CHEMICALS AND HIDE TANNING

Someone asked, “I saw you use lime, does that mean these are chemically-tanned hides, hard to get a needle through?” 

Read on. 

And thank you to this person for asking a super important clarifying Q.

First things first: every tanned hide is a chemically-tanned hide. Because the definition of “tanning” is to harness a chemical reaction within skin, in order to subvert the putrefaction (decay) process and transform the skin into a stable textile - leather, buckskin, or tanned fur.

So, what do we mean when we say, “chem-tanning?” This is the natural hide tanner's cheeky + yes, inaccurate way of saying “chromium-tanned.”  Why don't we just say, “chrome-tanned?” I don't know. I try. But I often find myself saying “chem-tanned” too. It's in the culture.

Then, what is chrome-tanned? Chromium(III) salts are prepared from chromium(IV) compounds, which are derived from chromite ore, which is a crystalline mineral.  Because it uses a mineral, chrome-tanning is a type of mineral tanning.  Like other mineral-tanning, it works by the ingredient absorbing into the hide and settling in the fibre network. 

This is, incidentally, the reason that chrome-tanned hides aren't easy to stitch (same goes for alum-tawed hides). There is a tangible substance sitting in the hide, filling up all the space. That's very different from smoke-tanning, which essentially empties a hide and makes a fluffy, easy-to-stitch material. In old hide tanning terminology, the quality of chrome-tanned leather is “more full in the handle” while smoke-tanned buckskin is “less full in the handle.”

But there's more. Chromium is stable at low pH (acidic). But raise the pH and you increase the reactivity of Chromium(III) and allow it to react with collagen (the structural hide protein).  So in chrome-tanning, a true chemical reaction also takes place.

To get there, you need to fire the chromite ore with an alkali in a kiln at 1200°C. Then after you've raised the pH, you lower it again to convert the alkali you've just made into chromic acid. You then reduce what you've just made, and you're left with basic chromium(III).

Chrome-tanning is thus an industrial process. And it begets another industrial process: heavy-duty dyeing, to change the bright blue chrome-tanned leathers into the earth tones we want to see.

Chrome-tanning is also a toxic process.  I'll let you read about it here. It is not hard to find information on the health impacts of factory workers or the environmental degradation of under-regulated chrome tanneries.