To the readers of this journal I would offer some talks on glue concerning its composition and its behaviour toward the different salts, etc. Glue is an animal product but does not in reality pre-exist in the animal organism. As there must of necessity be several transformations to take place in the process of manufacturing, therefore we distinguish glue yielding substance, crude glue, jelly, and glue.
By glue yielding substance we understand the raw material from which glue is made. Crude glue is the raw material free from all foreign matter and put in condition for boiling. Jelly is obtained by boiling the crude glue. After drying the jelly, we have what is commercially known as glue.
Chemistry of Glues
Chemically speaking, glue is composed of carbon 49.1 per cent, hydrogen 6.5 per cent, nitrogen 18.3 per cent oxygen and sulphur 26.1 per cent. Independent of impurities and accidental constituents glue consists of two distinctly distinguishable combinations: gluten and chondrinid. Experiments have shown gluten to possess much greater adhesive power than chondrinid. Pure gluten, when dry, is almost colourless, transparent, hard, and endowed with great but variable coherence according to the kind of material from which it is obtained. It is odourless and tasteless. Its specific gravity is greater than that of water. It exerts no influence whatever upon vegetable colours. It has a neutral reaction. It is insoluble, in spirits of wine, ether, fat and volatile oils. In cold water it softens, absorbing as much as 40 per cent.
The character and behaviour of glues, steps in the process of manufacturing, practical features of the constituents, comments upon glue factors of importance
A liquid which contains one hundredth part of its weight of gluten becomes sticky on cooking but when it has only the one hundred and fiftieth part the mass remains fluid. According to examinations made by skilled technologists the quantity of gluten contained in different kinds of glue varies between 68 and 81 per cent. Chondrinid, as regards its chemical composition, is poorer in nitrogen than gluten and contains more sulphur. Its formula approaches more closely that of albumin a substance which forms a constituent part of both the animal fluids and solids and exists nearly pure in the white of an egg.
Glue as found in commerce is a mixture of gluten, chondrinid and other substances not positively agreed upon. Its quality depends upon the crude glue and glue yielding material from which the jelly is produced. The jelly, no matter whether gluten or chondrinid, possesses before drying to glue different properties from a glue solution. It has less power of adhesion and spoils more quickly. At a temperature of about 72 degrees Fahr. jelly putrefies inside of 24 hours and decomposes while the glue solution will keep longer without deterioration. It is a known fact that glue from hides differs from bone glue in adhesive power, elasticity and fracture and a larger percentage of glue is obtained from the glue yielding tissues of old animals than from those of young animals. The older the animal from which the skin has been derived the more solid the glue will be. Glue from fish bladders though consisting mainly of gluten differs materially in its behaviour from hide or bone glue.
The behaviour of the glue solution toward different salts also deserves attention. By adding Epsom salts to a lukewarm fluid containing 15 or 20 per cent of glue the latter coagulates in consequence of 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 a glue solution, a large quantity of alum, the glue is precipitated as a transparent mass. Boiling with slaked lime deprives glue solution of its power of gelatinizing and changes it on evaporation to a gum-like colourless mass soluble in cold water. Tannic acid is a valuable and delicate test of the presence of glue and when added to a solution containing only one five thousandth part of glue the solution immediately assumes a cloudy appearance.
Testing by Tannic
Add The skins of animals, or that part from which glue is made, is composed of two parts, the corium or cutis, and the cuticle or epidermis. The latter consists of separate and distinct cells and constitutes the exterior covering in which the wool, fur or hair of the animal is rooted. It is of no importance to the glue maker. The corium or cutis is the portion which furnishes the material for glue. Beneath the corium are cells which often contain fat when falling into the hands of the glue maker. This fat must be removed otherwise it exerts a disturbing influence upon the manufacture of glue. To remove this fat is one of the objects of the lime bath used in preparing the glue stock.