Slag glass is an unattractive name for an often beautiful treasure. In fact I’d like to begin a campaign to rename it Smelt Glass or something less vulgar as it does it no justice.
The origin of slag glass is in the smelting of metal ores to separate the desired metal from the other impurities. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals.
This slag glass was usually discarded after the smelting process but in ancient times this slag was often melted and reformed into decorative items and jewellery as well as being ground to a powder and added to pottery glazes.
Agate or Malachite Glass
During Victorian times, glassmakers recognised the beauty of slag glass and developed a line of products manufactured so as to replicate the bands and stripes of the industrial waste product. Tiffany lamps are a good example of its use. These too may be washed ashore as sea glass, but are more transparent than the original smelter’s waste.