To be able to cast the previously built wax trees in precious metal, a kind of plaster mould has to be made by putting a metal cylinder over the sensitive wax tree. The so-called investment material, a substance reminiscent of plaster, is stirred under vacuum and fills the metal cylinders (so-called flasks) as a result. Just like plaster, the investment materials are powdery substances which, when mixed with water and then dried, have the required strength and moulding properties for casting with metals. Once the process has finished, you therefore have a metal cylinder filled with plaster and the wax parts as the core inside.
Different investment materials are also used for different alloys. One criterion can be, for example, the temperature resistance. A distinction is generally made between gypsum bonded and cristobalite bonded investments.
Conscientious work and consistent investment quality are also extremely important during the investing process. A lack of strength in the investment material can lead to burr formation or, in extreme cases, to a complete tearing of the entire plaster mould and the associated total loss of the wax moulds it contains.