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form like the glass used in higher-wattage halogen lamps, (e.g., automobile headlamps). Window and bottle glasses are diluted, low-melting forms of silica. Sodium and calcium oxides are used as diluents in sodalime window and bottle glass. Silica aerogel, silica smoke, fumed silica, and precipitated silica are names for amorphous silica powders that are important constituents of medical rubber-like goods, including breast implants. In fact, amorphous silica is used in almost all silicone elastomers, as well as in special-purpose isoprene (natural rubber) elastomers. Less often used is diatomaceous earth, the silica skeletal residue of diatoms, which are microscopic sea creatures (Heaney et al., 1.994; Iler, 1979a,b, 1981). Some forms of amorphous silica have been approved by the Food and Drug Administration (FDA) (D. Benz, FDA, personal communication, 1998) for use in pharmaceuticals and food; they are widely distributed in foods and foodstuff manufacture (Villota and Hawkes, 1986).

Silicon is a semimetallic element, located just below carbon in the periodic table, that is not found in nature in its elemental form. It is perhaps best known as the shiny semiconducting metalloid used to make computer chips. Silicon can be made by heating silica with carbon (coke or charcoal), typically in an electric arc furnace. At high enough temperatures, the elements silicon, carbon, and oxygen can exchange places, and the driving force of the reaction is the loss of gaseous carbon dioxide (CO2) leaving silicon and any excess carbon behind. The impure reaction product can be used to make silicones (see below) but requires extensive purification before it can be fabricated into computer chips. Although silicon is in a high-energy, unstable state with respect to its oxide, silica does not form spontaneously in air on its surface, as rust does on iron or aluminum oxide on aluminum. Like gold or platinum, silicon retains its shiny metallic appearance and electrical properties. However, silicon can burn in air to give a thick, white smoke of amorphous silica (LeVier et al., 1993).

Silicone refers to a large family of organic silicon polymer products with a main chain of alternating silicon and oxygen atoms. Typically, each silicon in the chain carries two methyl groups (CH3-, which can also be written as Me-, where C = carbon and H = hydrogen), and the material is called poly(dimethylsiloxane) (PDMS):

The tilde (˜) at the chain end implies that the sequence is repeated, typically with hundreds to thousands of silicon-oxygen links. As the number