249

CHAPTER IX.

LAKES.

LAKES form a class of pigments of considerable use in painting. They were among the pigments used by the early Italian painters, from whom their use has descended to the present time. Pliny gives some account of them, and from this description the origin of the name "lake" can be gathered. The early Italian dyers for certain colours used what was known as "lac," which was either the product now known under this name, or an analogous body. This lac requires the aid of tin and alumina compounds before the colour can be developed and fixed on the fabric which has to be dyed; during the process of dyeing some of the colouring matter of the lac combines with some of the tin and alumina to form an insoluble body, which forms a kind of coloured scum on the top of the dye-vat; this substance, known to the Italian dyers as "lacca," was collected, dried, and sold to artists. In the same way other laccæ were obtained when other natural dyestuffs were used; gradually methods of preparing these laccæ were discovered, by which they could be obtained direct from the dyestuffs themselves, without the necessity of troubling the dyer, and thus has arisen the preparation of the lakes, which name can be readily traced to the laccæ of the Italian dyers.

Lakes may be defined to be compounds of an organic colouring principle with a metallic body. The organic colouring principle may be obtained (as it was in the early times, and until very recent years) from natural colouring matters, such as lac, cochineal, Persian berries, fustic, Brazil wood, sapan wood, &c.; or it may be derived from the coal-tar colours, a source which has only lately come into prominence for lake-making, but which promises in the future to supplant the natural colouring matters for this purpose, as they very nearly have done for dyeing textile fabrics.

The colouring principle of most natural colouring matters is of an acid or phenolic character, and will combine with bases, such

REESE LAS

as tin, alumina, iron, lead, antimony, &c., to form coloured bodies. which are insoluble in water; as a rule, the affinity between the two bodies is so great that the lake is precipitated when a solution of a metallic salt is added to one of the colouring matter. Theoretically, a lake should be a compound of the colouring principle and the metallic base combined in equivalent proportions; but, practically, such a lake does not exist; usually, the base largely predominates. This excess is sometimes accidental, but often purposely made, the object being to modify the shade of the lake, as is the case with Dutch pink, rose pink, and one or two others. Then, again, in some lakes there may be small traces of the colouring principle carried down mechanically with the lake during the process of making.

Lakes are usually made by preparing a decoction of the colouring matter, and then adding to this a solution of the base; as a rule, the lake forms almost at once; at other times, the addition of a small quantity of a solution of carbonate of soda is sufficient to throw down the lake. By preparing alkaline solutions of the colouring matter the lake is thrown down at once on adding the solution of base; this method is not always applicable, as the alkali sometimes affects the shade of the resulting lake, as in making alizarine-lakes.

The colouring matters, or rather their colouring principles, may be divided into two groups. One contains coloured matter, and includes such substances as fustic, Persian berries, and cochineal, which may be called substantive colours, as the colour does not depend upon the mordant or base used; thus Persian berries will give a yellow lake with either alumina, tin or lead, although there are some minor differences in the tint or shade of the yellow so produced. The other group may be called adjective colours. It comprises substances like alizarine, fresh logwood, &c., in which the colour is only developed when the colouring principle is combined with a base, and differs with the base used; thus alizarine, when combined with alumina gives a red, while with iron it gives a deep dull violet; again, logwood with antimony gives a violet, with iron a blue-black, and with chrome a deep blue.

All lakes should be quite insoluble in any vehicle, such as water, oil, turpentine, or spirit, used to make them into a paint; on the other hand, a true lake is always more or less transparent when used as a pigment, and lakes are, therefore, mostly used as covering or glazing colours to modify the tint of an under cont of paint, and to obtain effects which are not obtainable with opaque pigments. Some lakes are rendered nearly opaque by

mixing the materials during the process of manufacture with some opaque white pigment, by which the body or covering power of the lake is increased, and at the same time the shade is more or less affected. If a lake dissolves in the vehicle, then all its properties as a pigment in regard to its body or covering powers are lost, and a coloured varnish only is obtained which will not do the work it is intended that the lake-paint should do.

In the following pages the lakes prepared from the natural colouring matters will first be described, then those prepared from the coal-tar colours.

RED LAKES.-These can be prepared from cochineal, madder, Brazil wood, barwood, and one or two other natural products; but those named are what are chiefly used commercially. Cochineal yields carmine, crimson, scarlet, Florentine, and a few other lakes. Brazil wood gives rose-pink and some of the cheap red lakes. Madder yields the madder lakes so much prized by artists, but which are too expensive for common house-painting. The other red colouring matters are, owing to special difficulties, not much used in lake-making. It is not intended to give a special description of colouring matters used in the preparation of lakes; if the reader requires such, he should refer to some work on dyeing, such as that by Knecht, Rawson & Löwenthal.

CARMINE. The best example of a lake is probably the pigment carmine, which is an almost pure lake, a combination of the colouring principle of cochineal (carminic acid) with alumina and tin. The exact method by which it is made from cochineal has never been published, and is probably only known to the few makers of this lake, although various descriptions of processes have appeared from time to time; some of these are very misleading, and have probably been intentionally made so. The pigment has been known for more than 200 years; its discoverer is unknown, although according to one statement it was a Franciscan monk. In 1656 a writer named Homberg published a recipe for making it.

The chemical nature of carmine is even now but imperfectly understood, although it and its source cochineal have frequently been examined. In the main, the various writers agree that carmine is a combination of the colouring principle of cochineal (carminic acid) with alumina; but there is always present small quantities of other bodies, which renders any investigation into its composition a matter of some difficulty.

Liebermann gave, in the Berichte der Deutsche Chemisch

Gesellschaft, vol. xviii., p. 1,969, the following analysis of a

cochineal-carmine :

:

More recently, in the Journal für Praktische Chemie, 1890, No. 3, Lafar published the following analysis of carmine:

The two analyses agree with one another as well as analyses of a commercial and variable product like carmine can be expected to agree. The lime and alumina in the ash are in the proportion of 2 Ca O: Al, O; this circumstance would point to the fact that carmine is not purely an alumina lake, but a lime-alumina lake, with some proteid matter. The tin and copper in the ash have probably been derived from the vessels in which the cochineal has been boiled, as it is often recommended in recipes for carmine-making, to make the decoction

of cochineal in two vessels; the other constituents are of no importance.

The following methods have been published for preparing carmine :

1. 1 lb. of cochineal is extracted by boiling in water for from 15 to 20 minutes, the decoction is strained off, 1 oz. of alum is added, and the boiling continued for a few minutes longer; the clear liquor is decanted off and 1 oz. of cream of tartar added; the mass is then allowed to stand for the carmine to settle.

2. Boil up 2 lbs. of cochineal, strain off the decoction, add 2 ozs. of alum, 3 ozs. of muriate of tin (a solution of stannous chloride), 2 ozs. of carbonate of soda, and allow to stand for 2 days, when the carmine will have been thrown down.

3. 1 lb. of cochineal is boiled with water and oz. of carbonate of soda; to the decoction is added 1 oz. of alum and 3 drams of cream of tartar; the mixture is allowed to stand for the carmine to be deposited.

4. The following process was patented by Wood, in 1856:9 ozs. of sodium carbonate, 8 ozs. of citric acid, and 27 quarts of water, are boiled together; then 1 lbs. of cochineal are added and the mixture boiled for 1 hours, strained, and clarified; the liquor is heated to the boil and 9 ozs. of alum are added; the mass is then boiled for 5 minutes longer and allowed to stand for 3 days, when the carmine precipitated is collected, washed, and dried.

A recipe was published by Madame Cenette, of Amsterdam, a noted maker of carmine, but this is defective, and carmine cannot be made by following it.

In the preparation of carmine it is advisable to use tin or tinned-copper vessels for boiling the cochineal in, as a small quantity of the metal dissolves in the liquor and exerts a beneficial influence on the carmine which is produced. Earthenware vessels may be used, but iron must be avoided, as any trace of iron in solution affects the shade of the carmine rather injuriously. The use of too much alum should be avoided, as it tends to reduce the colouring power of the carmine and also to alter the tint, turning it more crimson, while the shade ought to be a scarlet.

66

Carmine is a deep fiery-scarlet powder, slightly varying in tint; the best quality is known commercially as nacarat carmine." It is insoluble in water, alcohol, ether, turpentine, and all the vehicles used in mixing paints, but soluble in strong mineral acids. In caustic soda, caustic potash, and ammonia solutions, it dissolves with a deep crimson colour, from which