Glycerin

Glycerin was discovered in 1779 by Scheele as a by-product in the preparation of lead-plaster; for a long time the comparatively small quantity of glycerin met with in commerce was obtained in this way. It is now prepared from oils and fats by distilling them in a current of superheated steam, sometimes by first saponifying them with alkalies, or decomposing them with sulphuric acid, and then submitting them to this distillation treatment.

Glycerin generally occurs as a thick syrup with a sweet taste: when pure, it may be obtained in deliquescent crystals. Its empirical formula is C₃H₈0₃. It is a strongly hygroscopic or water-attracting substance, the pure water-free glycerin being capable of absorbing more than one-third its weight of water from the air. Commercial glycerin always contains water: the specific gravity of the liquid affords a rough method of estimating the amount. For pure glycerin at 15.6° C. has the specific gravity 1.265, while that which contains 20 percent of water is reduced to 1.213; with 30 percent it is 1.186, and with 40 percent 1.157. The presence of sugar, a not uncommon adulterant, may be recognised by the turbidity caused by mixing the glycerin, after evaporation to remove water, with chloroform. Glycerin containing lead darkens when sulphuretted-hydrogen water is added to it, while the presence of acids may be recognised by blue litmus-paper, which is not reddened by pure glycerin. The water-attracting property of glycerin induced me to use it as a substitute for honey in preparing moist water-colours so long ago as 1856, but I am given to understand that it was employed in 1847 by Messrs. Winsor and Newton. Even in cake-colours a trace of glycerin may be introduced with advantage, as it renders them less friable and more easily rubbed down with water.

It prevents size, glue, and white of egg from becoming brittle on drying, and on this account may be used in the preparation of linen, canvas, etc., as painting-grounds. Care must, however, be taken in every case not to add more glycerin than is necessary to effect the purpose in view. It is a useful addition to gum-water, 1 dram to each ounce of gum present being sufficient; some copying-inks contain it. Modelling clay may be kept moist by means of glycerin.