The first process is to cleanse the bones by immersing them in a pit or cistern of water, where they remain about 12 hours; the water is then drawn off, and fresh water added to them; this operation is sometimes repeated to remove any dirt. The water being withdrawn from the bones, a solution of lime, in the proportion of 1 bushel of lime to 500 gallons Of water, is poured into the cistern for the more perfect cleansing of the bones and the removal of any superfluous matter. After 3 or 4 days' saturation, the lime solution should be drawn off and fresh water added to get rid of the lime. Thus prepared, the bones are placed in a hollow globular vessel of wrought iron, called an extractor, which is filled with them by removing the interior plate which covers the manhole; this aperture is of an elliptical form, and allows the plate to be slipped round and re fixed in its place by turning a nut, which draws it up tight against the interior surface of the extractor, and the junctures are made air-tight by luting. The extractor turns upon a horizontal cylindrical shaft; one half of this shaft is made hollow, or consists of a strong tube, which tube also proceeds downwards towards the centre of the vessel to conduct the steam beneath the grating upon which the bones are laid.
The steam, of about 15 lb. pressure, is admitted by the cylindrical shaft, proceeds first to the bottom of the extractor, then rises up through the grating and amongst the bones, until the vessel is completely charged; previous to this, however, the air in the extractor is got rid of by opening a cock at the top of the extractor, and closing it after the admission of steam. While the steam is acting upon the bones the extractor is occasionally turned round by means of a hand-winch. When at rest, a quantity of fluid gelatine is collected at the bottom of the extractor, from whence it is discharged by means of a cock into a tub beneath, after opening the air-cock to enable it to run off. This done, steam is again admitted from, the boiler into the extractor to act upon the bones for another hour, when the second, portion of condensed liquor is drawn off. When the products thus obtained have become cold, the fit which has formed upon the surface is carefully removed by skimming, and the gelatinous portion only is to be returned into the extractor by means of a funnel through the cock on the top.
The steam, is then admitted to the extractor for an hour, after which it is finally drawn off into another vessel to undergo a simple evaporating process until it arrives at a proper consistency to solidify when cold, previous to which some alum is added to clarify it. When cold, this gelatinous mass is cut out into square cakes, and dried as usual in the open air.
(a) Common glue is extracted from hoofs, horns, and cuttings of the hides of various animals. For this process the materials are first steeped in water for 2 or 3 days, well washed, and afterwards boiled to the consistency of a thick jelly, which is passed while hot through osier baskets to separate the grosser particles of dirt or bones from it, and allowed to stand some time to purify further. When the remaining impurities hare settled to the bottom, it is melted and boiled a second time. It is next poured into flat frames or moulds, from which it is taken out hard and solid, and cut into square pieces or cakes, and afterwards dried in the wind in a coarse kind of net.
, (6) Substances intended for the glue-maker are macerated with milk of lime for 14 days, and dried by exposure to the air; they can then be transported to any distance without undergoing decomposition. The manufacturer generally treats the materials again with dilute milk of lime; afterwards they a]re carefully washed and exposed to the air for about 20 or 30 hours. They are then placed in a copper boiler having a perforated false bottom, which supports the materials and prevents their being burnt; the boiler is filled about two-thirds with water, and is piled up with the animal substances until they are level with the brim; a gentle but steady boil should be maintained, and the substances should be stirred from time to time. When the liquor on cooling forms a firm gelatinous mass, the clear portion is run off into another Vessel, and ft small quantity of dissolved alum is added. It is kept warm by means of hot water, and allowed to remain undisturbed for some hours to deposit its impurities; it is next run into the congealing boxes, and left to cool. When cold, the masses are turned put upon boards wetted with water, cut into small cakes, and these cakes are placed upon nettings to dry.
The dry cakes are then dipped into hot water, and lightly rubbed with a brush to give them a gloss, and lastly stove-dried for sale. This furnishes the best and palest glue. After the first liquor is drawn from the copper, the remnants left in the boiler are treated with fresh water, again and again, until no gelatinous natter can be extracted.
Break the glue into small pieces and soak for 12 to 24 hours in cold water; put the glue in the glue-pot, fill the outer vessel with water, and apply heat, For ordinary purposes it should run freely, and be of the con-: sistency of thin treacle. The hotter glue is, the more force it will exert in keeping the two parts glued together; in all large and long joints, the glue should be applied immediately after boiling. Glue loses much of its strength by being often melted; that glue, therefore, which is newly made is much preferable to that which has been used. When done with, add some of the boiling water from the outer vessel to the glue, so as to make it too thin for use. Put it away till wanted again, and by the time the water in the outer vessel is boiled, the glue in the inner is ready melted and of the proper thickness for use. Powdered chalk, brick-dust, or saw-dust added to glue, will make it hold with more than ordinary firmness.