At fusing temperatures glass melts into a semi fluid state and the contact surface of glass blends into neighbouring pieces, taking shape and surface impression from moulds that may be supporting the material.
Glass Flake Bowl - Using Fusing Technique
The molten state
varies according to glass type and fluxes involved. It is crucial that compatibility exists between additional materials, i.e.  colours, other glass types, or metal inclusions used in the fusing technique for the annealing process to reduce the stress of cooling to room temperature.

Moulds made from ceramic shell to help process glass in the fusing process can be used over again. They do not distort or shrink and process the glass at a faster rate than bulkier plaster investment moulds.