Chrome-tanning

This is the most common method of tanning leather for quality shoes – and for tanning leather in general.  Chrome-tanning with chromium (III) salts accounts for around 85% of global leather production (this was for 2014), For shoe upper leathers it’s 95%.

Chrome-tanning is a fast and relatively cheap tanning method in essence, and it’s certainly not the case that only really fine shoes are chrome-tanned. It’s used for everything from cheap shoes to the finest bespoke shoes, but the difference there lies in how the chrome- tanning is carried out and in the quality of the hides.

Whole hides have the characteristic blue-grey colour that chrome-tanning imparts. 

Chrome-tanning produces a soft, supple, water-resistant and durable leather that is easy to dye.  Chromium is an environmentally hazardous metal, and chrome-tanned leather should be disposed of as environmentally hazardous waste (which unfortunately isn’t easy to do in all parts of the world).  Less dangerous trivalent chromium is used in tanning, but when burned it turns into the toxic hexavalent chromium, which is a carcinogen. Trivalent chrome shouldn’t be let out into nature, which is why for example in Europe there are strict regulations on tanneries, which have closed systems where nothing is let out.

 

FOC-tanning (Free of Chrome)

The tanning type that has grown the most in recent years is driven by the automotive industry.  Previously only in developed countries, it is done increasingly in Asia and South America, where the tanning often doesn’t receive the same care as in the west, and therefore also has the most impact on the work environment and environment.  FOC-tanning accounts for about 5% off all leathers.

 

Vegetable-tanning

The third tanning method is the oldest.  Vegetable-tanning accounts for about 10% of the total leathers tanned in the world today.  Tannin extracts of barks, cones and/or leaves from various types of trees: oak, chestnut, mimosa, quebracho, olive and so on, are used.  Vegetable-tanned leather takes much longer and uses more water than chrome-tanning, but its disposal is harmless to nature.

Vegetable-tanned leather hanging to dry.  It’s a light brown when not dyed. Tärnsjö tannery

Depending on type of tannin, method of tanning, and type of hides, one can create numerous characteristics – from thin, extremely pliable hides, to very thick, stiff sole leather.  Vegetable-tanned leather is in general durable but stains more easily than chrome-tanned, and changes character much quicker.  For vegetable-tanned upper leather, if light-coloured it will turn darker over time, and if dark-coloured it will become brighter.

 

Combination-tanned

Combination-tanned, sometimes called vegetable re-tanned or semi-vegetable-tanned, when hides have been both chrome-tanned and vegetable-tanned, there is achieved a combination of both tanning properties.  The qualities of the finished leather depend upon the method. Normally it is chrome-tanned first and then vegetable-tanned, but the order can be reversed.

Edited from https://shoegazing.com/2022/08/28/guide-tanning-and-treatment-methods-of-leather/