Commercial glycerin is mostly obtained from the "spent lyes" of the soap-maker.
In the van Ruymbeke process the spent lyes are allowed to settle, and then treated with "persulphate of iron," the exact composition of which is a trade secret, but it is possibly a mixture of ferric and ferrous sulphates.
Potash lyes also may be bought direct, but in some cases they are sharpened or causticized by the soap-boiler himself from the carbonate.
Soft Soap. - Soft soaps are made with potash lyes, although in practice a small quantity of soda is also used to give the soap some consistence.
Distillation in a vacuum is practised in two forms: - if the pump draws off steam as well as air it is termed a "wet" air-pump; if it only draws off air, it is a "dry" air-pump. In the glycerin industry the lyes obtained by saponifying the fats are first evaporated with "wet vacuum" and finally distilled with closed and live steam and a "dry vacuum."