in quality; that for skin gelatine tends also to become less valuable. In such a case, as Rideal has truly remarked, to ensure that supplies to customers shall be always "up to sample," which is often a matter of contract--"exact and regular working, strict cleanliness, observance of temperatures and other physical data, and scientific supervision", are clearly necessary. "Rule of thumb" is never quite certain to produce the same article twice. In past years British methods of manufacture have been far too empirical. As in other industries, "rule of thumb" must inevitably be replaced by scientific principle. The advances in colloid chemistry of this last decade or so have, in the author's opinion, supplied the clue to this line of development. In the preceding pages emphasis has been laid upon the importance of the adsorption law, the lyotrope series, and the valency rule. The manufacturer or supervisor who understands and can apply these generalizations will find his task vastly easier and his factory more efficient. Much remains to be learnt, however, and the industry would certainly benefit by research work, for which there is a fertile field.
There is also considerable room for improvement in the methods of chemical engineering usually employed. Whilst the heat engineers have certainly done much to solve the question of evaporation and drying, there is still great scope in the more economical application of heat in extraction, and the last word can hardly have been said on the problem of clarification and decolorization. There is indeed almost as much scope for research by the chemical engineer as by the colloid chemist.
The industry also exhibits, in common with the leather and many other trades, the same tendency to save labour, both by careful arrangement of the factory and by the installing of mechanical labour-saving devices. Thus, lifts, runways, hoists, and trucks are increasingly used to move the solids, and pipes and pumps to move the liquors. As ever, there is scope for the mechanical engineer.
If some of these problems are vigorously tackled during the present reconstruction period, there is little doubt that the gelatine and glue industry will be in a much better position to cope with all possible competition in the future.
From what has been said in Section VIII. as to the wide uses of gelatine and glue, it will be seen that general prosperity in trade is conducive to better trade conditions in the gelatine and glue industry. It is similarly true that a general trade slump affects the glue trade adversely. The severe trade depression which commenced in 1920 has had this effect, and has made economic production much more difficult as well as more essential. As often is the case, the larger factories and firms can better face the difficulties, and there can be little doubt that if the depression be long continued there will be a tendency for the smaller factories to be closed down and for the larger firms to unite. As in the leather trade, both the War boom and the Peace slump have caused the gelatine and glue trade to develop along the lines of the great trusts. It may be reasonably expected, moreover, that these will be intimately connected with the leather trusts. This fact, together with the heavy freight charges on the raw material, tends also to make the skin glue factories gravitate towards the leather centres.