224. The oxides of lead have been used as mordants, but they tarnish the lustre of colours. Vogler obtained a beautiful black, by galling thread and cotton impreg nated with the salt of lead, putting them afterwards in a solution of sulphate of copper, and boiling them in a log wood bath.
225. Some metallic oxides have so strong an affinity for the substances to which they are applied, that they remain permanently fixed on them, and produce colours which are almost indestructible. In treating of the oxide of iron as a mineral substantive colouring matter, we noticed the readiness with which it enters into combina tion with stuffs, and the easy method of communicating, by means of it, a buff colour to cotton. We have now to remark, that the same substance may also be used as a mordant, to produce, with different colouring princi ples, violet, prune, and lilac colours. This oxide like wise constitutes the basis of black colours ; so that it may justly be regarded as one of the most useful sub stances employed in dyeing, whether in producing co lours by itself, or in acting as a mordant to madder red, with tannin, or the astringent principle.
226. The mordant of iron is applied under different forms. Some manufacturers employ the sulphate with out any addition; others compose it by dissolving iron in vinegar; some add to it a decoction of rye flour, while others mix it with urine, herring brine, &c. The lon ger the composition is kept, the better it becomes. At present, instead of the sulphuric, acetic, or other acids, is substituted, as we formerly observed, the pyroligne ous acid, which differs from the acetic by holding in combination a portion of empyreumatic oil.
227. Mordants are not procured from the earthy and metallic bodies alone; for, under certain circumstances, vegetable and animal substances serve as mordants for each other. Thus, in the complicated process for dye ing Adrianople red, the stuff is first impregnated with an animal substance; the astringent principle is then applied, after this preparation, the cotton is present ed to the aluminous mordant. In this case, therefore, the mordant is a triple compound of oil, the astringent principle, and alumine. The astringent principle merits
particular consideration, not merely on account of its extensive use as a mordant, but also as a colouring prin ciple. The class of substances included under the head of astringent principles, is rather vaguely defined. Fre quently some slight resemblance in taste only has been attended to, and under the name of astringents, alum, and many vegetables possessing very dissimilar proper ties, have been confounded together, both in medicine and the arts ; and more frequently still, every substance which renders a solution of iron black, has been consi dered as astringent. It does not appear, however, that either the property of corrugating the animal fibre, or producing a dark precipitate with the salts of iron, is sufficiently distinctive of the substances to which the term astringent is usually applied. Many vegetable products, which are decidedly astringent to the taste, afford no black precipitate with the salts of iron; and on the other hand, several substances which yield a copious black precipitate with the salts of that metal, exhibit no traces of astringency. The substance from which the astringent principle is usually extracted, is an excres cence formed on the branches of the oak, and known by the name of gall-nut : (See CI1ENISTRY) As this is almost the only substance employed to afford the astrin gent principle for the purposes of dyeing, we shall con fine our observations respecting astringents chiefly to its properties.
228. There are different kinds of the gall-nut; some inclining to white, yellow, green, brown, or red ; others are ash-coloured, or blackish. They also differ greatly in magnitude, and are either round or irregular, heavy or light, smooth or covered with protuberances. Those which are small, knotted, and heavy, are reckoned the best ; they arc known by the name of Aleppo galls, and come from Aleppo, Tripoli, and Smyrna. Galls are almost entirely soluble in water by long ebullition. Six teen drachms afforded Neumann fourteen of extract ; from the remaining two drachms alcohol extracted only four grains.