forming a layer of scoria or dross
By increasing the heat the cast iron centre portion of the granule first becomes fluid, and the granule bursts and falls by its own weight to the bottom of the crucible. At the same time the earths mixed with the ores melt and rise to the top, forming a layer of scoria or dross floating on the surface of the melted iron. Each granule of melted metal has therefore in falling to pass through the rising scoria; and it is in the passing through that the combination of the impurities of the metal with the alkaline earths takes place, so that the decarbonised iron on reaching the bottom of the crucible is cleansed of all impurities.
ex Thomas Spencer (of Newburn), his "On the Manufacture of the Uchatius Cast Steel," same volume, pp 146-154
Franz von Uchatius (1811-1871 *) developed Uchatius steel (mixing granulated iron and iron oxide), and a motion picture projector (ca 1853) used in lectures on ballistics.
tags: Thomas Spencer; Franz von Uchatius; impurities; marbled paper; scoria; steel