Well-dried glue is much less hygro-metric than badly-made glues or those made of inferior materials. The latter are liable to putrefaction. The water of composition seems to be injurious to the strength of glue, which increases in proportion to its dryness.
No. 1 extra C"glue.
Moisture (loss of weight at 212° F) ....
Gelatine, with a little) animal fibre and fats
Carbonate of lime.. ..
Sulphate of lime .. ..
Phosphate of magnesia
Alkaline salts ..
Silica, oxide of iron, etc.
Oxide of zinc ..
Analyses of 10 more samples of frozen and sheet glue, of common grades and from different makers, showed the proportion of water contained in them to vary from 14 to 18 per cent., averaging 17 per cent. And the proportion of ash or mineral matter varied from 3 to 6 per cent., averaging rather less than 4 per cent. Two of these samples contained about 1 per cent, of white zinc, and two of them contained sulphate of lime. Analyses of 2 samples of commercial gelatine averaged 16 1/2 per cent, of water, and 2.56 and 3.11 per cent, of ash respectively. There was no oxide of zinc or sulphate of lime in these gelatines. The presence of so much water was quite unexpected; and as the quantity is nearly the same in fresh and in seasoned specimens, it is not a makeweight, although steam is very freely used in the rooms where glue is packed by the manufacturers. The carbonate of lime comes from the quicklime used for cleaning and preserving the animal matter, or glue stock, while the sulphate of lime is formed by the addition of small quantities of sulphuric acid during the process of manufacture, to neutralize the lime that is carried forward by the solutions of glue.
The oxide of zinc is said to be added to prevent souring, or the acidity caused by decomposition, and it also improves the colour of the glue; but it is not generally used, as these analyses indicate. The impure glues, or those containing the most mineral matter, became almost insoluble after they had been broken into small pieces and heated in a hot-air bath (copper oven) at 212° F. (100° C.) for 2 or 3 hours, until they cease to lose weight; they then soften and become dough-like, but do not dissolve when boiled in water for some time. The purer gelatines were not so much injured, and one specimen, containing only 2.56 per cent, of ash, was not materially affected by this thorough drying. The solid sheet glue, while drying in this way, tumefied and became very porous: the frozen glue did not alter in structure. The conclusion drawn from these experiments was that the excess of lime combines with the gelatine, and perhaps with the extraneous animal matters of the glue, at the high temperature, forming a compound like lime-soap, as the whole quantity of lime is retained in the insoluble portion left after boiling the dried glue in water.
Such an explanation accounts for the difference noticed in the effect of drying upon gelatine and common glue.
In the selection of glue, the testing of it, so as to form some estimate of its adhesive qualities, is a matter of first importance. All glue in the cake is subject to be influenced by the moistness or dryness of the atmosphere, becoming soft in damp weather and crisp in dry weather, but different kinds are differently affected, and hence it is better to purchase in dry weather, as thai which is then soft is not of as good quality as that which is crisp; and it should be borne in mind also, when purchasing, that the most transparent is generally the best. It is always advisable, before purchasing, to submit to experiment a sample of the article offered. To do this, take a cake of glue, place it in a pan, and cover it with water; when, after some hours, if it be good glue, it will swell but not dissolve, while, if bad, it will partly, if not wholly, dissolve in the water. Another test is this: after being dissolved by means of heat, that glue is best which seems most cohesive, or which is capable of being drawn out into thin filaments or strings, and does not drop from the brush or glue-stick as water or oil would, but rather extends itself in threads as it falls from the brush or stick; and if the glue possesses the requisite properties, this will always be found to be the case.
During the boiling of bones for the removal of the fat, a portion of the cartilage enters into solution in the water. At ordinary bone-boiling establishments, the residual liquor, though containing more or less gelatine, is run off into the drains; but in establishments where size is made, the gelatinous liquor obtained by the prolonged boiling is drawn off by a tap from the lower part of the boiler, and is subsequently boiled down to the required consistence, either in the same or in another similar boiler, from which it is usually drawn off into cases for sale. A modification of this plan consists in crushing the bones, treating them with steam at high pressure, adding a proportion (2 per cent.) of hydrochloric acid to the semi-gelatinous mass thus formed, and re-boiling. The fat separates and floats in the liquor, to be easily collected and purified by being treated first with boiling water and a very small quantity of caustic soda, next with animal charcoal, and finally filtered.
The osseous cartilage may be removed from bones by suspending them in weak nitric or hydrochloric acid (1 part acid to 9 parts water) at 50° F. (10° C.) The acid causes an effervescence, by acting on the carbonate of A lime, and dissolves out the whole of that and the other earthy constituents without affecting the cartilage, which, while retaining the form of the bone, soon becomes soft and translucent. It is then washed in 2 or 3 changes of cold water, to remove all traces of acidity. It shrinks and darkens on drying, becoming hard and strong, but somewhat brittle, and losing none of its transparency. It now forms gelatine. It has been proposed to obtain gelatine from bones, etc, by means of benzine and other hydrocarbons. Sometimes it is found advisable to treat the material with lime before adding the hydrocarbon, but only rarely. After the bones have remained for a considerable time under the influence of the hydrocarbon, the fatty matters are dissolved, and the pure gelatine is found at the bottom of the vessel.