The sand casting process involves placing a model – usually plaster – in a two-part metal frame filled with foundry sand. The moulder calculates the parting line and creates casting channels and gas vents. The two frames are then joined together and hermetically sealed. When the part to be made is hollow, the moulder makes a sand core, suspended in the mould to allow the molten cast iron to pass through to a thickness of one centimetre. Making a mould takes time and precision. GHM specialises in the development of moulds for its art castings.