OK, I am wondering this.
If you mix up a pure formic acid/water/salt pickle, and you put oily skins in to it, you can proceed with shaving and tanning with no risk of grease burn.
But add potassium alum to that pickle to set hair and the skins burn, rip, and fall apart. The same applies to oxalic acid but less so.
What is it about alum, pickling acids, and natural animal fat that makes a skin fall apart? Is there a complex chemical reaction between the sodium sulfate, sulfuric acid, whatever other pickling acid is now present, and saturated animal fats which causes this?
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flesh the fat and membrane off.salt it.that takes care of the fluids .then pickle.no alum is needed.flesh tan and oil.keep it simple
to tell him to degrease the skin. Greasy skins should be degreased.
When you add the sulfates from alum you create oxidizing conditions. Adding nitrites/nitrates will do the same. One would never use Nitric Acid to pickle as it is a very potent oxidizing acid and will literally digest the hide. Sulfuric is an oxidizing acid as well but not quite as potent as Nitric. Most all acids are oxidizing to some extent. Formic, Oxalic and most pickling acids are not as strong oxidizers..