<Li> Mercuric oxide becomes vaporous mercury near 550 ° C (1022 ° F or 823.15 K), almost 600 degrees above mercury's melting point of - 38 ° C (- 36.4 ° F or 235.15 K). </Li> <P> Flux and slag can provide a secondary service after the reduction step is complete: they provide a molten cover on the purified metal, preventing contact with oxygen while still hot enough to readily oxidize . This prevents impurities from forming in the metal . </P> <P> Metal workers use fluxes in smelting for several purposes, chief among them catalyzing the desired reactions and chemically binding to unwanted impurities or reaction products . Calcium oxide, in the form of lime, was often used for this purpose, since it could react with the carbon dioxide and sulfur dioxide produced during roasting and smelting to keep them out of the working environment . </P> <Table> <Tr> <Td> </Td> <Td> This article needs attention from an expert in Mining or History . Please add a reason or a talk parameter to this template to explain the issue with the article . WikiProject Mining or WikiProject History may be able to help recruit an expert . (May 2009) </Td> </Tr> </Table>

Slag formed during smelting process in the extraction of iron