Specifically how to train the skin that you talked about, I want to know the method. Like your old school boxing method for skin.
There are multiple methods. Most of them revolve around toughening the knuckles, some of which I tried. This includes using Camphor Ice on skinned knuckles (Dempsey), rosin, or even an herbal lineament recommended by Bob Fitzsimmons which included Laudanum.
The methods you are most likely interested in "wash" the skin to be toughened periodically in various period products. Which one is recommended depends on the author. One author recommends diluted tanning solution. Another recommends "pickling brine." One American Revolutionary War era recipe makes a paste from salt and Apple Cider Vinegar. There's even a lost recipe referred to as "Tom Sayer's brew of turps, whiskey vinegar, horse radish and saltpetre." There is even some very old material that recommends using urine. In all of these cases that we can be certain of the ingredients, it always comes down to a very mild acid such as dilute vinegar. Tanning solution has Tanic Acid. Real "pickling" brine develops Lactic Acid through fermentation such as traditional sauerkraut/pickes/kimchee/etc. (modern shelf-stable pickled products typically use Acetic Acid). And, of course urine contains Uric Acid.
The effect of using these mild acids on the skin is to cross-link the collagen, which makes the skin leathery and tough, flexible but inelastic (bends but won't stretch), at least according to Dr. Keith Myers.
From what I can tell, the Lactic Acid and the Tanic Acid preparations were probably the most common. The Lactic Acid because fermented pickling was one of the most common ways to extend the shelf life of food until very recently and was easily available to, literally, everyone. And the, the Tanic Acid seems to have been common; one writer (ims) tells us that you can tell who was a professional boxer because they toughed the skin of their face with Tannin in order to reduce facial cuts and, in the process, stained their face a chestnut brown. And using mild acid (mild enough that you can drink it safely) seems to have been a short-term or "only occasionally" treatment. Just enough to cross-link the collagen, not enough to cause scaring from acid burns.
There was a thread here on MT some months ago where most everyone, including the OP, misunderstood what "brine from a salt-pork pickling barrel" was. Everyone thought it was super-saturated salt solution; i.e.: what we now think of as "brine." The OP took a lot of criticism and ridicule based on everyone's mistaken belief of what "brine" was.
The question is not "does this work." It absolutely does make the skin tougher, more leathery, and resistant to cuts and scrapes. Nor is the question, "will this take a lot of time, dedication, expensive ingredients, and require the supervision of a trained master?" The question is, "is it worth it?" If you are toughening the skin of your face, is it worth it to have tannin stained skin or to smell like sauerkraut brine? It was more than enough of an edge for professional athletes in the 19th Century. For modern amateurs and self-defense minded folks with a real 9-5 job? Well, maybe saurekraut isn't the best cologne.
Would these washes work on you to toughen the skin on your chest, arms, face, or hiney? Yes. Is it worth it to you? I don't know. Your call.
Peace favor your sword,
Kirk
