From the vats, the warm liquid is run into shallow, flat coolers consisting ef wooden boxes, about 14 ft. long, 12 in. wide, and 9 in. deep These are placed in a cold situation, so that the liquid may cool and gelatinize. This operation occupies from twelve to eighteen hours, according to the state of the weather ; at the end of this period, the glue will be sufficiently firm to be taken out as an elastic cake. For this purpose, the wooden boxes are inverted on a moist table, and the mass of glue is then divided into a series of blocks, or " gobbets " ; these blocks are next placed in a wooden frame, 12 in. by 7 in. by 6 in. deep, and provided with about a dozen slits, the whole much resembling a set of school-slate frames placed together. Each block is then cut horizontally into elioca, by means of a brasa wire stretched like a bow-saw on a frame, and inserted in the slits of the frame containing the block. The slices, thua cut, are laid out on sheets of galvanized iron wire netting, exposed on every side to the air, for the purpoae of drying and hardening. This part of the manufacture is moat expoaed to the riaks of poasible failure, by reason of the influence exerted by the weather. A sudden heat will aoften the glue, ao that it will run through the nets; moist weather will prevent the drying, and, if it ia warm at the same time, the glue may be so spoiled, and acquire such a bad odour, aa to be fit only to be thrown away ; a fog, or a thunderstorm may spoil the operation, 8,nd render all the previous trouble and expense totally useless. Glue which has once manifested a bad amell during the proceas of manufacture, even though dried afterwarda to a perfectly odourleaa solid, will reveal its quality when dissolved at any subsequent period ; nay, while still dry in the barrel, it will become disagreeably odoroua on every moist day ; and, what is worae, walla on which this glue has been used will give out the smell in damp weather. As a matter of course, such glue loaes much of its commercial value. In the course of drying, the slice of glue shrinks until its size is only half of what it was when in a moiat state; and it requires to be carefully watched and turned, to prevent uneven contraction, which would cause it to aplit. After being dried, the slices are ta.ken into sheds, 8,nd washed with boiling water, for the purpose of removing any dirt or dust, which may have adhered to their aurface in the course of drying. The glue is then packed ready for the market. The qualitiea commonly made are " town glue," worth about 6d. a lb. ; " atrong Ruasian dark," 9d.; " finest kid," is.
When glue-making is properly conduded, there should be no waste product. The large bonee, after boiling, may be aold to the button manufacturer (see Bones, and Buttons); the small bones are valuable for making bone black (aee Blacks), or for their fertilizing properties (ace Manures); hoofs are useful for making combs ; hair from the " fleahings" has many applicationa ; ths waste lima from the pits is an excellent manure.
A great improvement in the method of conducting the manufacture of glue ia the application of superheated ateam to the materials, by which a much larger proportion of glue is extracted, in a shorter time, and at less coat. Another improvement is the addition of Paris white (fine chalk) to the glue uaed by cabinet-makers. It has the following advantages :-1. It increases the adheaive qualitiea of the glue. 2. lt makes the glue look whiter, and thus givea to a browner glue, the lighter appearance of a more expeneive quality. 3. It is a pecuniary gain, since a substance
coating only lid. or 2d. a lb. is added to one costing Is. 3d. to ls. 8d. ; but it ia not to be considered as a common adulteration : the buyer loses nothing, aa the price is proportionally reduced, while the quality is improved. This is an American plan, and explains the milkineas of the glue made in the United States.
A novel feature in glue manufacture is the utilization of leather refuae for the purpose. Old leather, or leather refuae, is aubjected to the action of 15 per cent. of a mixture of slaked lime and water, in closed veasels, at a temperature of 121° (250° F.). In this way, the leather is completely decompoaed. Its principal constituents being tannic acid combined with gelatine, the tannic acid attacks the lime, forming tannate of lime, while the gelatine ia aet free, and is diasolved in the water. The high temperature required for the operation injures the glue, which is undoubtedly somewhat deteriorated by the previous action of the tannin on the leather from which it is derived. For these reasons, the glue obtained from leather is inferior in its adhesive qualities ; but it may do vet}, well for culinary purposes.
The Laplanders make an excellent glue frorn the akins of perches ; and it is probable that eel skina would serve the aame end. The largest skins are choaen, and dried ; they are then moistened with cold water, till they have become ao aoft that the scalea can be separated ; the latter are thrown away. Four or five of the skins are then put into a reindeer's bladder, or wrapped up in the aoft bark of the birch tree, in such a manner sa to exclude water. Thua covered, they are placed in a pot of boiling water, and kept at the bottom by means of a stone. After boiling for about an hour, they are uncovered, having, by that time, become soft or viscous. In thia state, they are employed for joining the two piecea of the Lapp bows ; if presaure be exerted till the glue is quite dry, the joint will never give way.
Good glue should be hard, and difficult to break with a hammer, though, when broken, it should yield suddenly to the furce, and present a aharp vitreous fracture. It muat be admitted, however, that some very fair glues, in thin piecea, will yield or bend a little, even when quite dry, before breaking. The colour, whether dark or light, should be bright, not too dark, and without any green tint. The substance should be tranaparent, and free from foreign particles, a,nd should be capable of absorbing a considerable quantity of water, aay, at leaat four to seven times ita own weight. Generally speaking, the amount of water thus absorbed will serve as an indication of the quality of the glue, provided that the resultant ma,ss is not too friable, and remains clear, or nearly so, and that when it haa been melted and allowed to cool, the jelly it forms is tolerably clear and fresh, and not liable to rapid spontaneous putrefaction. From careful experiments made with dry glue, immersed for twenty-four hours iu water,-at 15° (60° F.), and thereby transformed iuto a jelly, it was found that the finest ordiusry glue, or that made from white bones, will absorb twelve times its weight of water in twenty-four hours ; that glue from dark bones will absorb nine times its weight of water ; while the ordinary glue made from animal refuse will absorb but three to five times its own weight of water.