Chapter XLV: 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.