Abstract—In the formulation of gypsum‐bonded casting investments, small amounts of certain modifiers are sometimes added to increase the investment's thermal expansion.In the present investigation, a systematic study has been made of the effects on thermal expansion of adding boric acid and sodium chloride to the investment powder, and glycerol to the mixing water. The combination of 0.2 per cent of sodium chloride in the powder, and 10 per cent of glycerol in the mixing water, appears to offer most promise in the formulation of an investment with a high thermal expans