< COMMON FEATURES OF INVESTMENT MOULDS
PLASTER & GROG (TRADITIONAL INVESTMENTS)
Up until about the 1960’s, almost all lost wax casting of sculpture was carried out using plaster of Paris and fireclay grog investments for the casting mould. There were occasional exceptions, one investment used by a British foundry was composed of egg whites, ponsive clay, French sand and gum arabic [ref]. The majority of foundries however, based their refractory moulds on the standard plaster and grog model.
As indicated by the name, the two principal constituents of this investment system are PLASTER OF PARIS, and GROG FIRECLAY. Plaster functions primarily as a binder for the grog body, it also fixes any other non-adhesive ingredients that the founder cares to add in to improve the performance of the mould. Plaster of Paris also possesses an inherent refractory quality. This is derived from the plaster’s mineral content, which typically includes quantities of quartz, tridymite and crystobalite. The plaster grades normally used for making refractory moulds are the FINE CASTING or DENTAL type plasters. These two products are especially suitable for use in first coat preparations due to their fine particulate sizes.
Grog is a fired and ground ceramic material, and it has an appearance and consistency not unlike that of brick dust. Bagged grog is supplied by ceramics stockists in a variety of mesh sizes suitable for first layer or back up investment use.
To enhance and improve the basic plaster binder and grog refractory, founders often add in other materials. These additions, which are principally intended to increase the mould’s strength, refractory qualities and ability to dissipate evolved gases, can include animal hair, vegetable matter, ciment fondue and spent investment material (sometimes called LUDO). Although ludo slightly weakens an investment mould, it is often included not only for it’s refractory qualities, but also for the economy it offers by recycling previously used materials.
PLASTER & GROG MIXING AND APPLICATION >
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