Experimentation in Post-Processing of Sulphonic Acid Tanned Leather for Improvements in Durability and Rigidity


This is a living post, documenting experimentation with hair-off skins that have been preserved with a Sulphonic Acid based tanning agent, for the purpose of getting better results for derived leather products such as wallets, holsters, and other products requiring material rigidity and wear resistance.

Background: What is Leather?

Leather is the preserved and stabilized collagen grain structure derived from the skins of animals. Preservation and stabilization are not the same. Non-Stabilized skins are still called skins (usually rawhide, salted stock, etc.), not leather.

Preservation is simply making the skin inhospitable to microbiological life, but not actually making them resistant to microbiological attack. This can be drying /salting /acidification /basification – but is not a stabilization.

Stabilization is chemically changing the protein structures to be resistant to microbiological enzymes (inedible to bacteria) even when at ideal microbiological environmental conditions. Stabilization is called “tanning” or “tanned” in the industry, for example, “Tanned leather” instead of “stabilized skin.”

Collagen are tubular protein structures that make up the bulk of skin, which are the primary target of stabilization. They are strong fiber bundles that provide the strength, flexibility, and durability of the finished material.

The method of preservation selected will largely determine the characteristics of the resulting material.

The most ancient tanning agent used was the brains of the animals – a chemistry of stabilization I am not well studied in. Enzymes in the brain did work on the protiens of the skin, not breaking them down as bacteria would, but changing them to be more resistant to microbiological attack. The main drawback of this process is the scalability for industry, and the costs associated.

The second-most ancient and arguably still one of the most popular methods, is using vegetable tannins (origin of the industry’s use of the word “tanning”) to perform protein stabilization. Bark from hardwood trees such as maple, cherry, walnut, oak, are soaked in water to make a ‘tea’ – which the skins are submerged in for a period of 2-30 days to perform said stabilization. The main characteristic of this method of tanning is the attractive coloration, water resistance, durability, and rigidity. These can be formed with water, and when dry will maintain their shape.

The most common method of industrial scale tanning employs the use of Chromium III – known as Chrome Tanning. It is a quite toxic method of stabilization that results in flexible leather that’s most commonly used for clothing and upholstery. This method is excellent for large scale production, but it is not suitable for smaller tanneries.

Sulphonic Acid based Tanning Characteristics

Here at the Dinsmoor Tannery, we use a Sulphonic Acid tanning agent that has been synthetically manufactured. It has the the benefit of making excellent upholstery and softness of leather, but has the downside of any soft leather – low durability and no rigidity.

In processing, doing things such as not adding a lubricating oil and not breaking the fiber during drying will make the leather more rigid and durable, but beyond this, further improvement is desirable.

Sheep and goatskins

One of the difficulties with leatherwork using sheep and goatskins is their thinness. They have a finer grain structure than more commonly used cow hides and are thin as well. How do we get adequate strength out of a thinner material?

One way is to increase the actual thickness, by lamination. Layering multiple leathers to increase the thickness and bonding with either thread or – however this requires additional materials for the same effective strength. It would be better to get this strength by chemical and processing means without making them brittle.

Methods employed

I chose to employ methods of increasing the strength and rigidity of the leather after tannage:

Controlled pre-tannage fat conversion

The main goal of this is to get as much fat out of the skin as possible, as these may end up later rendering and causing the skin to become softer than desired. Generally there is a tradeoff in processing sheepskins with wool-on with the quality of the tannage and the limitations of processing that comes with needing the wool to remain on the finished leather. Allowing some of the fat to putrefy into substances easier to remove with chemical and physical action.

After dehairing in a sodium hydroxide bath of a pH above 12, the skin is placed in a neutral water bath for 48-72 hours with no additives. Going in, the skin will be swollen and requires gentle handling.

The skin’s high pH will leech into the water, and will raise the solution to around a 9-10. Bacteria growth will be retarded but present, and the fats will begin to putrefy, turning into a greasy slime on the flesh side of the skin which can be easily shed manually.

Surfactant cleaning

After physical removal of the surface slime and a rinse with fresh water and dish soap, then the skin can be transferred to a new bath at a pH of 4-5 with a non-saturated salt content and the addition of a bactericide to halt further enzymatic action by bacterial growth.

Solvent-based degreasing

After lowering the pH slowly to the pickling range, use a degreaser suitable for use in pickles and go through the normal pickling and shaving process.

Double-Tannage with Cherry Tannins and Metal Oxides

This part is new to me, and completely experimental.

I used dust from cherry wood, steel, and brass that came off my belt grinder, added a small amount of ferric chloride (to help dissolve some brass) and brought the solution to heat. Everything except the ferric chloride came off my belt grinder as waste from knifemaking.

My thinking is that the cherrywood will do a traditional vegetable post-tan on the leather, and combined with the iron and small amount of dissolved brass (copper) it would allow for an interesting coloration.

I left this solution in a bucket for 3 days, in an outer bath which was heated, a sort of double-boiler setup but without allowing it to get too hot. I stirred it multiple times a day, but a liquid drum would be much better and give better results for the evenness of the tannage and dying effect of the metals.

After removal from the solution, I dried it on a flat surface, and did leatherworking before it was completely dry.


I cold smoked the resulting leather under light smoke for an hour with cherry wood.


It ended up with a really cool slate color, and is quite rigid and strong compared to normal procedure. This same leather could have been super soft and supple. I will continue to experiment with this method and see if I can get some more detailed procedures down, rather than just “winging it”.