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Page:Popular Science Monthly Volume 21.djvu/328

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THE POPULAR SCIENCE MONTHLY.

divided into two classes: decoration by the sharp-fire, and decoration by the enameling furnace. The former method consists in the application upon the porcelain of coloring substances which are fixed and developed upon it at the same temperature as that at which the porcelain is baked. It is the method that gives the most highly prized results, for with it the color is covered by the enamel, and takes a high luster and deep setting, becoming, as it were, of one body with the object. The magnificent blue of Sèvres, certain browns, the blacks, and a few other shades are obtained by this process.

The color may be mixed in the paste, or put on the fashioned object, before enameling, or mixed with the glazing; or it can be put upon porcelain that has been baked and will then be baked a second time in the sharp-fire. This is the process we employ at Sèvres for our blues.

One of the most brilliant varieties of decoration by the sharp-fire consists in what is called the process of applied pastes, or in painting with barbotine upon the raw or biscuit-baked porcelain; by successive, rightly tempered applications we can lay on a considerable thickness of material, on which the artist can with the chisel give a fine finish and thus add great value to his decoration. When the barbotine is applied upon a tinted bottom, charming effects, making the porcelain look like real cameos, can be obtained in the clear material. If coloring oxides are added to the barbotine, a real picture can be obtained. Unfortunately, the number of colors that can be used in the sharp-fire is, on account of the excessive temperature, very limited.

In decoration in the enameling-furnace, the painting is always done on baked porcelain, consequently on enamel, and the heating takes place at a relatively low temperature. Since the glazing is not to be melted, as in the sharp-fire, an intermediate agent or flux is required to make the colors adhere. This is generally a silicate or a silico-borate of lead, or for metals a sub-nitrate of bismuth. When the temperature is raised, these substances melt, attack the glaze, and combine with it, determining by their reaction the adherence of the color. The temperature in these operations is determined by the nature of the fluxes and of the colors; and, as some colors are more sensitive than others, it is frequently necessary to bake in two successive and different fires.

The palette of Sèvres is complete, and is competent to reproduce with a marvelous excellence the chief works of the greatest painters. Notwithstanding the beauty of the sharp-fire colors, and the richness of the palette, much is still left to be desired in the decoration of French hard porcelain. The sharp-fire colors are too limited in number and too delicate to admit of any great variety of effects being drawn from them; and the colors of the enameling furnace, in spite of their richness, have a capital fault. They are opaque, and they cover the porcelain and hide all of its highly prized qualities.