File:KENT-7D67A7, Blast Furnace Slag (FindID 482545).jpg

English: Five fragments of Blast Furnace Slag of Post-Medieval date. The slag is irregular in shape with many broken edges. It is black to light grey in colour, with layering of colours in many areas. These five fragments are only a representation much more of the material that was found in the area.

Dr Justine Bayley comments "Blast furnace slag was produced in vast quantities as a by-product of iron smelting and shipped all over the country for use as hardcore, especially in areas that had no local sources of hard stone. It's used as ballast under railway tracks, and in smaller quantities gets dumped in muddy cart tracks and field gateways etc - so it can turn up almost anywhere. It's much less dense than medieval and earlier smelting slags as much of the iron is replaced by lime. This means you can extract much more of the iron present in an ore, but the disadvantage is that the slags are only liquid at significantly higher temperatures, which only became practicable with larger furnaces and water-powered bellows - a late to post-medieval innovation."