Leather dissolves soluble materials

Another characteristic of the condensed tannins was usually observed during leather tannage with these materials. Aqueous suspensions of tannin that were acidic from the tanning process gradually precipitated insoluble materials known as tanner s reds or phlobaphenes. These substances, derived from the tannin, were no longer soluble in water, but they could be dissolved in polar solvents such as ethanol or acetone or in aqueous base. Since most species of bark contain an extractive fraction that physically resembles the tanner s reds, they are referred to as phlobaphenes in the literature. Very little characterization work has been done on this fraction, and there is substantial reason to believe that the bark phlobaphene fraction contains a variety of water-insoluble polymers, some of them totally unrelated to the condensed tannin family. 

Gallo-tannic acid—Acidum tannicum (H. S., Br.)—is the best known of the tannins, and is obtained from nut-galls, galla (tf. S., Br.), which are excrescences produced upon oak trees by the puncture of minute insects. It appears as a yellowish, amorphous, odorless, friable mass has an astringent taste very soluble in water, less so in alcohol, almost insoluble in ether its solutions are acid in reaction, and on contact with animal tissues give up the dissolved tannin, which becomes fixed by the tissue to form a tough, insoluble, and non-putrescible material (leather). 

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