Mould

The earliest moulds for casting metal were made of stone. European early Bronze Age stone moulds consist of one main unit, a depression carved into a flat stone surface. Metal would simply have been poured into this depression to make a casting of the desired shape. Such moulds are often termed open moulds, but it is likely that they would have had some sort of cover to cut down the cooling rate and metal loss by oxidation. A number of such moulds have been found, most for producing flat axes. In the Middle Bronze Age, more complicated castings appear, cast in stone moulds consisting of more than one component (piece moulds), and usually made of steatite. The palstaves cast in these moulds were much more intricate in overall shape, but spearheads also had hollow sockets cast into them. Moulds for this purpose would have consisted of two main stone pieces forming the outside of the casting, and a clay core to form the inside of the socket. During the Late Bronze Age, piece moulds began to be formed of clay. Such moulds were broken to extract the casting and fragments have been found on archaeological sites. Piece moulds made of bronze appeared first in the Middle Bronze Age and continued into the Late Bronze Age. A complex mould must have provision for pouring in molten metal, for allowing bubbles of gas to escape and to allow for contraction in the metal on cooling. Moulds are designed with a small cup or funnel, into which the metal is poured. This may be connected to the main body of the mould by channels. The mould is filled right up to the lip of the cup. Bubbles can then rise through the mould and accumulate in the cup, away from the main part of the casting. Molten metal in the cup also acts as a reservoir to counteract the effect of metal contracting in the mould as it cools. When the casting is released from the mould, the metal solidified in the cup and in the channels must be trimmed off. These structures, respectively called the feeder or header, and gates or jets, have been found on archaeological sites. Where the various parts of the mould meet, casting seams or flashes are left on the casting. These may be observed on ancient bronze artefacts. Some very complex castings could not be made using piece moulds: these were cast in moulds formed by the lost wax or cire perdue method. See also casting, metallurgy (China).