The following patent US 2377076, filed in 1943 by Walter Ford, is such a nice piece of work in a former great company. The patent describes two shapes of molds wherein the glass is foamed at high temperature. Both shapes have been applied although without top cover like seen in the pictures hereunder.
The patent mentions that the demountable mold does not need a short heatup to release the foam during stripping.
Further, the patent gives a recipe for a coating, which is needed to release the foam from the mold without damaging the mold or the foam. The basic substance should release gas between 600°C and 900°C to facilitate parting. Hydrated Alumina (Al(OH)3) is such a substance found in Bauxite or Clay (Kaolinite) or which can be bought pure. Dextrin is added to improve adherance on the mould.