The process of melting metal ores in order to separate metal from impurities is called smelting. Often, fluxes such as limestone are added to the furnace to facilitate the process.
Because sand is a common impurity, the residue from smelting -- called slag -- is often silica-rich and can be combined with compatible glass (usually white) to create a streaky, opalescent glass known as slag glass.
Slag glass has been produced in a variety of colors, purple and green being most common. The degree to which the colored glass is mixed with the white glass results in different styles. Where the colors blend gradually it is called fused mix. Where there are distinct streaks of color or white it is called open mix.