Bone-size is, according to Thomas Lambert, prepared as follows: The bones are first degreased by the naphtha process, and then passed through the cleanser direct into the glue-boilers and steamed, as in the manufacture of glue. The resulting liquors are forced up to the clarifying vats and partially bleached with a current of sulphurous acid, passing through bag-filters to the evaporating troughs, and concentrated from 30 to 38 per cent., as required, and then jellied in casks.
The manufacturer with no benzine or glue plant at his disposal washes the bones in a revolving drum, and, after crushing, they are fed into a boiler and subjected to an alternate current of steam and water, the latter coming from a spray pipe fixed at the top of the boiler. The liquors are generally drawn in two portions, having a strength of 14 to 16 per cent. of glue. After separating the fat, which is refined and sold to the soap-maker, the liquors are run into a large wooden vat about 8 × 6 × 4 feet, fitted with a steam coil, partly bleached with liquid sulphurous acid, and then boiled down to the required strength.
For a common size the bones are crushed but unwashed, and are fed direct into the boiler and treated as above. The liquors are not bleached, and boiled to a strength of about 25 per cent. glue. The composition of the different grades may be given as follows:
Common size. 25 per cent. glue, 75 per cent. water, etc.
Medium size. 30 per cent. glue, 70 per cent. water.
Best size. 38 per cent. glue, 62 per cent. water.
A concentrated size is now prepared by many manufacturers. It is a bone-size, and is sold at a Baumé strength at 122° C.
No. 1. 15° Bé. at 122° C., 40.5 per cent. glue.
No. 2. 20° Bé. at 122° C., 44.5 per cent. glue.
No. 3. 25° Bé. at 122° C., 49 per cent. glue.
Under the name of concentrated size also are sold a series of powdered glues of different qualities. They are the off-color and twisted cakes, sorted out in the warehouse, and ground to a fine condition by passing through a mill, and their value is based on the quality of the cake ground.
Size rapidly ferments and becomes sour and mouldy unless some preservative is added. Sulphate of zinc is mainly used for this purpose.
Bookbinders’ Size.—I. Water, 2 quarts; powdered alum, 1 oz.; Russian isinglass, 2 ozs.; curd soap, 40 grains. Simmer one hour, strain through linen or a fine sieve, and use while warm.
II. Water, 2 gallons; best glue, 1 lb.; alum, 4 ozs. Prepare and use as above.