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 subst.i.tutes. 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 Indeed, size is often overboiled glue, made by crude and out-of-date methods.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 quant.i.ty 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 sh.e.l.lac 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.

SECTION IX.--THE EVOLUTION OF THE GELATINE AND GLUE INDUSTRY

The manufacture of gelatine and allied products has received a great stimulus in this country from the circ.u.mstances arising from the European War. The large restriction of continental--especially French and Belgian--supplies of gelatine, led to greater demands for the British-made product, and resulted not merely in a period of greater prosperity, but in a period in which much greater efforts were made to supply a high-grade article in larger quant.i.ties. Most manufacturers strove to make high-cla.s.s gelatine rather than low-grade glue, great extensions were made, and many new businesses were established. The development of the leather trades, more particularly in respect of greater production, caused a bigger supply of raw material for skin gelatine, and the slaughter of home animals for food caused a more plentiful supply of bones. At the same time it was realized that greater production not only reduced working costs, but also that a bigger turnover in any one factory involved a proportionately less capital outlay. These facts tend to counterbalance the heavy freight on the raw materials. Production is thus not only on a larger scale but more intensive.

One of the greatest difficulties of this industry is to produce a regular or standard article, for the raw material is so exceedingly variable in quality; that for skin gelatine tends also to become less valuable. In such a case, as Rideal has truly remarked, to ensure that supplies to customers shall be always "up to sample," which is often a matter of contract--"exact and regular working, strict cleanliness, observance of temperatures and other physical data, and scientific supervision", are clearly necessary. "Rule of thumb" is never quite certain to produce the same article twice. In past years British methods of manufacture have been far too empirical. As in other industries, "rule of thumb" must inevitably be replaced by scientific principle. The advances in colloid chemistry of this last decade or so have, in the author's opinion, supplied the clue to this line of development. In the preceding pages emphasis has been laid upon the importance of the adsorption law, the lyotrope series, and the valency rule. The manufacturer or supervisor who understands and can apply these generalizations will find his task vastly easier and his factory more efficient. Much remains to be learnt, however, and the industry would certainly benefit by research work, for which there is a fertile field.

There is also considerable room for improvement in the methods of chemical engineering usually employed. Whilst the heat engineers have certainly done much to solve the question of evaporation and drying, there is still great scope in the more economical application of heat in extraction, and the last word can hardly have been said on the problem of clarification and decolorization. There is indeed almost as much scope for research by the chemical engineer as by the colloid chemist.

The industry also exhibits, in common with the leather and many other trades, the same tendency to save labour, both by careful arrangement of the factory and by the installing of mechanical labour-saving devices.

Thus, lifts, runways, hoists, and trucks are increasingly used to move the solids, and pipes and pumps to move the liquors. As ever, there is scope for the mechanical engineer.

If some of these problems are vigorously tackled during the present reconstruction period, there is little doubt that the gelatine and glue industry will be in a much better position to cope with all possible compet.i.tion in the future.

From what has been said in Section VIII. as to the wide uses of gelatine and glue, it will be seen that general prosperity in trade is conducive to better trade conditions in the gelatine and glue industry.

It is similarly true that a general trade slump affects the glue trade adversely. The severe trade depression which commenced in 1920 has had this effect, and has made economic production much more difficult as well as more essential. As often is the case, the larger factories and firms can better face the difficulties, and there can be little doubt that if the depression be long continued there will be a tendency for the smaller factories to be closed down and for the larger firms to unite. As in the leather trade, both the War boom and the Peace slump have caused the gelatine and glue trade to develop along the lines of the great trusts. It may be reasonably expected, moreover, that these will be intimately connected with the leather trusts. This fact, together with the heavy freight charges on the raw material, tends also to make the skin glue factories gravitate towards the leather centres.

PART VI.--MISCELLANEOUS PROTEINS AND BYE-PRODUCTS

SECTION I.--BYE-PRODUCTS OF THE LEATHER TRADES

In the leather trades by far the most important and valuable bye-products are obtained from the hides and skins themselves, and all these are obtained before the tannage proper is commenced. The leather trades use only the dermis (corium) or true skin for the manufacture of leather, and as we have noted (Part I., Section II.) this prepared and purified dermis is called "pelt." The cuttings and tr.i.m.m.i.n.gs from the pelt form the most valuable bye-product of the leather trades, and are the raw material of the gelatine and glue industries (Part V., Section II.). Many portions of the pelt, indeed, such as ears, noses, and cows'

udders, are quite useless for any other purposes. Other portions, such as cheeks, faces, and even bellies, may be made either into glue or leather according to the state of trade. Hardly less important to the same industry are the cuttings of adipose tissue removed in "fleshing"

the hides and skins. These, though yielding less protein, yield also, however, the valuable animal greases (Part V., Section II.). To obtain both these products in a purer condition the removal of "flesh" after "soaking," but before "liming" (Part I., Section II.), has been favoured by some, especially in America.

Amongst the epithelial structures of the hides and skins, we have several protein bye-products which have some commercial value. The horns of cattle are now almost invariably removed before reaching the leather manufacturer, but have some little value. This part of the epidermis is not solid keratin. A "pith" is easily removed after boiling in water. The outer parts, too, are often coa.r.s.e and somewhat damaged, but if removed by sc.r.a.ping reveal often a rather beautiful structure of varying colour. There is some opening for this product in the manufacture of small articles of horn, but much of it, together with hoofs, is roasted and crushed for making fertilizers. The hair of cattle, goat, etc., has also a commercial value. This is removed after liming, and needs subsequent purification (Part I., Section II.). The hair is well washed with water, using either repeated changes or a continuous supply, the operation being carried out in paddles or similar machines which stir up the hair in the water. When clean, the hair is transferred to a centrifugal machine or "spinner," in which much adhering water is removed. This is a great a.s.sistance in drying out, which is the next and final operation. In drying, the hair is laid upon steam-heated boxes or pipes, and a current of warmed air pa.s.sed over or through it by means of a fan. It is better to have the hair "turned"

occasionally. This ensures quicker as well as more even drying. The product is made up into large bales and sold for the manufacture of felts, mattresses, etc. White hair is usually kept separate and commands a larger price. The power consumed in driving the washing machinery, the centrifuges and the drying fan, together with the fuel required for the drying steam, and the labour involved throughout, make it doubtful whether this bye-product is worth either the capital outlay or the working costs necessitated. Many manufacturers avoid this treatment altogether, therefore, and the wet limed hair is sold direct to the fertilizer factory. A less price is obtained, but much expense is saved.

Especially when the animals have only their short summer coats, this course is preferred.

In the case of the wool from sheepskins the product is much more valuable. The wool, indeed, is often the primary consideration.

Unfortunately this sometimes results in the neglect of the pelt. The removal of wool from sheepskins forms a special industry known as "fellmongering," which has been previously described (Part II., Section IV.). Pains are taken to clean the wool even before removal from the pelt, by the liberal use of water and the "burring machine." There is much variation in quality, and care is taken to keep the various grades separate, even during the "pulling" operation. From the fellmonger the wool pa.s.ses to the "wool stapler," and forms the basis of one of our most important mechanical industries, the manufacture of woollen cloths.

Wool is also removed from sheep by the periodic shearing, and in this case does not reach the fellmonger at all.

Apart from the raw material itself, there are few bye-products of the leather trades which are of commercial importance. The sludge from the pits of the limeyard contains, in addition to much lime and chalk, a certain proportion of protein matter. This is derived partly from the blood and dung a.s.sociated with the hide, partly from the solution of the corium hide substance, partly from the solution of the softer keratins, and partly also undissolved and loose hair. This bye-product is rather difficult to deal with, as it will not easily dry. It is indeed sometimes a problem to dispose of it, except in rural districts, where the farmers appreciate its manurial value and will usually cart it away for a nominal fee. Where possible, it is better to let it drain and settle on land, and pile it up in heaps to dry further. Soak-pit sludge has a distinctly greater value as manure, on account of the greater proportion of dung proteins. As some lime is often used in these pits, the product is a really useful fertilizer.

The only other bye-product of the leather trades is waste leather itself. For small pieces of leather there is always some little opening in producing small articles, such as washers for taps, etc., and there is also the possibility of shredding or pulping and making an artificial leather. The best leather subst.i.tutes, indeed, are made from waste leather. Nevertheless, there is always a certain amount of waste leather which only finds an outlet in the fertilizer factory. Such material is usually steamed or roasted to make it brittle, and then crushed in a disintegrator. It is then mixed in with other materials, but is sometimes solubilized by the action of sulphuric acid. Leather seldom contains less than 30 per cent. protein.

REFERENCES.

"Chemical Fertilizers and Parasiticides," S.H. Collins, M.S., F.I.C. (Companion volume in this series on Industrial Chemistry.)

"Wool Wastes," Part II., Section V., p. 75.

"Hoofs, Horns, Leather," Part III., Section II., p. 115.

"Gelatine, Glue, and Allied Products," T. Lambert.

SECTION II.--BYE-PRODUCTS OF THE GELATINE AND GLUE TRADES

From the skin gelatine and glue trades the most valuable bye-product is the grease, which is obtained from the "fleshings" of the adipose tissue. These fleshings are themselves a bye-product of the leather trades. The recovery and purification of this grease has been dealt with previously (Part V., Section II.). In the skin glue trade the only other bye-product is the residue from the extraction process (Part V., Section III.). This residue is known usually as glue "scutch," and is composed of the proteins of the skin which are insoluble in hot water. These insoluble portions are obtained from all layers of the skin. There is much hair often in scutch, the hyaline or gla.s.sy layer (grain), and the elastic fibres of the corium are also insoluble, and a proportion is derived from the fibres of the adipose tissue on the flesh side. All these portions are fairly rich in nitrogen, and the scutch has, therefore, considerable value to makers of fertilizers. It is liable to contain also a percentage of grease, which is usually removed by steaming under hydraulic pressure. This process recovers a valuable bye-product and increases the manurial value of the scutch. There is always left in scutch some of the gelatinous skin substance which, strictly speaking, should have been removed during extraction. There is, however, a practical limit beyond which it does not pay to do this. When this limit is reached the cost of steam in extracting, and also in evaporating and drying, together with the loss of time and labour involved by occupation of the plant, is greater than the value of the possible product.