366

DYEING AND CALICO PRINTING.

from which the very finest lakes can be prepared. For this purpose the clearliquid is drawn off with a syphon as far as possible, and the flakes areseparated from the rest of the liquid by filtration. The clear liquid is usedfor the preparation of ordinary lakes. The precipitation of the colour is thenext step : this can be effected by the addition of carbonate of potash or ofsoda, or caustic ammonia, with or without an admixture of tin-crystals.Chalk may also be used instead of the alkali, in which case gypsum is throwndown along with the alumina and colouring matter. If the whole of thealumina is thrown down the colour of the produ# is, of course, lighter. Alllakes in which chalk is not employed have a smooth frafture. Each precipitantmodifies somewhat the tone of the red produced. If a little salt of tin isadded during the precipitation, or at its end, the fire of the lake is heightened,as the tin appears to prevent the precipitation of a trace of a yellow colouringmatter. To retain this impurity in solution it is also well not to precipitatethe whole of the alum. When the bulk of the alum has been thrown down,the lake is separated by filtration from the yellowish supernatant liquid.

To prepare the deepest madder-lakes, known as “ crystallised ” and“ Munich ” lakes, the above-mentioned brown flakes are dissolved in thesmallest possible amount of caustic ammonia. The solution is filtered throughlinen covered with good filter-paper, and with this solution lakes of uncommondepth and brilliance can be prepared. The precipitation may be effected withalum, with salt of tin, or with a mixture of both, either warm or cold.

Splendid lakes can be prepared from artificial alizarin, and, doubtless, fromPerkin’s artificial purpurin. (See also p. 268.)

2. Lakes of the Red Woods.—The chief points to be attended to in thepreparation of these lakes are—That the deco#ion of the wood employed benot too recent; it must have been allowed to stand for some time, and todeposit certain impurities, before it can be used with advantage. The additionof a certain quantity of dilute solution of glue, or of carefully skimmed milk,is here advantageous. Alum, or tin-crystals, or a mixture of both,—of coursein known quantities,—is then added, and the precipitation determined bymeans of alkaline carbonates, &c., as dire&ed for madder. The commonlakes are let down with starch, sulphate of lime, China clay, sulphate ofbaryta, &c. These articles are added during the precipitation, and greatlyincrease the yield of the lake at the sacrifice of its brilliance.

Lakes of a great variety of shades may further be obtained by the substitu-tion of bismuth or antimony for tin and alumina.

3. Cochineal Lakes.—See pp. 359 and 360.

B. Blue Lakes.

A blue lake may be obtained by mixing a deco&ion of logwood, duly agedand clarified, with a little bichromate of potash, afterwards with a solution ofsulphate of copper, and lastly with ammonia.

C. Green Lake.

Extra# 1 lb. of raw coffee-berries with a gallon of pure water, and add asolution of 2^ to 3 lbs. of sulphate of copper.