Tanning processes largely differ in which chemicals are used in the tanning liquor. Some common types include:
Vegetable tanning
Or "Veg Tanned" leather is tanned using tannins extracted from vegetable matter, such as tree bark prepared in bark mills. It is the oldest known method. It is supple and brown in color, with the exact shade depending on the mix of materials and the color of the skin.
Vegetable-tanned leather is not stable in water; it tends to discolour, and if left to soak and then dry, it shrinks and becomes harder. This is a feature of oak-bark-tanned leather that is exploited in traditional shoe making.
In hot water, it shrinks drastically and partly congeals, becoming rigid and eventually brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armor after hardening, and it has also been used for book binding.
Veg tanning can take up to a year.
Chrome tanning
Chrome tanning was invented in 1858. Leather is tanned using chromium sulfate and other chromium salts. It is also known as "wet blue" for the pale blue colour of the un-dyed leather. The chrome tanning method usually takes approximately one day to complete, making it best suited for large-scale industrial use. This is the most common method in modern use.
It is more supple and pliable than vegetable-tanned leather and does not discolour or lose shape as drastically in water as vegetable-tanned. However, there are environmental concerns with this tanning method, as chromium is a heavy metal.
Aldehyde tanning
This is tanning using glutaraldehyde or oxazolidine compounds. It is referred to as "wet white" due to its pale cream color. It is the main type of "chrome-free" leather, often seen in shoes for infants and automobiles. Formaldehyde has been used for tanning in the past, which is being phased out due to danger to workers and sensitivity of many people to formaldehyde.
Chamois leather is a form of aldehyde tanning that produces a porous and highly water-absorbent leather. Chamois leather is made using marine oils (traditionally cod oil) that oxidize to produce the aldehydes that tan the leather to colour it.
Alum tanning
Leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Alum leather is not actually tanned; rather the process is called "tawing", and the resulting material reverts to rawhide if soaked in water long enough to remove the alum salts.
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