Tanning is the main process where the skins or raw hides are preserved and converted into leather.  It is a chemical treatment and although several methods can be used, the two main ones for upholstery leather are chrome tanning and vegetable tanning.

In Chrome tanning, the hides are treated with basic chromium salts.  This process yields a light blue color leather which can accommodate a wide range of colors when dyed.   Small amounts of other chemical might be used during the tanning process, but the function of these are to assist the tanning process by the chromium salts.

Vegetable tanning is carried out by treating the hides with water extracts of various plants or trees.  It is difficult to obtained dyed colors which do not change with exposure to light on vegetable tanned leather, but a vegetable tan leather generally possesses an enhanced grain pattern and esthetic properties that tend to be very good.  Some vegetable tanned leather tends to be susceptible to acid vapors in the atmosphere and can slowly deteriorate in strength on exposure.