The word enamel derives from the Old French "esmail" and the Old High German "smelzen", meaning "to smelt". This is the most crucial process in the making of enamel. To prepare enamel, the constituents of glass (flint, or sand, as well as red lead, and soda or potash) are heated together to form a clear flux. To this a coloring agent (usually a metallic oxide) is added: copper for green, cobalt for blue, iron for red or brown. The enamel is then allowed to cool and solidify in slabs. Before use, each color is separately ground into a fine powder, placed on a prepared metal surface, and fired in a kiln. When cool, the surface is polished to remove any imperfections and add brilliance to the colors. The difference between the various enameling techniques lies chiefly in the methods used to prepare the metal surface for the enamel.