If you want to try your hand at tanning some animal hides, a good tanning solution can be made as follows.
In a wood or glass or plastic container, stir 1 gallon warm water with 1 pound rock or pickling salt until salt is dissolved.
Slowly add 4 ounces (½ cup) 40% sulfuric acid, available at service stations as battery acid.
Always pour the acid into the water, never pour water into acid.
Use only non-metal utensils as metal will change the chemical nature of the solution. It doesn’t do the metal any good, either. Increase the recipe as many times as necessary to make enough to thoroughly cover whatever skins you are tanning.
Wear rubber gloves, unless you don’t mind slightly toughening your own skin. Every day, stir and rearrange the hides in the solution, to assure even pickling. This is actually a pickling solution but makes a cured hide very well.
Check skins by making a small slit next to the edge of the skin and check for color. As the hide pickles, it turns white. In a thick hide, the white works slowly towards the center, leaving a pink line, until pickling is complete. Skins can not be left too long in this solution, so don’t worry about leaving them in it until you can dry and soften them. It is best to over pickle than to under pickle as then they spoil. Use baking soda to kill the acid if spilled or splashed. After pickling is complete, thoroughly rinse hides in cool water, then add baking soda to the rinse water and rinse some more. Hang hide to partially dry, then shake and stretch out on a flat surface. Warm a good hide oil to a comfortable temperature and spread thoroughly over entire flesh side of the hides. Fold outer edges to the middle and roll the hide up from the neck edge toward the tail. Next day, start stretching and pulling the hides to soften as it dries. This makes good hats, mitts and boots as long as it is for a dry climate. Moisture will make the hide return to it’s former un-pickled state to a certain degree and you will have to rework the hide.
Battery Acid Refill jugs are at 40% so you can use that as stated in the instructions, but in a true grid down scenario with alot of vehicles immobilized, the acid out of a car battery has water added, so more will have to be used or a longer soak. Also you run the risk of discoloration due to lead in the battery, but if you arent tanning for looks this may work decently to adequately soften the hide, if not it may be more rigid and need to be used for non-clothing items.