Guide – Tanning and treatment methods of leather

Time for the promised follow-up to the previous post on leathers from different animals. Here’s a substantial guide to the main tanning techniques, dyeing methods and finishing types and treatments.

Read the article about different types of leather here.

 

For the sake of simplicity, I will only focus on cattle leather and the different methods used for this, but of course the same types of treatment are also used for many other types of leathers. There are also many other tanning and treatment methods that I won’t go into here, as I will focus on variants common in shoemaking.

 

Tanning methods

Chrome tanning

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 numbers 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 crap shoes to the finest bespoke shoes, but the difference there lies in how the chrome tanning is carried out and of course in the quality of the hides.

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 where it can impact local nature, which is why for example in Europe there’s very strict regulations on tanneries, who have closed systems where nothing is let out.

 

FOC-tanning (Free of Chrome)

The tanning type that has grown the most in recent years, not least driven by the automotive industry. Previously it was only done in developed countries, but it is done more and more also in for example Asia and South America, where the chrome tanning in many cases isn’t done in the same good way as in the west, and therefore also do the most good in terms of work environment and environmental impact. FOC-tanning stands for about 5% off all leathers in the world.

There’s various types of chrome-free tanning, where one use synthetic tanning agents without chrome. The preferences of chrome-free leather can vary, but can be very similar to chrome leathers in most regards. The tanning is more complicated where among other things the temperature needs to be more fixed than for chrome tanning.

 

Vegetable tanning

The third main type of tanning method is the oldest still existing one (to any industrial degree). Vegetable tanning stands for about 10% of the total leathers in the world today. One use tannin extracts of barks, cones and/or leaves from various types of trees: oak, chestnut, mimosa, quebracho, olive and so on. In general vegetable tanned leather takes longer time and uses more water than chrome tanning, but its disposal is normally harmless to nature.

Depending on type of tannin and method of tanning, and type of hides, one can create numerous characteristics of the finished leathers – 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 veg tanned upper leathers, in general, if light coloured it will turn darker over time, and if dark coloured it will become brighter.

 

Combination tanned

Combination tanned, sometimes called veg re-tanned or semi veg tanned, is when the hides have been both chrome tanned and vegetable tanned, to achieve a combination of both tanning methods properties. How the leather becomes depends on method and how much of each type that is done. Normally one chrome tan first and then veg tan, but it can be done the other way around as well.

 

Dyeing methods

Aniline leather

Aniline dyed leather hasn’t really been dyed with aniline for decades, since it’s toxic, but the name lives on as the method used is the same only with different substances. Aniline dyeing is when the leather is immersed in a bath of dye that penetrates the leather. In the past it was common for it to be dyed all the way through, struck trough, but today aniline dyed leather is more often only dyed part way through so it’s still light coloured in the middle. This is because it takes a long time to dye through, and also it can become a bit more sensitive and less taut in cases.

Aniline dyed leather will be an even shade, and will hold the colour very well. It’s also possible to get effects in aniline dyed leather by different methods, for example the so called museum cal leather is aniline dyed leather which is bathed with wet sponges during the dyeing process and thus gets its special character. Box calf leather is the name of aniline dyed calf leather with a specific type of finishing, traditionally including the natural milk protein casein, but all aniline dyed calf are not per se box calf, which is a common misconception.

 

Crust leather

This is leathers that are only dyed on the surface. Most common is that the whole hides are coloured by the tannery, and when finished it doesn’t look that different to aniline other than that it’s usually more matte, since intention is that manufacturer is to achieve the final finish themselves. That’s why crust leather is often called “burnishable” leather or similar. If you cut through chrome tanned crust you’ll see that it’s grey all except the outer layer.

In some cases, especially when one do painted patina on the shoes, the leather arrives at the shoe manufacturer undyed, so for chrome tanned completely grey, and the shoe gets its final shade by treatment with leather dye and shoe polish after the shoe is finished, or sometimes on the hides before the shoes are being built. Crust leather gets a patina quicker than aniline and can be transformed more with pigmented creams and waxes, but the base colour is still permanent.

 

Finishing types and treatments

Full grain leather

Full grain is leather of good quality that has not received any kind of external treatment in the form of sanding or cover. It’s simply leather that is of such good quality that it looks smooth and fine on its own, a tight grain surface with few blemishes, scars and stretch marks. Here you have the full part of the leather intact, and pros is that the grain area is the strongest part of a hide with the tightest fibre structure, which gives natural protection and is possible to restore once it gets scuffed.

Full grain calf leather absorbs shoe cream and wax well and can be polished to a good shine. An issue highlighted in recent years is that more and more full grain aniline and box calf leathers are being covered with thicker finishing layers to hide visual issues beneath, which doesn’t show when making the shoe or even until they are worn.

 

Corrected grain leather

There are lots of terms here that mean pretty much the same thing, such as polished binder, hi-shine, bookbinder, cavalry calf, PU-coated and so on. It involves sanding the upper and covering the surface with a plastic layer. Note that the leather doesn’t have to be of poor quality to be corrected grain or covered, but it isn’t possible to tell as you cannot see the leather itself, and standard is to take lower quality hides to be corrected grain since you won’t see the actual leather at all, only the plastic cover.

The advantage of corrected grain is that it is easy to maintain and looks shiny without shoe polish (it can have a matte finish as well though). But shoes in corrected grain can’t be polished in the same way as full grain, since the polish cannot penetrate the leather because of the covering layer but is only smeared around on top of it, and the major disadvantages is that it doesn’t breath at all or very little and that over time the treated surface cracks and this can’t really be fixed.

 

Top coated leather

This is a bit of a tricky definition, since a lot of proper great full grain leathers also are finished with various top coats, for example box calf, as mentioned above. However, normally when one talk about top coated leathers it’s when the hide is covered with a thick layer of paint which hides issues on the leather plus layers that can increase properties like water protection and be dirt repellent.

On top coated leathers the pores are less visible or not visible at all, and it can be done also on embossed leathers. Especially upholstery leather is often various types of top coated ones, since it has to be very abrasion resistant.

 

Smooth leather

All-natural skins, also calf skins, has a kind of uneven texture, so it’s a bit lumpy and wrinkled, but it’s a texture that’s often quite uneven and it’s not very common to use it as such. Shoes made of what is referred to as “regular” leather are indeed pressed smooth to the flat and even surface that it has.

 

Shrunken leather

Basically the opposite of the above, where one instead of smoothen out the natural texture enhance it, by shrinking the hide. This way one can create a more even texture which can look really beautiful.

 

Embossed grain leather

Embossed, or printed, leathers can in a way be called both full grain – since one don’t sand away anything but keep the entire grain surface – and corrected grain – since one do indeed modify the grain surface. However it has much more in common with regular smooth full grain leather than plastic covered ones, in all regards. Here one press a pattern into hides, either with a roll press or with large plates.

The reason that embossed leathers often are a bit more resistant against scratches, more waterproof and so on, is since one compress the outer grain surface so the already tight fibres becomes even tighter. The hides that becomes embossed grain leathers are ones that aren’t of good enough visual quality to be used for smooth leathers, and for tanneries who do high-end calf leathers where standards of the smooth ones have to be very high, a vast majority of all hides becomes embossed skins.

 

Suede

In all of the above finishing types and treatments of leather, the grain side is used, the outer side of the animal’s hide. For suede it’s instead the corium, the flesh side with looser fibres, which is sanded to the characteristic velvety finish. Nowadays very few suede leathers are actual the full section of the hide turned inside out, most are split suedes where the corium has been split away from the top part. What originally was called suede is today called full reverse suede (or versions of that), or roughout if the surface isn’t sanded as even, then the inside has the full grain intact.

One advantage of suede is that you avoid the usual creases that form on ordinary leather, and if you use a good waterproofing spray, the material can withstand harsh weather without problems. Basically all suede leathers are aniline dyed through.

 

Nubuck

Like suede, nubuck is sanded leather, but here it’s not the corium side that’s buffed, but the outer side, the grain. It gives a matte, rough finish that can take a beating, but can’t be polished with shoe cream. Since part of the protective properties of the grain disappears with the sanding, one needs to use waterproofing spray on nubuck. Just as embossed grain leathers, it’s hides that doesn’t have the quality to become smooth leather that are used for nubuck. For more info on nubuck, suede and roughout, read this article.