Tanning is the conversion of animal hide into leather. After cleaning and soaking, a tanning agent is applied that converts the gelatin of the hide into an insoluble material which cements the protein fibers together and makes them incorruptible. Until the end of the nineteenth century vegetable extracts containing tannins were used; then the process was greatly shortened by using chromium salts, and also formaldehyde (see methanal) and formic acid.