How leather works - how to make leather with brains, bark, pigeon droppings or chromium salts

by | Nov 16, 2009 12:00 AM ET

Leather has been around for thousands of years, and is still used today in millions of shoes, car seats, sofas, saddles, jackets, purses, belts and other clothing items. For any Harley rider or Cowboy, leather is essential.

Leather is completely different from rawhide. Rawhide is what you get when you scrape an animal skin and simply let it dry. If you have a dog, you know that rawhide is stiff and pretty useless as a form of fabric. Leather is rawhide that has been tanned. The tanning process changes the protein in the skin and makes it soft and flexible.

So how do they make leather? Here is how its done the "old fashioned way":

Step 1: Get the hair off by soaking in hydrated lime and then scraping.

Step 2: Scrape off the membrane on the inside of the skin.

Step 3: Soak in vinegar to neutralize the lime.

Step 4: Coat the skin in a mixture of brain and oats, as shown here (yes, the video is as unpleasant as it sounds when it comes to acquiring brain):

Step 5: Let it sit overnight, and then stretch and dry the leather. You have buckskin.

Bark tanning is another possibility (for example using extracts from oak bark), but you have to soak for months rather than days. This is where the word "tanning" comes from, by the way. Bark contains tannins. According to this article:

Tannin is a large, astringent (meaning it tightens pores and draws liquids out), molecule found in plants that bonds readily with proteins. When you apply tannins to your skin you can instantly see the skin contract. Put them in your mouth and your cheeks pucker. Medicinally, tannins are used to draw irritants out of your skin such as the venom from bee stings or poison oak. Next time you get stung, pull some fresh bark off the twig of a nearby tree, chew it up and apply it to the sting. The irritation will go away within seconds. Tannins are also applied to burns to help the healing and to cuts to reduce bleeding.

That article does a nice job of explaining the complete bark tanning process, including how to extract the tannins from the bark and how to do the months-long soaking process.

At the industrial level, chrome salts rather than brains or bark are used for tanning, as seen here:

Step 1: Start with the hide of an animal, typically a steer.

Step 2: Remove the hair by mixing the hides with water, sodium sulfhydrate and lime.

Step 3: Bathe the skins in acid, then in chrome tanning salts (e.g. chromium sulfate).

Step 4: Split the leather to get an even thickness.

Step 5: Use tree bark extract and a dye for a second tanning step.

Step 6: Glue the leather to glass for drying, and dry for four hours.

Step 7: Dye again to final color

Step 8: Polish the leather with a glass cylinder

Working in a tannery in Morocco is not the most glamorous job, as described here:

What you learn from this video is that pigeon droppings are another way (besides brains, bark or chrome salts) of tanning leather.

Today, much of the leather you see is synthetic leather, or pleather. Here's how it's made:

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