Glossary of Terms

We try to use plain English but it is not always possible when describing technical materials or processes. This document may help.


Glossary of Terms


Addition cross-linking (of silicone rubbers)-----The way in which the rubber forms its bonds to become a solid rubber, Usually using a platinum catalyst with no shrinkage on curing.

Alpha (plaster)---- A hard plaster made by the autoclave process, strong, dense, with long crystals.


Catalyst----Any agent that starts or causes a reaction. For instance causing resins to form chains, cross link and set.

Condensation cross-linking (of silicone rubbers)----The way in which the rubber forms its bonds to become a solid rubber, usually using tin as a catalyst. The most common method but produces a small amount of alcohol which can lead to a tiny amount of shrinkage.

Curative ----- Part of the overall compound that when added to the rest form a set resin or elastomer. Not to be confused with a catalyst. If there is too much/little of this it will have nothing to link with.

Beta (plaster) ---- A soft type of plaster made by the pan process, porous, casts & carves easily.

Dihydrate (of gypsum) ---- Containing 2 water, set plaster or gypsum feedstock

Elastomer ---- A material that has rubber-like properties.

Exotherm ----  The heat generated chemically when a plaster or some resins set.

Fibrous Plaster ---- Plaster where jute, sisal, glass fibre etc. has been added to the casts as they are made.

Gel Coat ---- The first coat of resin which will ultimately form the surface of the cast or laminate. Usually thixotropic, (doesn’t run).

Green (of plaster) ----- Plaster that has set but not dried out for the first time. It is about half the final strength it will reach at this point.

Grog -----any refractory material used to stabilise or add texture to clays or investments.

Gypsum ---- Calcium Sulphate Ca So4 2H20 a crystalline rock that becomes Plaster of Paris

Hemi-hydrate (of gypsum) ----CaSo4 ½ H2O, gypsum where most of the water of crystallisation has been driven out by heating... Plaster of Paris.

High expansion (HXP plaster) ----plasters that are made to expand to match the shrinkage of the material cast into or from them.


Investment -----Usually a plaster based castable material containing a “grog” (refractory material) along with other additives (usually plaster). For casting metals and glass.

Knife Point (of a plaster set) -----The knife point of a setting plaster is the stage at which, when you draw a knife through the plaster it doesn’t join back up.


Lost wax -----Lost wax is actually a process, the object to be cast is made in wax from a rubber mould (or sculpted) and then covered with an investment or ceramic shell. the wax is burned out and the metal poured into the space. Also used to cast acrylic in denture making etc.

Low expansion (LX plaster)----All plasters expand slightly on setting this is often an advantage but where engineering accuracy is needed expansion controllers are added during manufacture.

Marl -----A red clay found between seams of gypsum in the U.K. which can stain the gypsum pink especially in bad weather.

Micro-Bubble (of fillers)-----Tiny hollow spheres of glass (or fly ash). A very light filler. Being spherical, high loadings are possible before flow ceases.

Pillar & Stall -----A method of mining plaster in galleries leaving pillars hold the roof up, which are blown out later so the whole area drops by 3mtrs but has no noticeable impact on the surface landscape.

Prepolymer---The base of a compound that links with the Curative to form a set resin or elastomer.

Resin Bronze----Bronze powder in polyester resin used as the gel coat to make an imitation bronze. Sometimes called cold cast bronze.

RTV----Room temperature vulcanising: - A rubber that cures without applying heat.

Stone (of plaster)----Stone is an industry term for a hard plaster (see “alpha”)

Water of crystallisation----The water the plaster uses up to turn back into a dihydrate, (set). This water always remains in the set plaster and can be 20% of the gauging water.


Vicat Point (of set)-----This is the point at which a weighted standard Vicat needle doesn’t penetrate the setting plaster. (When you can’t press your thumbnail into the cast).



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