Abstract: An investment powder which is safer than conventional powders comprising tricalcium phosphate, and being substantially or entirely free of free silica in the respiratory portion yet providing an overall expansion at 750° C. of 1% or higher sufficient to prevent mould cracking during casting. A method of making a casting comprising forming a slurry by mixing a gypsum bonded investment powder with water, pouring the slurry into a stainless steel flask around a low melting point material model, allowing the slurry to set to define a mould, heating the mould to burn out the model and casting material into the mould wherein the stainless steel flask consists of a 400 series martensitic stainless steel.