CHAPTER I.

NATURE OF GLUE.

1. SOURCES OF GLUE.

The organisms of all animals, but more especially of the higher classes, contain tissues which are insoluble in cold, as well as in hot, water. However, by continued boiling they become dissolved, and yield on evaporation of the solution a glutinous, gelatinizing mass. By further drying this mass exhibits, according to the degree of purity of the material, a more or less transparent and brittle substance, which in its pure state is devoid of color as well as of smell; it swells up in cold water and dissolves by boiling in that liquid. This substance, _i. e._, the product of the conversion of the so-called glue or gelatine-yielding tissue, is what is known in the trade as _glue_.

Among the glue-yielding tissues, the following are the most important: Cellular tissue, the corium, tendons or sinews, the middle membrane of the vasa lymphatica and veins, the ossein or organic matter of bones, hartshorn, cartilage, the air bladders of many kinds of fishes, etc.

Neither glue nor gelatine exists ready formed in the animal organism, except under abnormal conditions as a phenomenon of disease, but they are the products of various transformations. The first of these transformations evidently takes place in drying the hide, since the result of boiling to glue a green hide prepared in the usual manner by liming, etc., but not previously dried, will be an entirely different product of less consistency than that obtained by drying the hide after liming and then boiling. A second transformation seems to take place in boiling the material, and a third in drying the jelly obtained, and this may explain the fact that the latter, which is not converted into actual glue, differs in its behavior from glue solution. The series of transformation does not end even with the actual glue, for it is a well-known fact that glue dissolved in water and boiled for some time does not gelatinize on cooling, but remains liquid. We have here to deal with organic combinations which are distinguished from the more solid organic compounds by passing more readily into decomposition. However, it is an established fact that glue is an organic combination presenting itself in different modifications. In the animal organism it occurs ready formed only under abnormal conditions as a phenomenon of disease, and hence it is only produced by first drying and then by continued boiling of the glue-yielding substance, and finally by evaporating and further drying the gelatinous mass obtained by boiling.

2. TRANSITION STAGES OF GLUE.

We therefore distinguish:

_a._ Glue-yielding substance.

_b._ Crude glue.

_c._ Jelly.

_d._ Glue.

_a._ The glue-yielding substance of the animal body is produced from proteïne substances, albumen, fibrine and caseïne, in a manner similar to that in which new substances are formed in the ripening fruit by the transformation and disintegration into constituent parts of others previously present.

_b._ By _crude glue_ are understood glue-yielding materials free from all foreign matter and physically prepared by drying. It forms an intermediate link between glue-yielding substance and jelly.

This distinction between glue-yielding substance and crude glue is justified by experience. If, for instance, fresh calves’ heads, such as the tanner cuts off after swelling the skins, be carefully limed and then boiled without previous drying, the result will be a turbid liquor containing, though everything be dissolved, no jelly whatever, or at least, very little.

_c._ _Jelly_ is obtained by boiling the crude glue. Its adhesive power is far less than that of solution of finished glue, and it will become more quickly putrid than the latter.

_d._ The finished product _glue_ is, in most cases, not a definite chemical compound, but a mixture of substances, with two of which scientific research has made us thoroughly acquainted.

3. CONSTITUTION OF GLUE.

Independent of impurities and accidental constituents, glue consists of two distinctly distinguishable combinations, namely, _glutin or gelatin_ and _chondrin_, the former being formed from the hide and osseous parts, and the latter from young bones while still in a soft state, and the “permanent” cartilages, such as those of the ribs and joints.

The manufacturer has it, of course, in his power to allow either of these substances to predominate in his product, but since experiments have shown glutin or gelatin to possess much greater adhesive power than chondrin, it is advisable to separate as much as possible the cartilaginous matter from other glue-yielding material.

As an accurate knowledge of these constituents of glue is of great importance to the manufacturer, brief reference will here be made to what scientific research has made known to us in regard to them.

Pure glutin or gelatin is obtained by treating buckshorn, etc., with water containing hydrochloric acid, until the phosphate of lime which serves, so to say, as a frame for the glue-yielding substance, is dissolved, and the organic tissue called _collagen_ or _ossein_, remains behind. After freeing the latter from fat by steeping in milk of lime and careful washing, it is boiled, and the resulting jelly, when cold, mechanically distributed in cold water, in which it softens but does not dissolve. By thoroughly stirring the mass the glutin yields its coloring matter to the water, the latter being replaced by fresh water until all the coloring matter is extracted. Then pour off the water and after dissolving the jelly in hot water, filter the solution through a cloth. By mixing the filtered solution with an equal volume of alcohol, a precipitate of pure glutin is obtained. By the precipitation with alcohol, the separating glutin carries down inorganic salts, especially phosphates, which may be present in the solution. To free it from them, dissolve it in a small quantity of lukewarm water, acidulate the solution with hydrochloric acid and bring it into a dialyser. The salts and the acids diffuse in the water which has from time to time to be renewed, and finally a jelly of pure glutin remains behind; this is evaporated to dryness in shallow vessels.

Pure glutin, in a dry state forms a glassy substance, almost colorless, transparent to translucent, brittle or slightly elastic, free from odor and taste, and remains unchanged in the air. Its specific gravity is greater than that of water. It is neutral, exerts no influence whatever upon vegetable colors and is insoluble in alcohol, ether, hydrocarbons or oils. In cold water it swells up, absorbing as much as 40 per cent., and becomes opaque, but does not dissolve. It dissolves in hot water and on cooling forms a jelly even if the solution contains only 1 per cent. of glutin. It gelatinizes at a lower temperature than chondrin.

An aqueous solution of glutin is precipitated by chlorine, platinic chloride, tannin and alcohol, but not by hydrochloric acid, acetic acid, lead acetate, alum and ferric sulphate. Concentrated sulphuric acid decomposes glutin, forming, besides other products of decomposition, chiefly glycocoll and leucine.

When heated, glutin softens, swells and diffuses an odor of burnt hartshorn. In the air, it takes fire with difficulty, smokes, flames only for a few minutes, and leaves a bulky charcoal difficult to incinerate, the ashes of which consist principally of calcium phosphate.

Glutin, when in the jellied state, and treated with alcohol, undergoes dehydration, under the influence of which it contracts greatly. It was by this means that Gonnor succeeded in reducing in a remarkable degree the size of a print obtained in a very hydrated film of glutin, and transferring it, so reduced, to stone, from which he obtained a new impression, quite similar to the first, but more or less diminished.

By taking these prints, on the contrary, with glutin very little hydrated, and afterwards steeping them in water, a dilatation of the plate is obtained, which enlarges the figures with the same regularity.

Pure _chondrin_ is prepared by boiling for from 24 to 48 hours the cartilages of the ribs, of the larynx with the exception of those of the epiglot, or of the windpipe and the bronchi.

Chondrin is precipitated from its solution by alcohol. The precipitate is redissolved in warm water, evaporated, and dried. It forms a semi-translucent mass of a slightly yellow color and resembles glutin as regards fracture and all external properties, but differs from it in being precipitated from its aqueous solution by mineral acids, acetate of lead, alum and ferric sulphate, and also by organic acids such as vinegar, citric and oxalic acids, none of which precipitate glutin.

As regards its chemical composition, chondrin is poorer in nitrogen than glutin, and contains more sulphur. Its formula approaches more closely that of albumen, which corresponds also with the origin of chondrin, for cartilages may be considered as transition-links between the proteïne and glue-yielding substances.

By the action of concentrated sulphuric acid upon chondrin, leucine is only produced but no glycocoll. By potassium hydrate chondrin is converted into glutin and yields then, like the latter, leucine and glycocoll. By boiling with concentrated hydrochloric acid chondrin is decomposed; a peculiar variety of fermentable sugar, to which the term chondroglucose has been applied, being formed.

It may finally be remarked that chondrin possesses less adhesive power than glutin and its presence in glue may be considered detrimental. To avoid its formation, the glue manufacturer should separate as much as possible cartilages from bones. Chondrin, however, is useful for size.

4. PROPERTIES OF GLUE AND ITS BEHAVIOR TOWARDS OTHER SUBSTANCES.

The product designated by the general term glue, is always a mixture of glutin, chondrin and other substances not yet accurately determined. Glue is formed by evaporating and further drying the jelly, and its properties depend on the crude glue and glue-yielding material used for the production of the jelly.

It may here be remarked that even if the quantity of glutin contained in the different products could not be determined by scientific means, the glue obtained from various materials can be readily distinguished by external characteristics. Every manufacturer knows that hides and bones yield a distinct quality of glue as regards adhesive power, elasticity and fracture, and that the jelly from glue-yielding substances of older animals is more solid and gives a larger yield than that obtained from the tissues of younger and weaker animals. Glue from the bladders and scales of fishes, though consisting mainly of glutin, differs materially from hide or bone glue.

Generally speaking, the jelly, no matter whether consisting of glutin or chondrin, possesses, before drying to glue, different properties from glue solution. It has less adhesive power and spoils more quickly. At a temperature of 68° to 72.5° F., jelly putrefies inside of 24 hours, smells of ammonia, and decomposes, while glue solution can be kept much longer without suffering deterioration.

The jelly absorbs ozone with avidity and is decomposed by it, this being the reason why an approaching thunderstorm may cause great damage by destroying the coagulating power of the glue liquors, or causing the glue to turn on the nets, _i. e._, to lose its consistency and become liquid and foul.

The behavior of glue solution towards different salts also deserves attention.

By adding potassium or sodium carbonate, neutral potassium tartrate, Rochelle or Epsom salts to a lukewarm fluid containing 15 to 20 per cent. of glue, the latter coagulates by the salt withdrawing the water from it. A lukewarm solution saturated with common salt, sal ammoniac, saltpetre, or barium chloride does not gelatinize.

By adding to glue solution a large quantity of alum, the glue is precipitated as a transparent mass.

Glue compounded at a high temperature with dilute acids, does not gelatinize by itself, but will do so on adding common salt.

Boiling with slaked lime deprives glue solution of its power of gelatinizing, and, on evaporation, changes it into a colorless gummy mass which is soluble in cold water and in saturated solution of common salt.

From a glutin solution compounded with oxalic acid, the latter can after some time be again separated by the addition of lime, the result being a non-gelatinizing fluid which, however, possesses great adhesive power. This is the so-called _meta-gelatin_.

Glue solution also loses its property of gelatinizing by repeated boiling and cooling (for about six days).

Tannin enters with the jelly, as well as with glue solution, into characteristic combinations which are formed even in solutions containing only 0.005 per cent. of jelly or glue. Glue is, therefore, an excellent agent for the detection of tannin.

When quite concentrated glue solution is treated with tannin, a heavy, flocculent precipitate of a dirty-yellow, caseous character is formed, which turns brown on exposure to the air and, after drying, constitutes a hard brittle mass, easily reduced to powder and soluble in hot potash lye, but insoluble in water, ether and alcohol. This precipitate, if not identical with, is closely allied to the combination of tannin with skin, called leather.

Glue exposed to a dry heat melts, diffuses a strong disagreeable odor of burned horn and leaves behind a charcoal which has a powerful discoloring effect like animal charcoal. When subjected to destructive distillation, glue yields an aqueous solution of ammonium carbonate and a thick brown oil consisting of a mixture of ammonium carbonate, sulphur, ammonium cyanide, etc.

The chemical composition of glue is such as to bring to mind that of starch and cellulose derived from the vegetable kingdom. It contains:

Carbon 49.1 per cent. Hydrogen 6.5 per cent. Nitrogen 18.3 per cent. Oxygen and sulphur 26.1 per cent.

which may be represented by the formula: C_{12}H_{10}N_{2}O_{4}.

The composition of glue differs but little from that of the glue-yielding substance. Isinglass is composed of:

Carbon 49.5 per cent. Hydrogen 6.9 per cent. Nitrogen 18.8 per cent. Oxygen 24.8 per cent.

This justifies the assumption that glue in its various transition stages does not represent different chemical combinations, but only modifications of one and the same combination distinguishable from each other by physical characteristics, as is the case with starch, which without suffering an alteration in its composition, appears as dextrine and grape-sugar, or as with cellulose, which, without altering its composition, can be transformed into amyloid and grape-sugar.