Friday, October 2, 2020

Acid and Acidity

I posted these questions to the Leather Chemists of America:

  • Is veg tan less acidic than chrome tan?   Holster makers believe this to be true - they avoid chrome tan against the metal of fire arms and report that they have seen pitting of the metal with prolonged storing.
  • Can an acrylic finish on chrome tan give it enough of a barrier that the acidity will no longer be a problem?
  • Can you simply rinse vegtan in calcium carbonate to neutralize it to a pH of 7?
  • I hope you do not mind a 'leather user' joining this forum, but I find so much useful information here! And I would like to use the knowledge I pick up here, to educate as much of us as leatherworkers as I can, as to the proper care and use of leather!

Here is a compilation of the answers:

"The common pH scale is from 0 to 14 with 7 being neutral. Below 7 is acidic, above 7 is basic or alkaline. The hair is removed from a hide with lime and sulfide which produces a pH above 13.

That is extremely alkaline. However, this condition is then transformed into an acidic state for most tannages. It is tempting and almost accurate to say that all leather is acidic, but there are some minor exceptions which do not normally apply to commercial leathers.

Before tanning, chrome leather is pickled in acid lowering the pH usually to 2.5 or lower (very, very acidic). After tanning the pH is raised to around 4 to set the chrome (actually it is the tanning step). In vegtan leather, the process for tannage is a lowering of the pH to set the tannins, but again the target pH is about 4 (a little less these days). Of course these are all wet processes and not the final pH of most leathers.

In chrome leathers the pH may actually end up much higher than in vegtan, but that depends a lot on the coloring (dye) process which is often the last wet process. If a heavy dye must be set, say black, often a lot of formic acid is used to fix that dye, so once again the pH may easily drop below 4. However, it is not just the acid that causes this leather to be very corrosive to metals, it is also the salt and the chrome itself which catalyzes oxidation of metals. Typically vegtan leather is very low in salt, as well as tannins being anti-oxidants.

Personally, I would want a Mossback, lining or other barrier between my gun, metal frame glasses, knife or other metal object stored in a leather protective or carrying case, sheath or holster, but I tend to be a little over the top. Certainly acrylic could do the job if applied adequately. Obviously, in the real world where naked leather is in contact with swords, guns, knives, eye glasses, etc., very routinely, little real damage is commonly seen.

If one would buy leather with low salt, low acid content and proper lubrication for this application (gun holsters) there would be little reason to be concerned. For chrome leather this would probably include a good, soft retannage in order to modify the surface. A good example would likely be found in military specifications for leather used in such applications. However, I have barked up this tree enough to know that no one is willing to hear the story. They just want to buy a piece of leather and go. So if they are wise enough to be concerned, tell them to check out Mossbacking and just use a similar gum, plastic or lining that they know to be non-corrosive.


As to dipping vegtan leather in carbonate of soda.....NOT A GOOD IDEA! This will raise the pH just fine, but that will de-tan the leather. Even de-tanning the surface is not a good idea.

Chrome tannage creates lots of salts in the leather! Sulphates between charged aminos and sodiums as the counter ions, for instance. The fact that most tannages are produced by water-born chemistry, yields a certain corrosivety to leather towards metals.

Vegtanned leather strap was also classically used for the evening-out of freshly sharpened edges of cutting steel, thus artisans feel that vegtanned leather is a natural companion to a sharp cutting instrument. The truth is that most tannages release acids from collagen, or associated materials like phenolic resins present.

My suggestion for cases made of leather is that they should breathe well for moisture release but should have a dry breathing film, water-barrier against the metallic surface being protected and enclosed in the case. I would suggest that the thinnest would be made by a water-emulsified nitrocellulose (Hydrolacker) application on the surface touching the metal, and nothing else! Moss-backs that consist of waxes and protein combinations could be OK, but acrylic resins (paint without pigment!) is too good a water vapor barrier and thus might actually aid galvanic corrosion by helping conduct electric currents.

I maintain that the corrosion of a metal surface is normally occurring through a water containing media, and hence breathe-ability in order to dry-out, is an important characteristic to cases made of leather. I hope this is useful and that the forum helps you."

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