We borrow from M. Lacroix his classification of colors, which is very practical with regard to their employment in painting: "Classification of colors with respect to iron.- Iron plays an important part in the composition of a great many enamel colors; for this reason I have taken it as a standard for my classification of colors into three groups.
"First Group. - Colors that do not contain any iron: First, the white; secondly, the blues; thirdly, the colors from gold.
"A horn or ivory knife is preferable for the use of colors of this group.
"A glass muller is still better than knives.
"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 com-, paring 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.
Thickness. The tint of enamel colors get darker when you increase their thickness. But you must beware of doing it too much. Light and fusible colors used too thick, blister in firing; it is prudent to give them only a medium thickness.
You should apply in drops those colors only that are specially designed for the purpose; permanent white, permanent yellow, and relief. They hold on earthenware, but their use on porcelain is liable to failure.
Mediums. Experience will prove that if too much oil of turpentine is added to the colors used, which is called adding "fat," they will craze in the firing. Make some trials by exaggerating this fault. You will remark nevertheless that colors applied very thin, although with much "fat," do not craze. The cracks caused in the firing, by the action of the resinous part of the oil, which evaporates and causes the white of the enamel to reappear, is called crazing.
Conduct of the Work. It is very important in the first painting to use the most fusible light colors, and those most easily developed in the first firing, which is the strongest. Commence always on a lighter scale than the final tint, for in pottery painting any color made too dark in firing cannot be made light again. When the mixtures have produced, for example, some browns or russet hues which have not glazed in the first firing, the glazing is brought back by a little fusible light grey, applied before the second firing for retouches. These short general instructions will be resumed and developed in the following lessons.