These substances may be considered as merely varieties of the same material, exhibiting no essential difference in composition or character; in fact, glue and gelatine pass insensibly into each other, glue being but an impure or discoloured form of gelatine, while size is practically identical with gelatine. The three substances differ only in their application and degree of purity; hence their preparation may be conveniently described under one head.
The materials seen by Dr. Ballard in use for the manufacture of glue are the following: -
(a) "Wet" materials: sheep-pieces or "spetches" from fellmongers; "fleshings " from leather-dressers and tanners; roundings of hides previously limed; the ears of animals; portions of bones to which tendons are attached; clippings of salted and alumed skins used for covering cricket-balls, etc.
(6) Dry materials: damaged pelts (Australian); ox feet salted (Australian and South American); calves' pates (German, etc.); horn " sloughs " (the pith or core of horns); clippings and roundings of parchment; glue pieces from fellmongers, leather-dressers, tanners, " pickers " hide works, and trotter-boilers; rabbits' pelts, and shreds from furriers.
Prior to making glue of them, all the soft tissues or materials used require to be limed. Such of them as come to glue-works from the leather-dressers and tanners, and some that come from the trotter-boilers, as well as the dry glue pieces and parchment clippings, have been limed already. But such as have not been limed are soaked first in pits containing milk of lime. After the liming, however, the lime has to be got rid of, or "killed." With this object, the limed materials are well washed with water. This washing is effected in tanks or vats, or in pits. At some works the washing is effected speedily in large barrels so arranged inside as to throw about the materials by revolution of the barrels. In the case of dry glue pieces, however, it is found sufficient to expose the material to the free action of the carbonic acid of the atmosphere, by spreading for a prolonged period on racks in erections, covered, but open at the sides, provided for the purpose. When thus prepared, the materials are ready for boiling.
But in some works they are subjected, after being washed, to pressure in a hydraulic press.
The boiling is effected in large open pans or boilers, of which there are usually several together. The pans are each capable of containing several tons of materials. In Young's works at Ber-mondsey the charge of each pan is 12 tons of fleshings with 1 ton of water, the produce of which is said to be about 25 cwt. of glue. A clear space is kept at the bottom of the pan by means of a false bottom of bars. A clear space in the middle is also kept by means of a vertical framework, which can be taken out and replaced at pleasure. The object of this frame and false bottom is partly to give free space for circulation of liquid during boiling, partly to prevent burning, and partly to assist the straining off of the liquid glue. The materials are boiled either by means of a fire beneath the pan or by means of open steam, or by means of both open and close steam. In some works both means (a fire beneath the pan and steam) are provided for the same pan. The pans are usually raised upon a platform approached by a ladder or steps, and are arranged under a roof or shed open at one or on all sides. When horn " sloughs " are used, it is customary to build them up around the outside of the central framework, before putting in the other materials.
When the boiling is completed, the fire is raked out, sufficient time is given for settling and partial cooling, and then the liquid glue is drawn out from the space beneath the false bottom along a wooden channel, in which lumps of alum are laid, to wooden troughs ("coolers") on the ground and about 1 ft. wide and deep, in which the liquid is left to solidify into a very firm jelly or size.
During the solidification, froth and some fatty matters rise to the surface, and in some works these are skimmed off; in other works they are left to solidify with the glue, and are dealt with in the next process.
This process consists in cutting the contents of the troughs into slices. The solidified material is taken in blocks from the troughs, and cut upon a bench into slices by women. When there is any scum on the surface of the blocks, it is first cut off and put aside to be returned to the pans.
The slices thus cut off are carried to sheds or erections open on all sides to the air, and are there laid upon nettings to dry spontaneously. When perfectly dry and hard, any mouldiness upon them is scrubbed off with a brush and warm water by women, after which they are laid on a rack to drain and dry, and are finally removed to a chamber heated artificially to between 85° and 120° F. (29° to 49° C.) for a final drying.
The matter left in the pans after boiling is termed "scutch." It is commonly thrown out of the pans in a heap upon the ground, sometimes under the shed where the pans stand, and sometimes in the open air, where it remains until removed to the manure-makers. Sometimes it is sent to the manure-makers in the condition in which it leaves the pan; at other works it is previously deprived of fat, and at others it is made into manure on the premises, without any previous removal of the fat it may contain. The "sloughs," when taken from the pans, are set aside in a separate heap for the use of bone-manure makers.
Size of very different qualities is made at glue-works. Some, destined for rough work, is made of similar materials to ordinary glue, while other varieties, of a fine quality, destined for the manufacture of gelatine and for use in soups, are made with especial care and precautions, and of very carefully selected pieces, such as " calves' pates.' It is important that after liming, the lime should be more completely removed than is necessary for glue-making, and for this purpose the pieces are first treated with a weak solution of hydrochloric acid. The boiling is effected in a similar manner to that of glue, except that free steam is more frequently used for heating the contents of the pans than in glue-making. The liquid size is either run out into little tubs for sale, or into a large vat, out of which it is taken and broken up for packing in tubs. The finest kinds of size for esculent purposes are made into blocks. Steam-jacketed pans are used in making such kinds. Some of the fine kinds of size made at ordinary glue-works go to the paper-makers. Size is sometimes made by first acting upon horn piths with hydrochloric acid, and then boiling them with water.