In the metallurgy of tin, its first process is that of grinding the ore. The ground ore is then washed, which removes impurities, tin being such a heavy metal that it is most easy to wash away the earthy matter, and even some of the foreign metallic ores often contained with it. When other metals of about the same specific gravity are contained in the tin ore, further treatment is essential. The ore is then roasted in a reverberatory furnace, whereby the sulphur and arsenic are expelled. It is then mixed with fuel and limestone and heated strongly in the re-verberatory furnace, so as to bring the whole into the state of fusion, which is maintained for upwards of eight hours. The lime unites with the earthy material still mixed with the ore and flows with them into liquid slag, while the coal reduces the oxide of tin to the metallic state.
The reduced tin falls by its own weight to the bottom, and is, at the end of eight hours, let out by tapping a hole in the furnace, which had been filled with clay. The tin yet is impure, containing iron, copper, arsenic and tungsten. That it may be purified, the blocks of tin are placed in a furnace and moderately heated until the tin melts and flows into the refining basins, while the greater part of the foreign metals remain in the solid state. The molten tin in the refining basins is then stirred with sticks of green wood, whereby gases are given off and the metal is maintained in a state of artificial ebullition. The upper parts of the contents of the basin are oxidized and removed from the surface, while the greater part of the foreign metals collect at the bottom.
The tin is allowed to partially cool, during which process it separates into zones, the upper consisting of quite pure tin, while the under is so impure it is necessary to return it to the furnace and melt again. The layer of tin is removed into molds in which it is allowed to solidify. It is then marketed as block tin, the purest specimens being called refined tin.