PLASTER & GROG (TRADITIONAL REFRACTORY MOULDING)
Up until the late 1960’s, almost all sculptural lost wax casting was refractory moulded using the plaster of Paris and fireclay grog system. There were occasional exceptions, for example one British foundry used a system based on 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 suggested by the process title, the two principal constituents of this refractory 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 (see below), that the founder cares to add 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, POTTERY 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, it typically has the appearance and consistency of brick dust. Bagged grog is supplied by ceramics stockists in a variety of mesh sizes suitable for both first layer and back up refractory mould layers.
To enhance and improve the basic plaster binder and grog refractory, founders often add in other material. 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 refractory material from used moulds (sometimes called LUDO). Although ludo slightly weakens a refractory mould, it is often included not only for it’s refractory qualities, but also for the economy it offers by recycling previously used materials.
INFO: An cost effective alternative to fireclay grog refractory is RED CLAY. This material primarily consists of crushed bricks and is usually available in a range of mesh grades. Sometimes referred to by the tradename 'Entecott', red clay is best known for it's use in preparing clay tennis courts.
WARNING: Refractory products may contain hazardous substances, read MSDS sheets, product data and follow OHS guidelines/safe working practices.
A part invested plaster & grog
refractory mould, the founder has
focused in coating the wax in fine
refractory material, the exposed wax
will be encased by secondary layers.
A cross sectional diagram of
a typical plaster & grog mould.
Note: wax drain (c), core pins
& core vent (a), riser (b).
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