human
in which the skins are treated successively with a chromic acid solution and with an acidified solution of
gpt
(sodium thiosulphate). The first bath was made up commercially of potassium dichromate and hydrochloric acid, so that, strictly speaking, it contained potassium chloride also. The second bath contained, in effect, sulphurous acid, which reduced the chromic acid in the skin fibres to the tanning chrome salts. Free sulphur is also formed in this bath and in the skin, and contributes to the characteristic product obtained by this process of tanning. Many minor deviations from the original process of Schultz have been introduced, but the main features have been unchanged, and this method of tanning is widely employed at the present time for both light and heavy chrome leather. In 1893 tanning by basic chromic salts was revived and the use of the basic chloride was patented by Martin Dennis, who offered such a tanning solution for sale. The validity of the patent has always been doubtful on account of the previous work of Knapp and others, but the process itself was commercially satisfactory, and the many variants of this and of the basic sulphate tannages are now generally known as the