Dead-white Enamel

—This requires greater nicety in the choice of its materials than any other enamel, as it must be free from every species of tint, and be perfectly white; hence the frit employed in this case should be itself composed of perfectly pure ingredients. But a frit should not be rejected hastily because it may be somewhat discoloured, since this may depend on two causes; either on some metallic oxides, or on fuliginous particles proceeding from vegetable or animal substances. Now the latter impurities may be easily removed by means of a small quantity of peroxide of manganese, which has the property of readily parting with a portion of its oxygen, and of thus facilitating the combustion, that is to say, the destruction of the colouring carbonaceous matter. Manganese indeed po e es a colouring power itself on gla , but only in its highest state of oxidize ment, and when reduced to the lower state, as is done by incombustible matters, it no longer communicates colour to the enamel combinations. Hence the proportion of manganese should never exceed what is just; for the surplus would cause colour. Sometimes, indeed, it becomes nece ary to give a little manganese-colour, in order to obtain a more agreeable shade of white; as a little azure blue is added to linens, to brighten or counteract the dulne of their yellow tint. A white enamel may be conveniently prepared also with a calcine composed of two parts of tin and one of lead calcined together; of this combined oxide, one part is melted with two parts of fine crystal and a very little manganese, all previously ground together. When the fusion is complete, the vitreous matter is to be poured into clear water, and the frit is then dried, and melted anew. The pouring into water and fusion are sometimes repeated 4 times, in order to secure a very uniform combination. The crucible must be carefully screened from smoke and flame. The smallest portions of oxide of iron or copper admitted into this enamel will destroy its value. Some practitioners recommend the use of washed diaphoretic antimony (antimoniate of potash, from metallic antimony and nitre deflagrated together) for white enamel; but this product cannot be added to any preparation of lead or other metallic oxides; for it would tend rather to tarnish the colour than to clear it up; and it can be used therefore only with ordinary gla , or with saline fluxes. For three parts of white gla (without lead) one part of washed diaphoretic antimony is to be taken; the substances are well ground together, and fused in the common way.