The process used to give a hard vitreous finish to clock dials and sometimes ornamental parts of clock cases. It should not be confused with painting. Enamel is a fusible vitreous silica-based compound which is mixed with water and applied to a sheet of copper or other metal and fused in an enamelling kiln, in which it melts to form a hard glossy surface. Obtaining the right thickness of enamel often calls for several coats, each applied and fired in turn after the previous coating has been rubbed down. Although normally opaque white, enamel can be coloured with different metallic oxides; decoration such as numerals, signatures and other decoration on the dials, can be added in other colours and fused. The technique was extensively used for French clock dials, earlier ones being built up from individual enamelled plaques for each hour numeral. In cloisonne enameling, the area to be decorated is divided up by a design made in fine wire soldered to the ground metal and forming a series of compartments or cloisons, which are then filled with different colours of enamel and fired.