The vegetable bronze is obtained from logwood in a similar manner, a solution of stannic chloride being generally used to precipitate the lake when a deep bronze is required, and alum for a pale gold shade. By using mixtures of the two salts intermediate shades are produced. The precipitate produced by alum may be shaded by means of potassium bichromate. Hæmatoxylin forms with chromium oxide a deep bluish black compound which has such intense colouring power that it is used to colour writing ink. The ink is prepared by adding a little potassium bichromate to a decoction of logwood. If a very small quantity of this dark precipitate is mixed with the lake precipitated from logwood extract by alum solution, colours are produced possessing the peculiar metallic lustre and a shade depending upon the quantity of potassium bichromate added. The addition of this salt must be very carefully made, as a very small excess is sufficient to render the colour so dark that it is useless as a bronze. The shade of the precipitate depends upon the concentration of the liquid and other conditions, so that it is impossible to give precise quantities. In practice, the safest and most convenient method is to dissolve the bichromate in a large quantity of water and add very small quantities of the dilute solution to the logwood extract along with the alum. After each addition a portion of the precipitate is rapidly mixed with the above-mentioned solution of soap and wax, to which a little size has been added, and then spread upon paper. If the desired shade has not yet appeared, a little more bichromate is added and another test made, and the process repeated until the proper shade is attained.
Pigments for colouring wall papers and fancy leather are not easily made which produce such fine effects at so small a cost as the vegetable bronze pigments, which deserve the greatest regard from the colour maker and leather and paper manufacturers.
Appendix—The Brocade Pigments (”Brocatfarben”).—Under this designation powders have been recently introduced characterised by a strong metallic or glassy lustre, and very suitable for certain purposes, such as the manufacture of wall papers, since they enable remarkably fine effects to be produced. They consist of mica in a tolerably fine state of division. Mica is a mineral which occurs frequently in nature and which very readily splits into thin sheets; it occurs in various colours. In thin sheets mica is colourless and completely transparent; in somewhat thicker pieces it generally shows a distinct metallic lustre similar either to that of gold or silver. When ground to fairly fine powder, it has the same gold or silver lustre, and gold or silver brocade colours are distinguished in commerce. They are made by grinding mica which has been previously sorted according to its appearance. The powdered mica can be sold as powder or may be mixed with a binding substance in order to be ready for use. Gum Arabic is generally used as the binding medium, but it can be replaced by the cheaper dextrine. When printed upon paper the brocade pigments produce the effect of a bronze, and in addition to their cheapness they have the great advantage of being completely unaltered by the air.
PIGMENTS OF ORGANIC ORIGIN.
CHAPTER XLVIII.
LAKES.
The pigments commercially designated lakes generally consist of an organic colouring matter united with a metallic oxide. In isolated cases other compounds of colouring matters are included under the term, such as indigo sulphonic acid, and yet more rarely pure colouring matters are included, for example carthamine red. But the overwhelming majority of lake pigments are compounds of a colouring matter with the oxide of a metal. For this purpose the oxides of tin, lead and aluminium are commonly used.
Lakes are generally made by mixing the solution of the colouring matter with the solution of a salt of the metallic oxide, and precipitating the oxide by an alkali. The colouring matter is separated along with the oxide and forms with it a substance known as a lake. It has not yet been decided whether the lakes are true chemical compounds of the colouring matter with the metallic oxide, or whether the colouring matter is simply held fast by the surface attraction of the finely divided oxide. In favour of the latter view is the fact that a larger or smaller quantity of the colouring matter can be united with a given quantity of the oxide.
The lakes vary greatly in durability; some, such as the madder lakes, can be counted among the most durable pigments, whilst others have very little permanence, as for example, the logwood lakes.
The majority of the colouring matters used in the preparation of lakes are of vegetable origin, but several are derived from animal sources. The properties of the colouring matters used for this purpose vary greatly; each material demands a special treatment for the production of lakes. Therefore, in describing the methods by which lakes are made, we shall give first the properties of the colouring matter in question, and then proceed to the preparation of the lake. In the preceding portion of the book the pigments have been arranged according to their colour; the lakes will be described in the same order. The yellow, red or green lakes of different origin will be treated in the same section in order to facilitate reference to any particular pigment.
White lakes are not known, neither are there lakes which can be described as black. With these exceptions lakes of all colours can be made and also of all shades. As an example of this madder lake may be given, which is found in commerce in a great variety of shades; it can be made from the palest rose-red to the deepest purple red, or, more properly, madder red, which is a characteristic shade. The shades of any lake are obtained by mixing in a white pigment just as in the manufacture of mineral pigments. Whilst, however, in the latter case the white pigment must often be mechanically mixed with the colour, the shading of the lakes is accomplished in their preparation. Generally speaking, a pale shade of a lake is obtained by increasing the amount of the salt, the oxide of which is used to precipitate the colouring matter. The oxides in question are white (with the exception of lead oxide); thus in the pale shades a small amount of colouring matter is precipitated upon a large amount of oxide, whilst in the deep lakes the reverse is the case, a large quantity of colouring matter is precipitated upon a small quantity of oxide, and the shade thus appears very deep.