Scientific American 21, 12.2.1848
Ultramarine is used in enamel where very bright blues are wanted, but there are few instances where zaffer (a mixture of cobalt) will not answer equally as well, that is if it is used with borax, and a little calcined flint to take off the fusible quality of the borax. The ultramarine required no preparation when used in enamel painting, previously to being mixed with a flux.
Ultramarine ashes are used where light semi-transparent blues are wanted but these are often adulterated with precipitations of copper which turn green on fluxing and great care is therefore necessary in its use.
Zaffer is used to produce blue, green, purple and black colors on enamel. Zaffer, or as it is more usually known ny the name of cobalt, assumes a strong blue color approaching to purple as it vitrifies. Cobalt is known to be good or bad only by trial and comparison with some that has proven to be good.
RED ENAMEL. - The protoxide of copper affords a fine red color, only it must be taken at the proper point of fusion. If this is raised to too high a temperature it may be brought back by adding some combustible matter such as tallow or charcoal. It is possible by pushing the heat in this way to reduce entirely apart of the oxide and the particles of the metal scattered over it, will look like a stone called aventurine. To procure the protoxide of copper pure, boil equal parts of sugar and the acetate of copper in four parts of water, a powder of red is deposited which after two hours boiling is set aside to settle and the precipitate washed and dried.
PURPLE COLOR. - A preparation of gold is used to make the purpureus color, which is done by dissolving fine gold in aqua regia and some tin dissolved in the same kind of spirit, and after all effervescence has ceased, drop some of the gold solution into some water and some of the tin into the same, and a red precipitate will be the result which must be washed and dried and is then fit for use. Of course, a great or small quantity can be treated accordingly. This is the precipitate of gold. Crude tin is often used to precipitate the powder of gold without dissolving it in muriatic or nitro-muriatic acids.
A powder for red enamel may also be produced by using salt of tartar to deposit the gold instead of the salt of tin, and if it be not well washed from the salt, it will not answer for without washing it is the aurum fulminies from its explosive qualities. Volatile salts will also deposite the gold powder, and for a full description of which see our articles on Electro Gilding in former numbers of this paper.
Gold precipitated from acqua regia by copperas (sulphate of iron) has a fine appearance and is bright.
YELLOW COLOR. - Silver preparation is used to produce a yellow enamel. Clovet says that a fine yellow is made by pure oxide of silver painted in a thin coat and fused along with some metallic flux, and another powder for this purpose is to calcine one ounce of the filings of silver to half an ounce of sulphur in a crucible, the sulphur being added after the silver is made red hot.
GREEN COLOR. - Copper in enamel painting is used for making both green and red colors, also blue. For green the oxide of chrome is most generally used and is allowed to be the best, but the ferretto or the sulphate of copper six times calcined is very fine. The appearance of the red powder must be red. If copperas is dissolved in water and pearl ashed added, the precipitate makes a good yellow preparation and if mixed with cobalt makes a cheap green mixture.
Antimony is also used for yellow. Powdered bricks have also been used for compounding yellow colors in enamel, but they act just as ochre does in this respect and require a greater force of flux than the pure ochres or calcined iron. When tehy are used they should be chosen of the reddest color and of the softest and evenest tenure.
Blue clay fuluxed makes a splendid deep and dark color and powdered brick and other fluxes make imitation agate.
Black enamel is made with the peroxide of manganese or peroxide of iron and cobalt. - Clay alone with protoxide of iron gives, according to Clovet, a fine black.
Lead is often used as a flux, but it should be carefully avoided for all ware intended for domestic use, and therefore for those enamels that are intended to stand exposure to acids or the effects of the atmosphere, it should be mixed always with calcined pounded flint or crystal.
A flux for common purposes is composed of red lead one pound, pearlash 6 ounces, and two ounces salt calcined together and afterwards reduced (for using as a flux) to powder.
A flux to vitrify a large portion of cobalt may be made of lead one pound, 6 ounces of pearlash, 4 ounces borax and 1 of arsenic. - This is a soft flux and will glaze with less heat than the preceding one, treated in the same manner.
A white flux is made with 1 pound of powdered flint glass, 6 ounces of pearlash, salt 2 ounces, borax one ounce. This is the flux that should be used along with those substances for making blue, crimson and purple and also for pure transparent white.