Colors for painting under the glaze are specially prepared for the purpose from various metallic oxyds. As mentioned in the previous chapter, the colors used under the glaze have less flux or fusible matter in their composition than those used for painting overglaze: this is necessary, because in the former the colors are expected to bear a much greater degree of heat in firing than in the latter. They are so prepared as not to fuse except at the temperature required in the manufacture of the pottery to which they are applied. The variety of underglaze colors is less than that to be found in overglaze colors, as the oxyds from which colors may be made which will stand so great a degree of heat, are few in number.

The oxyds from which the colors are prepared are called the bases of these colors. The colors used in underglaze painting are made from the following bases:

Blue from the oxyd of cobalt. Co. O.

Green from the sesquioxyd of chromium. Cr². O³.

Browns from the sesquioxyd commonly called the peroxyd of iron, Fe². O³., and from the sesquioxyd of manganese. Mn². O³.

Yellow from the compound of titanium with oxygen, called titanic acid. Ti. O².

Black from the oxyd of uranium.

Red from the suboxyd, or red oxyd of copper. Cu². O.

We may add, upon the authority of M. Debette, that “pure black is obtained by taking 1 part of oxyd of uranium diluted in 22 parts of glaze; ordinary black with oxyd of manganese or oxyd of iridium; bluish black with a mixture of oxyd of cobalt and of manganese, and smoky grey with chloride of platinum. Rose is obtained by diluting, in glaze, gold which has been dissolved in aqua regia. For the blues, oxyd of zinc and alumina may be mixed with the oxyd of cobalt. For greens may be mixed oxyd of cobalt and oxyd of chromium.”

The fluxes used for these bases are given in the following paragraphs, translated from the Encyclopédie-Roret:

“The matters which enter into the composition of the fluxes and which cause the adherence of the metallic oxyds are quartz, feldspar, borax and boracic acid, nitre, the carbonates of potash and of soda, red lead and litharge, and oxyd of bismuth. At Sevres, they employ, for under-glaze painting, seven kinds of fluxes, which suffice for all the colors. The majority of these fluxes are composed of quartz, oxyd of red lead and of boracic acid, and to some is added a small quantity of carbonate of soda.

“The seven fluxes of Sevres are, first, the flint flux, which is prepared by melting quickly in a crucible and then flowing upon a metallic plate a mixture of 3 parts of red lead or of litharge, and 1 part of Etampes sand.

“Second. The flux of greys prepared with 6 parts of red lead or litharge, 2 parts of Etampes sand, and 1 part of pulverized borax.

“Third. The flux of carmines is made of 1 part red lead, 3 parts of Etampes sand, and 5 parts of pulverized borax.

“Fourth. The flux of purple is made with 3 parts of red lead or of litharge, 1 part of Etampes sand, and 5 parts of crystallized boracic acid.

“Fifth. The flux of violets is made of 27 parts of litharge or red lead, 2 parts of Etampes sand, and 11 parts of crystallized boracic acid.

“Sixth. The flux of greens employed, as well as the two preceding M. Salvetat, was prepared by him with 8 1-9 parts of red lead or litharge, 1 part of Etampes sand, and 2 parts of crystallized boracic acid.

“Seventh. Last the flux of the metallic substances, which is of sub-nitrate of bismuth, obtained by decomposing in water the nitrate of the acid of bismuth, to which is added 1-12 of borax.”

These fluxes are mixed with the basic oxyds in greater or less proportion, according as the colors are intended for work, which will require a greater or less degree of heat in firing.

Underglaze colors in powder suitable for painting on pottery, can be procured of several different manufactures. As mentioned before, the variety of these colors is not so great as is to be found in overglaze colors, but as they can be readily mixed the number is sufficient for all practical purposes.

In my own work I have made almost exclusive use of French colors, those manufactured by M. Lacroix. These colors are finely ground and of considerable variety of tint. A list of those which have been tried and found to give satisfactory results, may be given here:

There are many others, but these are mentioned as among those that have been tried and found to fire well, and the colors given in this list will be sufficient for the production of all the tints needed in underglaze painting. In faience painted after the Haviland method, fine white clay takes the place of white.

In painting on the biscuit white paint is sometimes used. This can be procured of English manufacture, but is not, I believe, made by M. Lacroix. The colors manufactured by Messrs. Hancock & Sons, Worcester, England, are also very satisfactory. The tints of these and the colors of other English manufacturers corresponds with those of M. Lacroix, which have been mentioned. The latter manufacturer, however, furnishes a much greater variety of colors, and the reds especially are superior to any English reds I have seen. M. Lacroix’s list includes at least three reds, one of which, Rouge T., can be relied upon to produce as good a scarlet as is possible under the glaze, when used under the proper conditions. The English carmines are very satisfactory. The English green, called French green, corresponds in tint to that of M. Lacroix, called Vert foncé, No. 1, while mazarine blue, of the Worcester and Phillips manufactures, and Cobalt blue of others corresponds to the Bleu de Roi of the French, and for work on biscuit white ware stands the fire rather better. The Victoria green, of the Worcester colors, is especially to be commended for a light green which stands the fire remarkably well.

Phillips’ English colors are also very good. His mazarine blue, especially, is one of the finest blues I have ever met with. Emery’s and Harrison’s colors fire very well, but are not so finely ground as the first mentioned.