141. The principal consumption of logwood is for blacks, to which it gives a lustre and velvety cast, and for greys of certain shades. It is also employed very extensively for different compound colours, which it would be difficult to obtain of equal beauty and variety, from colouring matters of a more permanent nature. It is frequently mixed with Brazil wood, to render colours deeper, the proportion of the two woods being varied according to the shade desired. In short, no colouring matter with which \ v e are acquainted, is capable of af fording so great a variety of dyes, though this advantage is much diminished by their want of permanency.
142. The bark of the quercus nigra of Linnxus af for a ) clime (lye, n pith is at present very generally emplue ed. This sustanee It as first prepared as a dye be Dr Banclott , 1 c I .ten g git it it the name of quer tr.n, which it is iiu%t 111.1%i_ sally yet. ognized. •• The bark el the ion reit, higi a appears," according to his ac e( unt, to cot list of ilot e.its ore oats. 1st, The e•pi tle or t Ntk rnal co t, 11 lough which the several ex retiui s of the tree arc transmuted, which, in part at le st...imere to i s outer sin lace, and become almost by comic usation. 2d. The middle or cellular coat, in ie Well the colouring matter principally resides. 3d, The interior or (toucan part, consistu.,., chiefly of lami na. Mimeo be the re in n nor to rent vessels, te hich be come more Laid and lit sous as they are placed tic:nest to tile w, oil) part el the tree, and have therefore less room to e o tarn the colouring matter." 143 Velure a xtractihg the colour from the bark, the epidermis, or exterhal covering, ought to be removed, by s i %mg T.,c reinainihg parts being then properly gr.( und by milt-stones. separate partly into a light fine pot e.et, and partly into stringy filaments or fibres, hid) last )ield but about half as much as the powder, and therefore care should be always taken to employ both together, and as nearly as possible in their natural proportions, othertt ise the quantity of colour produced may either greatly exceed or fall short of what is expected. The quercitron bark thus prepared and proportioned, says Dr Bancroft, will generally yield as much colour as eight or ten times its weight of the weld plant, and about four times as much as its weight of the chipped old fustic. The colouring matter, continues he, most nearly resembles that of the weld plant, with this advantage, however, that it is capable alone of produc ing more cheaply all, or very nearly all, the effects of every other yellow dyeing drug; and, moreover, some effects which are not attainable by any other means yet known.
144. The colouring matter of qucrcitron readily dis solves in water, even at a blood heat. If the infusion be strained and left at rest, a quantity of resinous matter subsides in the form of a whitish powder, which pro duces the same effects in tycing as the part remaining m solution. The clear effusion being evaporated and drier!, affords an extract equal in weight to about one twelfth of the bark from which it was obtained. Much arc, howeeer, must be employed in procuring this ex tract, so as to make it produce colours equal in beauty to those obtained directly from the bark itself. If tree evaporation be carried on rapidly, and the heat be too great. the colour is tarnished, probably, as Dr Bancroft conjectures, from the absorption ill oxygen, the colour thus undergoing a sort of semi•combustion. On the other hand, if the evaporation be conducted too slowly, the colouring matter suffers another change, and soon spoils by keeping.
145 The decoction of cm•rcitron is of a yellowish brown colour, which is darkened by alkalies, and bright d by acids. A of alum being added to it, separarcsa small portion of the colouring matter, which str.si•.es in the form of a deep yellow precipitate. The solutions of tin pre durr a mere copious precipitate, and of a beautiful lively yellow colour. Sulphate of iron causes a copious ohtc precipitate, sulphate of. copper, a yellow of an olive cast.
146 The weld plant, (rescda luteola.) seems to have been employed from the remotest times as a yellow dye. Two sorts of it are distinguished, the cultivated and the wild ; the former is preferred, as it yields mote colotor ing matter. The wild species differs from the cunt% at ed, in producing taller and situ:16er stalks. The w hole of the plant is used in dyeing.
147. A strong decoction of weld is of a yellow colour, inclining to bro‘sn; if it be greatly diluted witn water, its yellow, which is more or less pale, has a gre enish tinge. The addition of an alkali deepens the colour of the decoction, and alter a certain time a little ash•co loured precipitate is thrown down, which is insoluble in alkalies. The acids in general render its colour paler, and produce a little precipitate which dissole es in alka lies, and gives them a yellowish brown colour. A aim occasions a yellowish precipitate, and the liquor retains a fine lemon colour. The solutions of tin produce a co pious bright yellow precipitate. Sulphate of iron pro duces a copious dark grey precipitate, and the superna tant liquor is brownish. Sulphate of copper causes a brownish green precipitate, and the liquors preserve a pale green colour.