GLUE 681 



matters employed in the manufacture of glue, constitute a peculiar branch of industry. 

 Those who exercise it should study to prevent the fermentation of the substances, and 

 to diminish the cost of carriage by depriving them of as much water as can conveniently 

 bo done. They may then be put in preparation by macerating them in milk of lime, 

 renewed three or four times in the course of a fortnight or three weeks. This process 

 is performed in large tanks of masonry. They are next taken out with all the adhering 

 lime, and laid in a layer, 2 or 3 inches thick, to drain and dry, upon a sloping pave- 

 ment, where they are turned over by prongs two or three times a day. The action 

 of the lime dissolves the blood and certain soft parts, attacks the epidermis, and dis- 

 poses the gelatinous matter to dissolve more readily. When the cleansed matters are 

 dried, they may be packed in sacks or hogsheads, and transported to the glue-manu- 

 factory at any distance. The principal substances of which glue is made are the 

 parings of ox and other thick hides, which form the strongest article, the refuse of 

 the leather-dresser ; both afford from 45 to 55 per cent, of glue. The tendons, and 

 many other offals of slaughter-houses, also afford materials, though of an inferior 

 quality, for the purpose. The refuse of tanneries, such as the ears of oxen, calves, 

 sheep, &c., are better articles ; but parings of parchment, old gloves, and, in fact, 

 animal skin in every form, uncombined with tannin, may be made into glue. 



The manufacturer who receives these materials is generally careful to ensure their 

 purification by subjecting them to a weak lime steep, and rinsing them by exposure 

 in baskets to a stream of water. They are lastly drained upon a sloping surface and 

 well turned over till the quicklime gets mild by absorption of carbonic <acid ; for, in 

 its caustic state, it would damage the glue at the heat of boiling water. It is not 

 necessary, however, to dry them before they are put into the boiler, because they dis- 

 solve faster in their soft and tumefied state. 



The boiler is made of copper, rather shallow in proportion to its area, with a uniform 

 flat bottom, equably exposed all over to the flame of the fire. Above the true bottom 

 there is a false one of copper or iron, pierced with holes, and standing upon feet 3 or 

 4 inches high ; which serves to sustain the animal matters, and prevent them from 

 being injured by the fire. The copper being filled to two-thirds of its height with 

 soft water, is then heaped up with the bulky animal substances, so high as to surmount 

 its brim. But soon after the ebullition begins they sink down, and, in a few hours 

 get entirely immersed in the liquid. They should be stirred about from time to time, 

 and well pressed down towards the false bottom, while a steady but gentle boil is 

 maintained. 



The solution must be drawn off in successive portions ; a method which fractions 

 the products, or subdivides them into articles of various value, gradually decreasing 

 from the first portion drawn off to the last. It has been ascertained by careful expe- 

 riments that gelatine gets altered over the fire very soon after it is dissolved, if the 

 heat of 212 is maintained, and it ought therefore to be drawn off whenever it is suffi- 

 ciently fluid and strong for forming a clear gelatinous mass on cooling, capable of 

 being cut into moderately firm slices by the wire. The point is commonly determined 

 by filling half an egg-shell with the liquor, and exposing it to the air to cool. The 

 jelly ought to get very consistent in the course of a few minutes ; if not so, the boiling 

 must be persisted in a little longer. When this term is attained, the fire is smothered 

 up, and the contents of the boiler are left to settle for a quarter of an hour. The stop- 

 cock being partially turned, all the thin gelatinous liquor is run off into a deep boiler, 

 immersed in a warm water bath, so that it may continue hot and fluid for several 

 hours. At the end of this time the supernatant clear liquid is to be drawn off into 

 congealing boxes, as will be presently explained. 



The residue, or undissolved matter in the boiler, is to be again supplied with a 

 quantity of boiling water from an adjoining copper, and are to be once more subjected 

 to the action of the fire, till the contents assume the appearance of dissolved jelly, and 

 afford a fresh quantity of strong glue liquor, by the stopcock. The ' grounds ' 

 should be subjected a third time to this operation, after which they may be put into 

 a bag, and squeezed in a press to leave nothing traextracted. The latter solutions are 

 usually too weak to form glue directly* but they may be strengthened by boiling with 

 a portion of fresh skin-parings. 



Fig. 1115 represents a convenient apparatus for the boiling of skins into glue, in 

 which there are three coppers upon three different levels : the uppermost being acted 

 upon by the waste heat of the chimney, provides warm water in the most economical 

 way ; the^second contains the crude materials, with water for dissolving them ; and the 

 third receives the solution to be settled. The last vessel is double, with water con- 

 tained between the outer and inner one; and discharges its contents by a stopcock 

 into buckets for filling the gelatinising wooden boxes. The last*made solution has 

 about one-five-hundredth part of alum in powder usually added to it, with proper 

 agitation, after which it is left to settle for several hours. 



