VEGETABLE BRONZE PIGMENTS.
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.
PIGMENTS OF ORGANIC ORIGIN.