Not sure how effective Dempsey's gourmet pine tar chewing was to strengthen his jaw, but I believe he was knocked out only one time, back in 1917, by Fireman Jim Flynn, and several boxing writers have called this KO questionable. According to some of the Dempsey bios he trained in a cage 4 feet high to develop his crouching technique. :bbb
The ko isn't really questionable, just surprising with hindsight. He got sparked, that's pretty much all there is to it.
Yes its true, dempsey would soak his face and hands in meat brine habitually and chew pine tar, tree sap to make his jaw stronger...they dont make em like that anymore haha. I'm pretty confident it would work. Have you ever seen dempsey cut badly in a fight? The only one I can think of is the Bill Brennon fight where Brennon split open dempsey's left ear with a right hook. Dempsey's face looks like leather...even in his old age his fists were awfully big too