An early and simple mold developed for casting metal tools and weapons and, later, glass and brick. It consisted of a single block of stone, or occasionally clay, with the shape of the required artifact cut into it. Only very simple objects can be cast in this way, especially when one surface must be flat. The molds continued in use after more sophisticated versions had been developed, mainly for the manufacture of blanks for coins. The molds were probably not technically open, since this would result in oxidation of the surface of the metal, so probably a flat stone or other cover was placed over the mold during cooling. Glass was cast in open molds by the Egyptians as early as 5 BC.