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.
SPECIAL INFORMATION CONCERNING PAINTING COLORS.
MODE OF USE—MIXTURES—CONCORDANCE OF ENAMEL WITH MOIST AND OIL COLORS, AND THEIR USUAL TECHNICAL NAMES.
Whites, belonging to the first group. White is obtained by permanent white, (for high lights), and Chinese white, a color of very limited use in painting, it being preferable to keep the white of the china when possible.
Permanent white, alone or mixed with other colors for heightening, which is called high light, or relief, requires perfect grinding. It should be tried by repeated and well fired tests before using it for important works. It is lifted up with the point of the brush, and laid without spreading. It could not bear two firings; it is put at the second firing, which is always less powerful.
Blues. (First group.) In his character as a chemist, M. Lacroix gives us, in his work already quoted, the general reason for excessive care in using blues. “All the blues, with very few exceptions, derive their color from cobalt.... As the mixture of cobalt and iron produces, proportionably, tints varying from light grey to black, it is well to take great precautions in painting when blues are used with reds, fleshes, browns and ochres. It follows as a natural consequence, that when you wish to have some beautiful shades of blue, you must avoid using brushes which have already served for one of the iron colors, and have not been properly cleaned.”