No precipitate Yellow or brown precipitate
Leather Produce a "bloom" No "bloom"
Conc. sulfuric acid Yellow or brown Red or pink
Lime water Gray or blue ppte. Pink to brown ppte.

Pyrogallol tannins contain approximately 52 per cent of carbon; while the
catechol tannins usually contain 59 per cent to 60 per cent, the difference
being due to the absence of glucose from the molecule in the latter types.

The two types are distributed in plants as follows: pyrogallol tannins in
oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol
tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia,
and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The
so-called "pseudo-tannins" (i.e., compounds which do not tan leather but
possess other properties like tannins) are found in hops, tea, wine,
fruits, etc.

SOME COMMON TANNINS

Ordinary commercial "_tannin_," or "_tannic acid_," is a compound of one
molecule of glucose with five of digallic acid. It is found in many plants,
and is prepared commercially from the Turkish oak-galls and the Chinese
sumac-galls. It exhibits all the characteristic properties which have been
listed above for tannins in general and responds to all the characteristic
reactions of a pyrogallol tannin. It is extensively used for the
manufacture of blue-black ink, and in many technical processes.

=Catechu tannin= and =catechin= are compounds of the catechol tannin type.
The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc.
It is not a true tannin, since it does not convert hide into leather; but
when heated to 120 deg. or above, it is easily dehydrated, forming catechu
tannin which is identical with that which is obtained directly from gambier
and Bombay cutch (products made by evaporating water extracts from the bark
of various tropical trees). This latter is a true tannin, which is much
used in dyeing and other technical processes.

"=Quercitannic acid=," obtained from oak bark, etc., is likewise a catechol
tannin. It yields no glucose on hydrolysis.

A great many other tannins are known, and their possibilities for technical
use in tanning, dyeing, etc., have generally been investigated; but so
little has been learned about their composition and relation to the plant's
own needs, that it seems unnecessary to discuss them in detail here.