A name given to several pigments formed by precipitating colouring matter with some earth or oxide. The principal lakes are carmine, Florence lake, and madder lake; the first of these has been already described under its initial letter. See Carmine.
Florentine lake is prepared from the sediment of the cochineal, which is deposited in the preparation of carmine, and the red liquor also remaining from the same; these are boiled with the requisite quantity of.water, and afterwards precipitated with the solution of tin: this precipitate must be frequently edulcorated with water. Exclusive of this, two ounces of fresh cochineal, and one of crystals of tartar, are to be boiled with a sufficient quantity of water, poured off clear, and precipitated with the solution of tin, and the precipitate washed. At the same time two pounds of alum are also to be dissolved in water, precipitated with a lixivium of potash, and the white earth repeatedly washed with boiling water. Finally, both precipitates are to be mixed together in their liquid state, put upon a filter and dried. A cheaper kind of crimson lake is prepared,- Brazil-wood may be employed instead of cochineal, and treated in the foregoing manner.
Several modes of preparing fine red lakes from the madder of different countries were communicated to the Society of Arts by Sir H. C Englefield, to whom the Society awarded a gold medal for the same. The following is his process of preparing it from the Dutch crop-madder: - Two ounces troy of the finest quality is to be inclosed in a bag of fine and strong calico, large enough to hold three or four times as much; put it into a large marble or porcelain mortar, and pour on it a pint of clear soft water, cold; press the bag in every direction, and pound and rub it about with a pestle, as much as can be done without tearing it, and when the water is loaded with colour, pour it off. Repeat this process by adding fresh water till all the colour is extracted. Heat all the liquor in an earthen or tinned copper vessel, or what is better, a silver vessel, until it just boils; then pour it into a large earthen vessel, and add to it one ounce troy of alum, dissolved in a pint of boiling soft water, which must be thoroughly mixed. Pour in about an ounce and a half of a saturated solution of sub-carbonate of potash; a precipitation will ensue; let it stand till cold, when the supernatant clear, yellow liquor maybe poured off from the red precipitate.
A quart of boiling water should again be poured on it and well stirred. When cool, the colour may be separated from it by filtration through paper in the usual way; and boiling water should be poured on it in the filter till it passes through of a light straw colour, and free from an alkaline taste. The colour may now be gently dried, and when quite dry it will be found to weigh half an ounce; just a fourth part of the weight of the madder employed. If less alum be employed, the colour will be somewhat deeper: with less than three-fourths of an ounce, the whole of the colouring matter will not unite with the alumina. One ounce of alum to two ounces of madder is the best proportion. Spanish madder affords a colour of rather a deeper tint than the Dutch madder, but does not appear of so pure a red as the Zealand crop-madder. The lake produced from the foregoing process from Smyrna madder is remarkable for the richness and depth of its tint: the colour may be obtained from the fresh roots of madder, and will prove of equal if not superior quality to the vdry. Upon the whole, the author of these processes considers the essential advantage of them to consist in the trituration or pressing of the root in cold water.
Almost all vegetable colouring matters may be precipitated into lakes, more or less beautiful, by means of alum or oxide of tin.