Leather tanning technology

Which chromium metal is safe in use? Which if its compound or salt is better in leather tanning?

Chromium Cr is a transition metal which is found in different form as trivalent and hexavalent form chromium sulfate is commonly used in wet blue (tanning) leather production. There is much inaccurate information in circulation regarding the use of chromium salts and their associated safety for the tanning of leather.
Approximately 80% of global leather production is tanned with safe chromium III salts. It is a highly effective tanning agent producing leather with a flexible range of properties making it suitable for many end uses.
Chromium is a transition metal that can exist in a number of different oxidation states each with distinctive properties.
Metallic chromium
this is a steel grey, hard metal found as chromite ore (it does not occur in the metallic state naturally). It is used to harden steel, to manufacture stainless steel, and to form alloys. It is also used in plating to produce a hard corrosion resistant surface.
Trivalent chromium (Cr III)
Trivalent chromium compounds occur naturally in the environment. They are found in rocks, soil, plants and volcanic emissions. Chromium salts are present in foodstuffs and are a necessary nutrient for the human body as trivalent chromium is required for the normal metabolism of fats and sugars. Nutritional supplements are currently on sale containing chromium picolinate. Chromium (III) sulfate is considered safe to use in leather manufacture.
Hexavalent chromium (Cr VI)
Hexavalent chromium is the hazardous form of this element. It can be formed when trivalent chromium is oxidised. This usually occurs in the presence of oxygen combined with other factors such as extremes in pH. The salts have a characteristic yellow colour and are classified as carcinogens. Chromium VI is not used in the tanning of leather.