An organic–inorganic polymer in which some of the carbon atoms are replaced by silicon atoms. They may be divided into condensation-cured silicones (vinyl polydimethylsiloxanes) and addition-cured silicones (polydimethylsiloxanes). Crosslinking of condensation-cured silicones is achieved by the use of an activator paste containing tetraethyl silicate; the by-product of this reaction is alcohol which detrimentally affects the storage stability; impressions made with this material should therefore be cast immediately. The setting reaction of addition-cured silicones is via a platinum catalyst and a silanol. Because there is no by-product, addition-cured silicones have greater dimensional stability. The wearing of latex gloves can cause the inhibition of the setting reaction of addition-cured silicone putty. Silicone impression material is manufactured in a range of viscosities (light-, medium-, heavy-bodied, and putty) controlled by the amount of filler incorporated.