Leather is made through a lengthy, complicated process that hasn’t changed much over the centuries - including what is used to tan the leather. Tanning is the step in the leather-making process where a chemical reaction transforms an animal hide (skin) into leather. This chemical reaction changes the protein structure of the rawhide into a stable material that will not putrefy and will be suitable for a variety of applications. There are several options used to tan leather, but two options are most common:
When chromium sulfate is used to tan leather, the chromium penetrates the fibers of the collagen in the hide. Raw, chrome-tanned hides appear blue, so leather in this state is called “wet blue.” The process is faster when chromium sulfate is used to tan leather (compared to vegetable tanning) and also produces a more flexible, stretchable leather.
(Plant Tannins) When naturally occurring tannins (from plant leaves and bark) are used to tan leather, the tannins bind to the collagen proteins in the hide, coat them, and cause them to become less water-soluble, which makes them more resistant to bacteria. The process of stretching and applying the tannins over the course of several days makes the hide more flexible than before, although not as flexible as mineral-tanned leather. After tanning, depending on the desired outcome, the leather will undergo various finishing processes.
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