“Second Group.—Colors with but little trace of iron. This group includes the yellows and greens, several of which colors contain iron in small quantities.
“Third Group.—Colors with an iron basis, or of which iron is one of the coloring parts: First, the reds, fleshes, red browns, and violets of iron; secondly, the browns, yellow browns, ochres, blacks and a greater part of the greys.”
The enamel colors usually designated by the name of iron colors are: All the browns; the greys, excepting platina grey; the blacks, minus iridium black; the ochres; the reds, and the violets of iron.
The enamel colors said to be with a golden basis are: The carmines; crimson lake; the purples, and the violets of gold.
Tests.—The amount of flux added to the coloring oxides for the manufacture of enamel colors varies according to the color; this difference, joined to the diversity of the chemical elements, causes actions in the firing which may modify certain colors and even make them disappear entirely; it is said expressively that they have been eaten up, devoured by the fire. We shall cite, as an example, the mixture of ivory yellow with carmine, as one of those which decompose in the firing. Other causes act likewise on enamel colors during firing; the intensity, more or less great, of the heat, the thickness, and the amount of oil in the color, the way it is used, etc.
In order to well understand the various influences, and to secure yourself against accidents, you must be continually making tests of the mixtures yourself; it is the only way to paint with safety.
It is indispensable for the test to be double, that is, on two small bits of precisely the same manufacture of china as the piece you wish to paint. The same mixture is made on both small pieces, they are both dried, and one only is fired in order to be able to judge what change is caused by the firing, by comparing it with the unfired test you have kept by you. Besides, you will be able to make sure of a satisfactory result by comparing your experiments with the test tiles and plates of fired colors.
Mixed colors should be stirred with the brush when used; without this precaution they would separate; light blue would rise on dark blue, yellow on green, ivory yellow on carnation.
Some hints follow which it will be advantageous to verify and to carry out by tests. They apply generally to painting on porcelain or earthenware for the ordinary muffle.
Fusibility. Hard colors (those which require the greatest heat to make them fuse) spoil and often destroy those of a softer flux (that fuse more easily). The flux added by the manufacturer to the coloring oxide lightens the tint of the color; dark colors are therefore generally harder than light ones. In the palette of M. Lacroix the colors more fusible than the rest, although taking the same time to fire, are light sky-blue, light carmine A, pearl grey, warm or russet grey, and ivory yellow, all light colors.