Material added to a furnace or crucible charge to aid in the melting and removal of unwanted waste products (or gangue) such as silica, and to prevent further oxidation. Iron oxide, for example, reacts with silica (and other minerals) to form a glass-like slag, consisting mainly of fayalite (with a free-running temperature about 1170̊C), which either rises to the top of the iron or steel in a crucible, or can be run or ‘tapped’ off. There seems to be little evidence before the late medieval period for the use of additional fluxing agents such as lime to lower the free-running temperature and improve the yield of iron.

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