DYEING. The object of this beauti ful art is to fix certain coloring matters uniformly and permanently in the fibres of wool, silk, linen, cotton, and other substances. The moderns have obtained from this Continent several dye-drugs unknown to the ancients ; such as cochi neal, quercitron, Brazil wood, logwood, annatto ; and they have discovered the art of using indigo as a dye, which the Romans knew only as a pigment. But the vast superiority of our dyes over those of former times must be ascribed principally to the employment of pure alum and solution of tin as mordants, either alone or mixed with other bases ; substances which give to our common dye-stuffs remarkable depth, durability, and lustre. Another improvement in dyeing of more recent date is the appli cation to textile substances of metallic compounds, such as Prussian blue, chrome yellow, manganese brown, &c.
There are a few dyeing material's which impart their color to different stuffs with out any previous preparation, and these have been technically termed substantive colors; by far the greater number, how ever, of coloring materials, only impart a fugitive tint under such circumstances, and require that the stuff to be dyed should undergo some previous prepara tion, in order to render the color perma nent; that is, capable of resisting the ac tion of air, light, and water. The sub stance applied with this intention is called a base or mordant, and must possess an affinity for the fibre of the stuff on the one hand, and for the coloring materials on the other. The mordant often effects another important object; that of chang ing or exalting the color at the same time that it fixes it. The principal mordants are aluminous earth and oxide of iron, and these are usually applied in the state of acetates. Oxide of tin is a valuable mordant ; it is generally applied as ni trate or chloride. _ As an instance, we may mention the mode of dyeing calico red by means of madder, a decoction of which, if applied to the unprepared goods, would only give them a dirty red tinge, neither agreeable nor permanent.
If the calico be previously passed through a weak solution of acetate of alumina, and then dried at a high temperature, and afterwards washed, a portion of the alumina is retained in chemical combina tion with the fibre of the calico ; and when thus prepared and submitted to the action of a hot decoction of madder, and again washed, it comes out of a fine red, which is fixed in consequence of the at traction of the alumina for the peculiar principle which gives color to the mad der. If the mordant be oxide of iron in stead of alumina, the color which is then produced is purple ; and various shades and colors are obtained by mixing mor dants, by using more or less of them, and by applying the colored solutions in various states of concentration. Some times articles are dyed by a similar pre cipitation of colored metallic oxides in the fibre ; thus yellow is obtained by passing cloth impregnated with acetate of lead through a solution of chromate of potash • a double decomposition ensues, and yellow chromate of lead is precipi tated in and combined with the vegetable or animal fibre. Blues are prddueed by passing the goods previously mordanted with iron through an acidulated solution of ferrocyanate of potash ; these are gene rally called chemical colors, though not in fact more so than the others. Scarlet is exclusively produced by the coloring matter either of the cochineal or of the lac insect, which is 'fixed by oxide of tin, or by alumina, and heightened by the action of tartar.
Indigo. This dye-drug, when tolera bly good, contains half its weight of in digotine. The cold vat is prepared com monly with water, copperas, indigo, lime, or sometimes carbonate of soda, and is used almost exclusively for cotton and linen ; immersion in acidulated water is occasionally had recourse to for removing a little oxyde of iron which attaches it self to the cloth dyed in this vat.