Scoria Blocks

During the copper smelting process, impurities such as fragments of rock and iron ore melt and float on the surface of the denser liquid copper metal. The molten slag from the initial melting of the ore was a waste material known as scoria containing little copper. For the subsequent seven cycles of roasting and melting the ore, the slag was sufficiently rich in copper to be worth recycling and so it was fed back into the earlier stages of the process.

Roughly 2 tons of scoria were produced for every one ton of copper. The molten slag was cast into blocks which were used as a cheap building material. They were given free to the employees of the Cornwall Copper Company to build their houses and sold for sixpence for 20 blocks. The gasses released during the smelting process made the slag layer quite frothy which is why the blocks are porous.