Tannin generally refers to tannic acid.

Tannin is a tannin obtained from gallnuts. It is a yellow or light brown light amorphous powder or scale; odorless, with a slight special smell and an extremely astringent taste. Soluble in water and ethanol, easily soluble in glycerin, almost insoluble in ether, chloroform or benzene.

The aqueous solution turns blue-black when it meets the iron salt solution. Adding sodium sulfite can delay the discoloration. Industrially, tannic acid is widely used in tanning leather and making blue ink. Tannic acid coagulates proteins. People chemically treat raw pigskin and raw cowhide with tannic acid, which can coagulate the soluble protein in the rawhide.

Extended information:

Properties of tannic acid

Pure tannic acid is a light yellow powder that is easily soluble in water. Persimmons sold in the market are usually soaked in lime water in advance or have a layer of lime applied to the skin. Because lime can solidify tannic acid and become insoluble in water, in this way, tannic acid will no longer trouble the tongue, and persimmons will no longer be astringent. Heating can also solidify tannic acid.

Therefore, some people like to soak persimmons in hot water to remove astringency and eliminate bacteria. Sometimes, even if you don't use an iron knife to cut pears and persimmons, there will be some black spots on the skin. This is another chemical change. Because the molecules of tannic acid contain many phenolic hydroxyl groups, they are very sensitive to light and are easily oxidized into black oxides.

Another alias for tannic acid is tannic acid. Tannic acid has traces all over nature, and many tree barks contain tannic acid. In my country's Sichuan and Yunnan areas, there are some tumors growing on a kind of gall tree (also called salt skin tree) - gallnut. The gallnut contains a lot of tannic acid.

Baidu Encyclopedia - Tannic Leather

Baidu Encyclopedia - Tannic Acid