glue from such heterogeneous materials; one blending may be a success and another a failure. The raw material has been divided into three great divisions: (1) sheep pieces and fleshings (ears, &c.); (2) ox fleshings and trimmings; (3) ox hides and pieces; the best glue is obtained from a mixture of the hide, ear and face clippings of the ox and calf. The raw material or “stock” is first steeped for from two to ten weeks, according to its nature, in wooden vats or pits with lime water, and afterwards carefully dried and stored. The object of the lime steeping is to remove any blood and flesh which may be attached to the skin, and to form a lime soap with the fatty matter present. The “scrows” or glue pieces, which may be kept a long time without undergoing change, are washed with a dilute hydrochloric acid to remove all lime, and then very thoroughly with water; they are now allowed to drain and dry. The skins are then placed in hemp nets and introduced into an open boiler which has a false bottom, and a tap by which liquid may be run off. As the boiling proceeds test quantities of liquid are from time to time examined, and when a sample is found on cooling to form a stiff jelly, which happens when it contains about 32% dry glue, it is ready to draw off. The solution is then run to a clarifier, in which a temperature sufficient to keep it fluid is maintained, and in this way any impurity is permitted to subside. The glue solution is then run into wooden troughs or coolers in which it sets to a firm jelly. The cakes are removed as in the case of bone glue (see above), and, having been placed on nets, are, in the Scottish practice, dried by exposure to open air. This primitive method has many disadvantages: on a hot day the cake may become unshapely, or melt and slip through the net, or dry so rapidly as to crack; a frost may produce fissures, while a fog or mist may precipitate moisture on the surface and occasion a mouldy appearance. The surface of the cake, which is generally dull after drying, is polished by washing with water. The practice of boiling, clarification, cooling and drying, which has been already described in the case of bone glue, has been also applied to the separation of skin glue.
Fish Glue.—Whereas isinglass, a very pure gelatin, is yielded by the sounds of a limited number of fish, it is found that all fish offals yield a glue possessing considerable adhesive properties. The manufacture consists in thoroughly washing the offal with water, and then discharging it into extractors with live steam. After digestion, the liquid is run off, allowed to stand, the upper oily layer removed, and the lower gluey solution clarified with alum. The liquid is then filtered, concentrated in open vats, and bleached with sulphur dioxide.[1] Fish glue is a light-brown viscous liquid which has a distinctly disagreeable odour and an acrid taste; these disadvantages to its use are avoided if it be boiled with a little water and 1% of sodium phosphate, and 0.025% of saccharine added.
Properties of Glue.—A good quality of glue should be free from all specks and grit, have a uniform, light brownish-yellow, transparent appearance, and should break with a glassy fracture. Steeped for some time in cold water it softens and swells up without dissolving, and when again dried it ought to resume its original properties. Under the influence of heat it entirely dissolves in water, forming a thin syrupy fluid with a not disagreeable smell. The adhesiveness of different qualities of glue varies considerably; the best adhesive is formed by steeping the glue, broken in small pieces, in water until they are quite soft, and then placing them with just sufficient water to effect solution in the glue-pot. The hotter the glue, the better the joint; remelted glue is not so strong as the freshly prepared; and newly manufactured glue is inferior to that which has been long in stock. It is therefore seen that many factors enter into the determination of the cohesive power of glue; a well-prepared joint may, under favourable conditions, withstand a pull of about 700 ℔ per sq. in. The following table, after Kilmarsch, shows the holding power of glued joints with various kinds of woods.
| Wood. | ℔ per sq. in. | |
| With grain. | Across grain. | |
| Beech | 852 | 434.5 |
| Maple | 484 | 346 |
| Oak | 704 | 302 |
| Fir | 605 | 132 |
Special Kinds of Glues, Cements, &c.—By virtue of the fact that the word “glue” is frequently used to denote many adhesives, which may or may not contain gelatin, there will now be given an account of some special preparations. These may be conveniently divided into: (1) liquid glues, mixtures containing gelatin which do not jelly at ordinary temperatures but still possess adhesive properties; (2) water-proof glues, including mixtures containing gelatin, and also the “marine glues,” which contain no glue; (3) glues or cements for special purposes, e.g. for cementing glass, pottery, leather, &c., for cementing dissimilar materials, such as paper or leather to iron.
Liquid Glues.—The demand for liquid glues is mainly due to the disadvantages—the necessity of dissolving and using while hot—of ordinary glue. They are generally prepared by adding to a warm glue solution some reagent which destroys the property of gelatinizing. The reagents in common use are acetic acid; magnesium chloride, used for a glue employed by printers; hydrochloric acid and zinc sulphate; nitric acid and lead sulphate; and phosphoric acid and ammonium carbonate.
Water-proof Glues.—Numerous recipes for water-proof glues have been published; glue, having been swollen by soaking in water, dissolved in four-fifths its weight of linseed oil, furnishes a good water-proof adhesive; linseed oil varnish and litharge, added to a glue solution, is also used; resin added to a hot glue solution in water, and afterwards diluted with turpentine, is another recipe; the best glue is said to be obtained by dissolving one part of glue in one and a half parts of water, and then adding one-fiftieth part of potassium bichromate. Alcoholic solutions of various gums, and also tannic acid, confer the same property on glue solutions. The “marine glues” are solutions of india-rubber, shellac or asphaltum, or mixtures of these substances, in benzene or naphtha. Jeffrey’s marine glue is formed by dissolving india-rubber in four parts of benzene and adding two parts of shellac; it is extensively used, being easily applied and drying rapidly and hard. Another water-proof glue which contains no gelatin is obtained by heating linseed oil with five parts of quicklime; when cold it forms a hard mass, which melts on heating like ordinary glue.
Special Glues.—There are innumerable recipes for adhesives specially applicable to certain substances and under certain conditions. For repairing glass, ivory, &c. isinglass (q.v.), which may be replaced by fine glue, yields valuable cements; bookbinders employ an elastic glue obtained from an ordinary glue solution and glycerin, the water being expelled by heating; an efficient cement for mounting photographs is obtained by dissolving glue in ten parts of alcohol and adding one part of glycerin; portable or mouth glue—so named because it melts in the mouth—is prepared by dissolving one part of sugar in a solution of four parts of glue. An india-rubber substitute is obtained by adding sodium tungstate and hydrochloric acid to a strong glue solution; this preparation may be rolled out when heated to 60°.
For further details see Thomas Lambert, Glue, Gelatine and their Allied Products (London, 1905); R. L. Fernbach, Glues and Gelatine (1907); H. C. Standage, Agglutinants of all Kinds for all Purposes (1907).
GLUTARIC ACID, or Normal Pyrotaric Acid,
HO2C·CH2·CH2·CH2·CO2H, an organic acid prepared by the
reduction of α-oxyglutaric acid with hydriodic acid, by reducing glutaconic acid, HO2C·CH2·CH:CH·CO2H, with sodium amalgam, by conversion of trimethylene bromide into the cyanide and hydrolysis of this compound, or from acetoacetic ester, which, in the form of its sodium derivative, condenses with β-iodopropionic ester to form acetoglutaric ester, CH3·CO·CH(CO2C2H5)·CH2·CH2·CO2C2H5, from which glutaric acid is obtained by hydrolysis. It is also obtained when sebacic, stearic and oleic acids are oxidized with nitric acid. It crystallizes in large monoclinic prisms which melt at 97.5° C., and distils between 302° and 304° C., practically without decomposition. It is soluble in water, alcohol and ether. By long heating the acid is converted into its anhydride, which, however, is obtained more readily by heating the silver salt of the acid with acetyl chloride. By distillation of the ammonium salt glutarimide, CH2(CH2·CO)2NH, is obtained; it forms small crystals melting at 151° to 152° C. and sublimes unchanged.
On the alkyl glutaric acids, see C. Hell (Ber., 1889, 22, pp. 48, 60), C. A. Bischoff (Ber., 1891, 24, p. 1041), K. Auwers (Ber., 1891, 24, p. 1923) and W. H. Perkin, junr. (Journ. Chem. Soc., 1896, 69, p. 268).
- ↑ The residue in the extractors is usually dried in steam-heated vessels, and mixed with potassium and magnesium salts; the product is then put on the market as fish-potash guano.
