Tannage with a polyphenolic substance followed by the application of dissolved metal salt, specifically vegetable tannins followed by aluminum salts, produces unexpectedly high thermal stability and acid resistance in the final leather. Several explanations have been proposed for these observations, but none realistically account for the synergistic effect, or the specific need to apply the tannage carefully using the polyphenolic first, followed by the application of a metal salt. By demonstrating through laboratory experiments the need to modify the more traditional ideas about straight mineral tannage with respect to the effect of the metal salt reaction, a new, more logical concept of polymer formation is developed and supported through laboratory experimental evidence.