Whilst only the finest varieties of cochineal can be used to make fine carmine, the cheaper sorts can be used for Florentine lake. The cochineal used for carmine is boiled but a short time with water, so that the residue contains considerable quantities of colouring matter, which may amount to half of that originally present. The same materials are used as in the preparation of carmine, and in this case, too, stress should be laid upon their purity. The weight of the alum is usually 10 to 15 times that of the cochineal; a little stannous chloride and cream of tartar are also added to brighten the shade. All these materials are boiled with the cochineal, soda is added to the clear solution until effervescence no longer occurs, the separated lake is then washed. The lake may be made with magnesia instead of alumina, it is added in the form of magnesium sulphate; the more magnesia is used the paler is the lake. The proportions in which the materials are employed vary with each manufacturer. The following quantities have always given the author a favourable result: Cochineal 10 parts, alum 150 parts, water 250 parts; or cochineal 10 parts, magnesium sulphate 5 parts, alum 0·5 part.
In making Florentine lake a fairly deep colour should be produced. This may be mixed without difficulty in the wet or the dry state with a white pigment, thus producing any desired shade, even to the most delicate rose-red.
Ammonia-Cochineal.—This preparation, which is chiefly used by dyers, is obtained by treating cochineal in a well-closed flask with strong ammonia, which dissolves the colouring matter. After about a month alum equal to about 3 per cent. of the amount of cochineal is added without separating the undissolved residue from the solution, and the whole is evaporated at a gentle heat in a tinned pan until it becomes a stiff paste on cooling. The mass solidifies more readily if a small quantity of starch paste is added. ”Cochineal paste” is cochineal which has been treated in this manner; it is generally brought into the market in the form of cakes or small slabs.
In addition to the true cochineal other species of the same insect contain a red colouring matter and have a restricted use in dyeing. The most important of these is the Polish cochineal (Coccus polonicus), which lives on the roots of the scleranthus. It has not been proved that these insects contain the same colouring matter as true cochineal, but it is certain that the colouring matters of Polish and also of Russian cochineal (Coccus euvæ ursi) are far inferior in beauty to that of true cochineal.
CHAPTER LII.
THE COLOURING MATTER OF LAC.
From the punctures of the insect Coccus lacca on certain East Indian trees, especially those belonging to the genus Ficus, flow at the same time resin and colouring matter in such quantities that the insects are frequently enclosed and large red masses are formed on the trees. The insects live in some measure in these masses, the females lay their eggs in the spaces and the larvæ are said to feed on the red sap contained in the mass. When the larvæ have left the incrustation, generally in November, it is broken off from the branches and collected. It is now known as stick-lac; it consists of large masses of resin of a fine deep red colour. In the interior the cells of the insects may still be perceived. On chewing, it becomes soft and colours the saliva a deep violet. When stick-lac is boiled in water a portion of the colouring matter dissolves. Good stick-lac contains 10 per cent. of colouring matter and 80 per cent. of resin.
Seed-lac has the same origin as stick-lac, but frequently the greater part of the colouring matter has been extracted; it rarely contains more than 2·5 per cent., and is consequently of little value for colouring purposes.
Lac Dye.—The colouring matter of stick-lac is generally separated from the resin in India and comes into the market under the name of lac dye. It is made by stirring coarsely powdered stick-lac in large vessels for several hours with warm water. Almost the whole of the colouring matter dissolves, whilst the resin remains as a ruby red mass which is melted and brought into the market under the name of shellac. The solution of the colouring matter is evaporated in shallow vessels in the sun or boiled down in pans; the residue is made into cakes. At present soda solution is used instead of pure water to extract the colouring matter; a larger yield is obtained. Lac dye usually contains 45 to 50 per cent. of colouring matter, 25 per cent. of resin, and, in addition, earthy substances which are to be regarded as intentional admixtures.
In the preparation of lac dye by Stephen’s method coarsely powdered stick-lac is boiled with soda, and the remainder of the colouring matter contained in the resin extracted by repeated boiling with water. All the extracts are then united and precipitated by alum. The colouring matter separates as a lake which still contains a large quantity of resin.
According to the patented process of Henley, the colouring matter of seed-lac is extracted by hydraulic presses. The lac is filled into press bags which are placed between iron boxes heated by steam. On applying pressure the melted resin, shellac, comes through the bags, whilst the colouring matter remains behind. If this method were employed on the large scale, presses similar to those used in stearin candle works would be suitable.