Tanning hides is a craft that has been a part of human culture as long as there has been human culture. The very earliest hunter-gatherer tribes wore skins, and cured them without harsh chemicals. These techniques are still commonly used by folks who do leather work or hunt. Though chemical kits are available for home use, chemicals can be dangerous. Even a hide as large as a deer can be tanned naturally and by hand.
Commercial tanning is done with chemicals, and makes leather inferior to naturally tanned skins (see Reference 1). "Brain" tanning and "bark" tanning use emulsified oils and tree bark, respectively, in place of toxic chemicals.
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What You'll Need
Depending on the size of your pelt, you'll need a large frame or a smaller board. A scraper will be necessary, for removing fat and meat from the inside the pelt. For a large skin, you'll also need salt. For bark tanning, you'll need about twice as much bark as the weight of your pelt.
By tacking a small pelt to a board, or stretching a large skin on a frame, air-dry the skin, flesh side out. Scrape all the flesh and fat from the inside of the pelt. Take care to prevent spoilage of the skin. Until it's tanned, skin will rot the same way meat will, so once it's dry, store it in a cool, dry place. Don't expose the skin to sun or heat. Large skins will need to be salted. Apply granulated salt to the flesh side of the fresh skin, 1 lb. of salt to 1 lb. of skin.
Next, soak your pelt in water. A large hide might take several days to soften. In this case, you'll need to change the water each day to prevent spoilage of the pelt. A top-loading washing machine can be helpful, but use cold water and stop the machine before it enters the spin cycle.
Barks used for tanning include oaks, fir, willow, chestnut, birch, alder, and hemlock. Grind the bark as finely as possible, then boil it in water to leach the tannin. Soak the pelt in a container large enough to allow the liquid to reach all parts of it. For the first tanning bath, dilute the liquid 1 to 1. Too strong a mixture for the first bath will cause the outside of the skin to tan first and prevent curing of the inside. The first bath should take about a week. Agitate frequently.
The second bath should be stronger than the first, and strengthened every few days until the skin has been tanning for five or six weeks. Vinegar can be added to acidify and strengthen the mixture further.
You'll know the skin is thoroughly tanned when in a cross-section the brown color goes entirely through the leather.