ZAFFRE, or SAFFRA, is the residuum of cobalt, after the sulphur, arsenic, and other volatile matters of this mineral have been expelled by calcination. It is therefore a kind of calx of cobalt, of a grey or reddish blue colour. The use of cobalt is to produce a very fine blue colour, when it is melted with fusible and vitrifiable matters. This blue colour produced by the vitrification of zaffre proceeds from the earth or calx of a semi-metallic substance contained in cobalt, called by chemists regulus of cobalt. This is proved by melting zaffre with a reducing flux, like any other roasted ore; by which means the above-mentioned semi-metallic regulus of cobalt will be obtained. The scoria in this fusion has also a blue colour, which proceeds from a portion of the calx of this regulus that is not reduced, but is vitrified along with the scoria. The calx therefore or metallic earth of the regulus of cobalt is the sole cause of the blue colour produced by zaffre. But as the quantity of regulus contained in cobalt is variable, therefore some zaffres furnish more blue than others. The heterogeneous fixed matters contained in cobalts, contribute, according to their quantity, not only to the greater or less intensity of the blue colour, but also to its lustre and beauty. For which reason, those who manufacture zaffre from cobalt, make frequent assays of the roasted ore, by mixing it with vitreous matters, to discover the intensity and beauty of the blue colour. Good cobalts calcined would form
too deep a blue, and almost a black glass, if they were not previously mixed with a certain quantity of vitreous fritt. In the manufacture of zaffre, therefore, the calx of cobalt, the strength of which has been previously determined by assays, is mixed with such a quantity of sand, or of powdered flints and quartz, that with the addition of some saline flux, a deep blue glass may be formed. The zaffre that is commonly sold, and which comes from Saxony, is a mixture of calx of cobalt with some vitrifiable earth, as we have said. It is of a grey colour, as all calxes of cobalt are before vitrification. Some zaffres are dearer than others, according to the intensity of the colour which they are capable of producing. Zaffre is employed in the manufacture of pottery and of porcelain, for painting the surface of the pieces of ware upon which it is applied together with some saline flux, previously to the baking or glazing, that the same fire may also vitrify this colouring material.
The blue of zaffre is the most solid and fixed of all the colours that can be employed in vitrification. It suffers no change from the most violent fire. It is successfully employed to give shades of blue to enamels, and to the crystal-glasses made in imitation of some opaque and transparent precious stones, as the lapis lazuli, the turquois, the sapphire, and others of this kind.