Chemistry

[Leerwerker]

I recently asked the following questions of the Leather Chemists of America and with each I am also giving you the answer I received ...

Q1) When vegtan leather turns darker in light, does that

mean the leather is getting oxidized?

A1. No the leather is not oxidizing, the tannins are. The veg. tannins are what give the leather its stability, color and much of its leather character. They are polyphenolic compounds (in most cases), so they are subject to chemical change such as oxidation, espcially in the presents of light, oxidizers (such as permanganate or peroxide) and air (oxygen). The changes depend a great deal upon the type of tannins, moisture, temp., other materials present, etc.

Q2) I have some vegtan that have been bleached bone white

in the sun, and the moment I bring water in contact with

the leather, it gets back its nice honey colored brown

darker color. What is happening?

A2. Not sure what is happening here. The color changes suggest that there is a unique tannin present or some residual bleaching agent, etc. Clearly the moisture content is exerting the controlling influence, but that is not terribly uncommon, though odd for simple veg. tanned hides. Perhaps this is Tara tannin? Any idea? What is unique about this leather, ie source, character, properties???

Q3) I use some Potassium Permanganate in water to 'paint'

on leather - it has the same effect as a dye, but you can

have a lot of control over how much you want to oxidize

the leather and how dark you want it to be.

Does this drastically change the pH of the leather? Or

harm it in a irreversible way?

A3. Permanganate is a strong oxidizer. It will start with the tannins which oxidize easily, but then move on to destroy oils, carbs, protein and more. Just like a woman bleaching her hair until it falls out, it would be pretty easy to do permanent harm, but it all boils down to how much, how strong, how warm, etc. This is not likely to change the pH, but certainly will change some other more serious chemistry.

Q4) A customer of mine wants to bring in a pair of saddle bags

he has soaked in kerosene to 'clean'.

I am a but horrified to think about this, but maybe I am

all wrong and this is a tried method of cleaning leather?

Can you help - the chemistry of it all seems wrong, but I

do not have the necessary knowledge to judge ...

A4. Kerosene should clean out oils and grease for sure. But then how to clean out the awful smell of the kerosene??? Obviously working with highly combustible organic solvents such as kerosene has some serious risks and concerns, though the use of kerosene in the old days was probably much greater than any of us can imagine, plus many solvents very similar to kerosene but with different names are still in use in places.

You do not say what the guy wants you to do with the leather. Remove the kerosene? Liability would be pretty great for any recommendation that we might make here, but clearly the answer would lie in using a lighter solvent that would evaporate more rapidly and completely than the kerosene. Obviously these types of solvents could have even greater flammability and inhalation risks. The big question then is whether you feel safe with your knowledge and skill level to tackle such a task.

It is also noteworth that the leather is going to be bone dry after these treatments and need to be re-lubricated or fatliquored.

[UKRay]

What a fascinating Q&A. I can't honestly say I had ever thought of asking any of those questions but they (and the answers) certainly gave me pause for thought. Thanks Leerwerker.