CHAPTER XI.

TESTING GLUE AND GELATINE.

It is of importance that the manufacturer as well as the dealer should know how to test the quality of a glue. This may be done by chemical means and in a mechanical way.

_Determination of moisture._ For this purpose a weighed quantity of the sample, finely powdered, is for fourteen hours exposed to a temperature of 217° to 230° F. It is then cooled under the dessicator and reweighed. The content of moisture is then calculated from the loss in weight.

_Determination of ash._ The origin of a sample of glue may be traced by examining the ash for phosphates of lime and magnesia, bone-glue containing both, while skin-glue is free from phosphates. Reduce a portion of the sample to a fine powder, and weigh it in a tared constant crucible. Heat slowly over a Bunsen flame until carbonized, then remove the crucible to the muffle, and heat to bright redness for 10 hours. Cool under the dessicator and weigh. The increase in weight of the crucible is the ash of the sample. This will vary from 1 to 2 per cent. in a gelatine, 2 to 3 per cent. in a good glue, 6 to 8 per cent. in a common quality.

_Determination of Acidity._ Kissling elsewhere determines the acidity by suspending 30 grammes of the sample in 80 Cc. of cold water for 10 to 12 hours in a flask connected with a condenser. The volatile acids are then driven over by a current of steam into a graduated cylinder. When the distillate amounts to 200 Cc., distillation is interrupted, and the distillate treated with standard decinormal alkali. When the distillate contains sulphurous acid, a known quantity of the standard alkali is previously added to the cylinder.

An undue quantity of acid may be detected by the taste. Glues may be alkaline from the addition of excess of lime in the manufacture to correct the sourness of the jelly. For uses where colors are concerned the glue must be neutral to litmus; for adhesive uses this does not matter unless the alkalinity or acidity is due to defective preparation (Samuel Rideal).

_Determination of Glutin._ The percentage of glutin in a glue solution is determined by precipitating it with tannin. The dense white precipitate formed is thrown on to a tared filter, washed with hot water, dried and weighed. The calculation is made on the basis that the tannate of glutin has a percentage composition of 42.74 per cent. of glutin and 57.26 per cent. of tannin.

Bisler-Beumat while employing the same principle prepares two solutions: _a._ 10 grammes of pure tannin to the liter. _b._ 10 grammes of pure isinglass and 20 grammes of alum to the liter. The ratio in which the tannin is precipitated by the isinglass solution, which Risler considers as pure glutin, is then determined by titration. The tannin solution is then diluted so that exactly an equal volume of glue solution is precipitated by it.

In order to test a sample of glue, dissolve 10 grammes of it together with 20 grammes of alum in a liter of water, heat being applied if necessary. Next 10 cubic centimeters of the tannin solution are taken to which an equal bulk of glue solution is at once added, as one may be sure that this quantity is not sufficient for the precipitation of the measured quantity of tannin, because no glue found in commerce is as pure as isinglass. The vessel containing the mixed liquids being well shaken and the precipitate having subsided, another cubic centimeter of glue solution is added to the tannin solution which is next filtered through a moist cotton filter. If _one_ drop of the glue solution still produces a precipitate in the clear filtrate, another cubic centimeter is added to the tannin solution, and then again filtered, these operations being repeated until the filtrate is no longer rendered turbid by the glue solution.

With the known relation of the tannin solution to pure glue (isinglass), a conclusion may be formed from the number of cubic centimeters of glue solution used as to the content of glutin in the sample examined.

The percentage of glutin varies, of course, according to the quality and origin of the glue. Bone glue of good quality contains from 50 to 52 per cent. and skin glue from 65 to 75 per cent.

S. Dana Hayes analyzed two samples of American glue of best quality and obtained the following results:

a. b. Water (escaping at 212° F.) 16.70 16.28 Glue substance 79.85 80.42 Calcium carbonate 1.42 1.33 Calcium sulphate 0.41 0.34 Magnesium phosphate 0.35 0.31 Alkaline salts 0.17 0.12 Silica, ferric oxide, etc. 0.09 0.08 Zinc oxide 1.01 1.12

The chemical modes of testing glue give only the quantity of glutin contained in it, but do not prove that the substance combined with tannin corresponds to the actual adhesive power of the glue, for it is possible that a glue containing a large quantity of glutin may possess but little adhesive power, and a jelly from which the glue is formed may contain an equal amount of glutin with the latter, but not possess an equal power of adhesion.

It is certain that the determination of the glutin alone is not a criterion of the quality of glue. In the absence of a reliable method of direct analysis, attempts have been made to deduce the quality of glue from indirect properties.

One of these methods consists in immersing the sample of glue in a large quantity of water at 59° F. for a considerable time. The glue swells up, absorbing 5 to 16 times its own weight of water. The more consistent and elastic the glue in this state is found to be, the greater its adhesive power, and the larger the quantity of water absorbed the more economical the glue will be in use. This method does not give thoroughly reliable results, and should only be employed with bone-glue, as skin-glue does not behave in a similar manner.

A more reliable method is to test the strength of a glue by the method first proposed by Lipowitz, which is based on the weight-sustaining power of a jelly of known strength and temperature. The test is executed as follows: Soak 5 parts of the sample in water, then dissolve it in sufficient hot water to make the weight of the solution equal to 50 parts, and allow to stand for 12 hours at 64.4° F. in a glass cylinder of uniform width to gelatinize. Cover the glass cylinder with a tin cap perforated in the centre, Fig. 65. Through the perforation is passed freely a stout iron wire, at the lower end of which is soldered a piece of tin shaped like a saucer, the convex side of it resting on the surface of the jelly. To the upper end of the wire, which together with the tin saucer should weigh 5 grammes, is soldered a funnel weighing 5 grammes, and capable of holding up to 50 grammes of fine shot. The apparatus is gradually loaded with shot until the saucer-shaped piece of tin is forced into the jelly. The greater the strength of the jelly, the heavier will be the weight required. From the determined weight of the shot used, the relative value of the glue as to its adhesive power is determined.

The following results have been obtained by comparative experiments with this apparatus:

Variety of Glue. Weight required to force the saucer down.

Breslau 1704 grammes = 3.74 lbs. Russian 1446 grammes = 3.18 lbs. Cologne 1215 grammes = 2.67 lbs. Muhlhausen I. 727 grammes = 1.599 lbs. Nördlingen 724 grammes = 1.592 lbs. Muhlhausen II. 387.5 grammes = 0.85 lbs.

The results of these experiments agree to a remarkable degree with the market prices of the varieties of glue, which is not the case with glues tested by the methods previously mentioned. The results of these tests are given in the table below:

Table Key:

Variety of Glue.

1. Best White isinglass in three qualities 2. Translucent yellowish bone glue in tablets and easily soluble 3. Pale yellow glue similar to No. 2 4. Brown-reddish, brittle in fracture and soluble 5. Clear translucent glue of medium brown color 6. Brown-yelow glue in thick tablets and possessing but little translucency 7. Pale brown-yellow glue, readily soluble and elastic before breaking 8. Pale amber-colored glue with but little translucency 9. Brown glue, solution turbid 10. Amber-colored glue, opalizing and readily soluble 11. Thich tablets or dark-brown glue, solution very turbid 12. Dark horn-like glue with little translucency 13. Very translucent glue of a light brown color and very clean solution 14. Translucent dark-brown glue, giving a very clear solution

----+----------------+--------------+------+-------------+------------- |Loss of water | 100 parts |Glutin| Water |A 100 per cent. |in drying for | of glue are | per | absorbed in |solution of |several hours at| precipitated |cent. |24 hours by 5|glue will bear |239° to 248° F. |by tannic acid| |parts of glue|a weight of ----+----------------+--------------+------+-------------+------------- 1. | 20 to 21 | 74.62 | 55.69| — | | | | | | 2. | 13.2 | 76.2 | 56.8 | 40 |64 grammes | | | | | (2.25 ozs.) 3. | 13.0 | 70.0 | 52.2 | 35 |60 grammes | | | | | (2.11 ozs.) 4. | 10.0 | 71.0 | 52.9 | 12 |Does not | | | | | gelatinize. 5. | 11.0 | 71.5 | 53.3 | 20 |20 grammes | | | | | (0.705 oz.) 6. | 12.5 | 68.0 | 50.7 | 27 |15 grammes | | | | | (0.52 oz.) 7. | 13.0 | 66.6 | 49.7 | 30 |36 grammes | | | | | (1.26 oz.) 8. | 9.5 | 68.5 | 51.1 | 33 |60 grammes | | | | | (2.11 oz.) 9. | 10.0 | 82.0 | 53.7 | 30 |50 grammes | | | | | (1.76 oz.) 10.| 9.5 | 73.0 | 54.4 | 35 |56 grammes | | | | | (1.97 oz.) 11.| 13.5 | 64.0 | 47.7 | 18 |23 grammes | | | | | (0.81 oz.) 12.| 9.0 | 72.6 | 54.2 | 29 |12 grammes | | | | | (0.42 oz.) 13.| 13.5 | 70.0 | 52.2 | 30 |40 grammes | | | | | (1.41 oz.) 14.| 15.0 | 66.0 | 49.4 | 25 |42 grammes | | | | | (1.48 oz.) ----+----------------+--------------+------+-------------+-------------

This table shows the following facts:

1. The percentage of water in the 14 dry varieties of glue examined varies between 9.0 and 21. The loss of water from isinglass is surprisingly large, especially as it cannot be explained by an artificial admixture of water, since the six varieties examined reabsorbed the same percentage of water from the air. The percentage of water in the other varieties of glue differs but little.

2. The various varieties of glue required different quantities of tannic acid for their precipitation, the amount for 100 parts of glue varying between 66 parts and 76.2, or calculated to per cent., between 49.4 and 56.8.

3. Placed in cold water, glue swells up and absorbs from 12 to 40 parts of water. The behavior of the various varieties differs very much in this respect, and in most of the experiments, with the exception of variety No. 4, the percentage of glutin is in exact proportion to the quantity of water absorbed.

4. The strength of the gelatinized glue varies between 12 grammes (185.18 grains) and 64 grammes (987.67 grains) for a 10 per cent. solution. This property corresponds neither with the absorption of water nor the percentage of glutin.

Variety No. 4, in the table, contains 52 per cent. of glutin, but does not gelatinize, its strength being therefore equal to 0, while variety No. 14, with 49.4 per cent. of glutin, therefore less than No. 4, shows a strength of 42. As no close connection between the properties mentioned in the table can be recognized, it is best not to be guided by these properties alone, but to test also the behavior of the glue in practical use. Such test consists in estimating the adhesive power of the glue from the weight required to tear asunder two pieces of wood glued together and dried. But as the results obtained by this purely practical test must necessarily vary on account of the impossibility of having two surfaces of wood always exactly alike, and the uncertainty of applying every time the same quantity of glue, Weidenbusch’s method may here be given. It is based upon the principle that sticks of plaster of Paris cast of exactly the same material and of uniform size break under the same weight when placed upon two supports and loaded in the centre. If now such plaster sticks be saturated with glue solutions prepared according to the same method, but from different qualities of glue, a greater force will be required for breaking them, and this force will be the greater the better the quality of glue is.

The plaster sticks are prepared as follows: Reduce pure crystallized gypsum to a fine powder, pass the latter through a sieve having 324 meshes per square centimeter and heat it to between 284° and 302° F. For casting the sticks moulds of soap-stone are used. The moulds are made by boring in a piece of soap-stone, at a distance of about 1 centimeter from each other, holes with a diameter of 6 millimeters on top and 7½ millimeters on the bottom.

The gypsum is weighed off in portions of 1 gramme each, mixed with 1 gramme water and cast in the moulds. The solidified sticks are first dried at a moderate heat and then over calcium chloride, and kept for use in an air-tight vessel.

The glue solution is prepared as follows: The glue dried at 212° F. is weighed, soaked over night in water, then melted in a small glass in the water-bath, and finally enough water is added so that the solution contains 10 per cent. of dry glue.

The plaster sticks are for one or two minutes immersed in the glue solution heated to 212° F., and then placed vertically upon a glass plate until superficially dry, when they are completely dried at 212° F. It is recommended to color the glue solution with indigo, the uniform saturation of the sticks being thereby more easily recognized.

The apparatus for testing the strength of the sticks consists of a brass ring _a_, Fig. 66, having two notches to receive the stick, and its diameter is divided into two equal parts by an indicator. The ring is supported by a pin, by means of which it is secured in a stand. The apparatus is completed by an iron or glass cup which is suspended by three cords, _i_, and the hook _f_ to the plaster stick _b_. The hook _f_ is placed in the position indicated by the indicator. Mercury is now poured into the cup until the plaster stick commences to break. The weight required is recorded and compared with a standard glue. During the experiment the cup is suspended by the three cords _h_ to the ring _a_, and is thus caught when the plaster stick breaks. On the lower end of the cup is a clip for emptying the mercury into a vessel so that none of it is lost.

The “Artillery Werkstätte” at Spandau has adopted the severing strain of two blocks of wood glued together. The test is executed as follows: Three parts of glue (but not less than 250 grammes) are mixed with 6 parts of water and boiled in a steam bath until the weight of the boiled glue amounts to only 5/9 of the original mixture. The reason for this continued boiling is to ascertain whether the glue to be tested retains the required adhesive power even after six hours continued heating in a steam bath as frequently happens in the workshop. With the glue thus prepared the following breaking test is made:

Blocks of hard or soft wood 420 millimeters long and 40 × 40 millimeters cross section are cut in two, so that each piece thus obtained is 210 millimeters in length. These two pieces are then again glued together across the grain with the glue to be tested. The block is then placed in a dry room at a temperature of 62° to 68° F. for 72 hours, when the joint is tested as to its resisting power. At a distance of 180 millimeters from the joint a hole is bored in the block. Through this hole is pushed a bolt furnished on the lower end with a hook to which a scale is suspended. The block of wood is clamped to a table so that the joint projects 1 centimeter beyond the edge. The scale is at the start loaded with 25 kilograms, the load being every five minutes increased 5 kilograms till fracture takes place.

Two such blocks, one of hard and one of soft wood, are subjected to the test, and a serviceable glue must stand at least an average load of 70 kilograms.

_Determination of adulterations._ White lead, sulphate of lead, zinc white, or chalk in quantities varying from 4 to 8 per cent. are frequently mixed with the melted glue to improve the appearance of the finished product. Thus, according to analyses by A. Faisst, Russian glues contained in 100 parts foreign admixtures as follows:

I. II. III. IV. Zinc white 1.66 Chalk 2.40 2.95 3.79 2.10 Sulphate of lead — 4.16 2.35 3.18 —— —— —— —— 4.06 7.08 6.14 5.28

The so-called patent glue which is opaque and of a white color is produced by adding considerable quantities of white lead to ordinary glue.

According to Barreswil, glue is frequently mixed with lead acetate solution to protect it from putrefaction. Such an addition, as well as the presence of white lead or sulphate of lead, is detected by the introduction of sulphuretted hydrogen into very dilute glue solution. In the presence of lead acetate a black precipitate of lead sulphide appears in the clear solution; if white lead or lead sulphate is present, the white powder settling on the bottom is blackened by the formation of lead sulphide.

For the detection of other earthy additions, prepare a very dilute solution of the glue in question and allow it to stand quietly for a few hours. The heavy additions subside, and after decanting the supernatant fluid is collected upon a small filter and examined by the customary analytical methods.

It is difficult to say what quantities of earthy constituents actually constitute an adulteration, since it is claimed by many practical men that a considerable content of earthy parts is of advantage as regards the cementing power of glue. Generally speaking it may, however, be said that a glue containing more than 6 to 8 per cent. of earthy constituents must be considered adulterated.

For many purposes, especially if the glue comes in contact with colored materials as, for instance, in book-binding, a content of free acid would exert an injurious effect upon the colors, destroying or changing them. It is therefore advisable to test the glue with blue litmus paper which, in the presence of free acid, is reddened.

In testing a large number of samples of glue, Kissling obtained the following results:

Number Smallest Largest Average SKIN GLUE. of samples. percentage. percentage. percentage.

Water 15 13.4 18.1 15.7 Ash 16 1.0 4.13 2.15 Fat 21 0.01 0.090 0.037 Volatile acids, free } 8 {0.084 0.238 0.178 Volatile acids, fixed } {0.084 0.334 0.191

BONE GLUE.

Water 25 11.5 17.7 13.4 Ash 26 1.16 5.07 2.46 Fat 5 0.047 0.217 0.113 Volatile acids, free } 7 {0.088 1.451 0.655 Volatile acids, fixed } {0.097 0.721 0.460

However, those who from their practical knowledge are most competent to judge the commercial value of a glue, scarcely require such complicated tests, as by taking the sample in their hand and looking at it, they can in most cases tell its quality. Great hardness, a clear, rattling sound when struck, and resistance to breaking are signs of good quality, and if the cake is cut thick, it shows that the jelly possessed great consistency. Many kinds of glues are intentionally cut thin so that they will dry before spoiling. The derivation of a glue from a sound jelly is recognized by the fine cutting lines. When the jelly possesses but little gelatinizing power and is in the first stages of decomposition or putrefaction, or formation of sugar has already set in, it cannot be poured into moulding boxes, as it would putrefy before it has a chance to gelatinize. Such sick jelly is poured in thin layers upon glass or metal plates so that it may acquire sufficient solidity to allow of its being cut up into leaves and dried upon nets. If the edges of the glue are deeply indented and raised, the jelly, before drying, possessed but little concentration (25 to 30 per cent.), and if notwithstanding this, it acquired sufficient consistency to allow of its being cut, it must have been very sound. Glues cut thick and showing no indentations and raised edges, are derived from excessively concentrated jellies (30-35-40 per cent.). Such jellies lose in quality in evaporating.

A high degree of transparency is a favorable sign as regards the purity of a glue, substances inducing putrefaction having been eliminated. Hence this property should be worthy of recommendation, but the consumer having been disappointed in the use of thin glass-clear glues, distrusts also the thick-cut transparent product, preferring a turbid, translucent or opaque article. For this reason the manufacturer is forced to render glass-clear glue turbid with coloring matter.

The color of the glue is also a means of judging it. To be sure, chemically pure glutin is a colorless substance, but glue is always colored more or less dark brown. Although this coloration does not in the least impair the adhesive power, the manufacturer endeavors to produce a product of as light a color as possible, and this is best effected by bleaching with sulphurous acid. By this means the glue not only acquires a lighter color, but also becomes more stable, substances inducing putrefaction being destroyed by the acid.