The nature of lignin is not fully understood, but it is considered to be a complex cross-linked, highly aromatic structure of high molecular weight, i.e., about 10,000. It cannot be hydrolyzed by acids, but is readily oxidizable. It is soluble in hot alkali and bisulfate, and condenses readily with phenol and thio compounds. Lignin is not a compound but a system, and its composition varies both with the method of isolation used and with the species, age, growing conditions, etc., of the tree or lignified material. It is more or less completely removed during chemical pulping operations in paper manufacture but none is removed by mechanical pulping. It is further removed or modified by bleaching sequences to give pulps of greater brightness.
So-called sulfite cellulose (which is more appropriately called "sulfite lignin extract"), when adjusted to a pH of between 3.0 and 5.0, has a tanning action resembling that of the vegetable tannins. When used alone, however, it produces a brown leather that tends to be thin and hard, with low tan fixation. It is used chiefly at the end of some vegetable tannages to improve the fullness and firmness of the leather, especially in those cases where the leather is to be sold by weight. (17 , 72 , 198 , 268 , 306 )