Under this apparently contradictory term, substances come into the market which produce a peculiar metallic lustre. When applied under certain conditions, the appearance is similar to that produced by real bronze. The vegetable bronze pigments are lakes as pure and free from foreign admixtures as possible. The lakes obtained from red wood or logwood can be used for this purpose.

From the red woods (see p. 384) a magnificent bronze pigment can be obtained, which is either pure golden yellow or possesses a greenish golden metallic lustre not unlike the colour of the wing cases of the rose-bug. To obtain either shade a pure lake is first made by extracting red wood with boiling water, adding a little carbolic acid to the decoction (0·01 per cent. of the quantity of liquid) and allowing to stand for several weeks. The liquid is syphoned off from the deposit, heated, and alum added equal in quantity to 10 per cent. of the wood used. The mixture is then allowed to stand for about a week, the precipitate is filtered off, washed, and, if necessary, dried. If the bronze is to be used in the form of water colour, the precipitate is dried to a thick paste and mixed with about 10 per cent. of its volume of thick gum solution, so that a viscid mass is obtained which can just be applied with the brush. When the coating is made so thick as to hide the surface of the bronzed article, it has when dry the golden green colour.

In using this lake to prepare a pigment similar to gold bronze it must be almost completely dried, and then mixed with the liquid obtained in the following manner: White soap is melted on the water bath with the smallest possible quantity of water, and when completely dissolved the same quantity of white wax is stirred in, finally water is added, so that the cooled liquid has the consistency of a moderately thick varnish. When this liquid is ground with the requisite quantity of the still damp lake and the mixture applied to paper, wood, or leather, and after drying rubbed with a glass ball, it gradually acquires a very fine golden bronze colour. This method of bronzing is largely used in the manufacture of wall papers and for colouring fancy leather. To protect this coating against the action of water, it should be varnished when dry.

These bronze pigments may also be used in varnish. The lake is then completely dried and ground with varnish in such quantity as to give a thick mass which can just be brushed on.

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.