The rawhide still needs to become a stable leather that will not perish, and the Tanning process achieves this. This process is not primarily concerned with adding colour or changing the handle (though these may be a side effect) but making sure that the hide’s structure is fixed and stable to create an article that will last. It is only at this point that it can be called Leather.

In contrast, parchment is an animal hide that is not considered leather because it does not undergo Tanning. Whilst it is dry, in its characteristic stiff paper-like quality, it is relatively durable. But as it is not tanned, it will break down and decay, especially if damp or wet.  

There are three main Tanning methods, Mineral Tanning (usually using Chromium), Vegetable Tanning (using oak and spruce bark, among other tannins), and Synthetic Tanning. Over history, there have been other types of Tanning, such as oil tanning, salting, and even just sun-dried, but these all have limitations for modern applications. Most European leather is produced from a chrome tanned hide, as this creates the most reliable and versatile leather.  

Veg-Tanning has its roots in history, and modern Veg tanning can produce beautiful looking natural hides. The process can take many months, and the resulting leather requires ongoing maintenance and care, but the articles can be excellent for specific end uses. After initial Tanning, these hides usually have a characteristic brown hue, which is why the concept of Tanning is synonymous with a brownish product for many people.  

This underlying tint can make lighter colours challenging to achieve, restricting the typical colour palette of Veg-Tanned hides.  

Synthetic Tanning is a modern development that reduces or even removes chromium use. Synthetics is a relatively new area, which shows promise. Often the Synthetics are used in combination with Chromium to tan, and in a few cases, there are Free of Chrome (FOC) tanned leathers. The FOC leathers do have some drawbacks such as shrinkage and a tendency to water damage, but as Syntans improve, they will become more widespread. After initial Tanning, synthetic tanned hides are known as wet-whites because of their appearance.  

Chrome Tanning is used in over 80% of the worlds leather production because of its ability to create a durable and supple article. The tensile strength of this is about twice that of a Veg-Tan article. The Chromium 3 (Trivalent) used in the leather process is a naturally occurring element found in Rye Bread, peppers, and even multi-vitamins in the USA. As with all chemicals, there is tight regulation concerning usage, further refined in the European REACH legislation of 2015.  

The Chrome tanned hides have a faint blue hue once they are tanned, hence they are known as Wet-Blues.  

Purpose of tanning leather: 

• The skin is made up of one third protein (collagen). Tannins preserve the flexibility of these protein strands and the entire tanning process prevents the protein filaments from hardening and becoming tacky on drying. ‘Parchment’ is not tanned and hence different to leather. It is hard and translucent. 
• Tanning makes the leather resistant to decay (self-decomposition by microorganisms). 

• The swelling of the fibres on wet leather is prevented. 
• Temperature resistance is significantly increased. Untanned skin shrinks in water at 62 ° C. Chrome-tanned leather is resistant up to 100 ° C. The fibres do not stick to each other and the leather does not shrink. 

 Sammying (Pressuratura) is used to press out extra water and even out the moisture content through the material. Shaving (Rasatutura) is used to thin and equalize using a leather shaving machine which cuts off leather fibres from the flesh side. 

Sorting (Scelta in Wet-Blue). The leather undergoes another Quality Check and is sorted according to the quality. Hides which do not have blemishes or marks are best quality and can be processed to aniline leather. The ones which have marks may be processed into corrected grain.