A good animal charcoal will have a dull appearance, and be of a deep colour; it should be used in fine grains and not in the form of a powder.
The charcoal from the filter presses is washed free from glycerine (which is returned to the treated lyes), cleansed from foreign substances by the above treatment and revivified by carefully heating in closed vessels for twelve hours.
Glycerine obtained by other Methods of Saponification.—French saponification or "candle crude" glycerine is the result of concentration of "sweet water" produced in the manufacture of stearine and by the autoclave process. It contains 85-90 per cent. glycerol, possesses a specific gravity of 1.240-1.242, and may be readily distinguished from the soap-crude glycerine by the absence of salt (sodium chloride). This glycerine is easily refined by treatment with charcoal.
The glycerine water resulting from acid saponification methods requires to be rendered alkaline by the addition of lime—the sludge is separated, and the liquor evaporated to crude. The concentration may be performed in two stages—first to a density of 32° Tw. (20° B.), when the calcium sulphate is allowed to deposit, and the separated liquor concentrated to 48° Tw. (28° B.) glycerine, testing 85 per cent. glycerol and upwards.
Yield of Glycerine from Fats and Oils.—The following represent practicable results which should be obtained from the various materials:—
| Tallow | 9 | per cent. of 80 per cent. Glycerol. |
| Cotton-seed oil | 10 | " |
| Cocoa-nut oil | 12 | " |
| Palm-kernel oil | 18 | " |
| Olive oil | 10 | " |
| Palm oil | 6 | " |
| Greases (Bone fats) | 6-8 | " |
The materials vary in glycerol content with the methods of preparation; especially is this the case with tallows and greases.
Every care should be taken that the raw materials are fresh and they should be carefully examined to ascertain if any decomposition has taken place in the glycerides—this would be denoted by the presence of an excess of free acidity, and the amount of glycerol obtainable from such a fat would be correspondingly reduced.