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SILICONE RUBBERS

Silicone rubbers are composed of silicone-oxygen polymer chain links, catalysed with a tin or platinum based compound. Like POLYSULPHIDE rubbers, silicones come in a variety of setting and curing rates, and can be modified to suit by adding in thixotropic and other agents.

Silicone rubbers share many common features with other RTV’s, for instance they are typically supplied as two part (A&B) packs. The rubber can be used to create all types of moulds (poured, brushed-on, blanket etc). Silicone rubbers often have a number of specific characteristics that can make them especially useful for specialised moulding operations. Silicone compounds tend to release very easily from moulded patterns (silicone is also used as a lubricant), and the rubber rarely stains or reacts with the underlying material it is applied to. This makes silicone rubbers especially useful for moulding delicate or valuable items. The more expensive silicone products exhibit a very low degree of shrinkage on curing and this feature, combined with the other factors mentioned, generally makes silicone the first choice rubber for use in museum and restoration workshops.

Another feature of certain silicone rubbers (especially those catalysed with platinum compounds), is their ability to perform well across a wide temperature band (extremes of -300 to +575°F [-150 to +300°C are possible). This characteristic allows suitable silicone rubbers to be used as a direct mould for low temperature metal casting (lead, pewter etc). Silicone rubbers are also especially good for creating reproductions in resin.

Silicone rubbers are typically more expensive than other RTV rubbers and PVC moulding compounds. This is especially the case if a platinum based catalyst is used. Foundries that use silicone rubbers as their standard moulding medium tend to specialise in small, intricate casts. Thoough silicone rubber appears to be superior product in comparison to the other reproduction materials described in this section, in practice the discernable difference between a metal cast produced  from a silicone mould and one produced from a polysulphide mould, is in most instances likely to be visually negligible.

Silicone rubbers are resistant to ultra-violet and ozone degradation, they are also fire resistant. If well stored (see POLYSULPHIDE RUBBERS for details), silicone moulds should be serviceable for some years, although some products start to degrade after four years or so (consult individual manufacturer’s data). Silicone rubbers make excellent, if potentially expensive art foundry moulds.

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