The precipitates of gold, and the colcothar or other red preparations of iron, are called _tender_ colours. The heat must be no greater than is just sufficient to make the enamel run upon the piece, for if greater, the colours will be destroyed or changed to a different kind. When the vitreous matter has just become fluid it seems as if the coloured metallic calx remained barely _intermixed_ with it, like a coloured powder of exquisite tenuity suspended in water: but by stronger fire the calx is _dissolved_, and metallic colours are altered by _solution_ in glass as well as in acids or alcalies. The Saxon mines have till very lately almost exclusively supplied the rest of Europe with cobalt, or rather with its preparations, zaffre and smalt, for the exportation of the ore itself is there a capital crime. Hungary, Spain, Sweden, and some other parts of the continent, are now said to afford cobalts equal to the Saxon, and specimens have been discovered in our own island, both in Cornwall and in Scotland; but hitherto in no great quantity. Calces of cobalt and of copper differ very materially from those above mentioned in their application for colouring enamels. In those the calx has previously acquired the intended colour, a colour which bears a red heat without injury, and all that remains is to fix it on the piece by a vitreous flux. But the blue colour of cobalt, and the green or bluish green of copper, are _produced_ by vitrification, that is, by _solution_ in the glass, and a strong fire is necessary for their perfection. These calces therefore, when mixed with the enamel flux, are melted in crucibles, once or oftener, and the deep coloured opake glass, thence resulting, is ground into unpalpable powder, and used for enamel. One part of either of these calces is put to ten, sixteen, or twenty parts of the flux, according to the depth of colour required. The heat of the enamel kiln is only a full red, such as is marked on Mr. Wedgwood's thermometer 6 degrees. It is therefore necessary that the flux be so adjusted as to melt in that low heat. The usual materials are flint, or flint-glass, with a due proportion of red-led, or borax, or both, and sometimes a little tin calx to give opacity.