A process for forming a mould or a core for casting molten metal by machining (e.g. cutting or drilling) to the desired shape a bonded particulate refractory material wherein the shear strength of the refractory particles is less than the shear strength of the bonds between the particles. The preferred particulate is aluminosilicate microspherers (e.g. "flyash floaters") and the binder is preferably based on silica (e.g. fumed silica/sodium hydroxide). The bonded particulate material is easily machined to form accurately dimensioned moulds or parts thereof.
