Slag is a material added by Thermal Expansion. It is a byproduct of various recipes of the Induction Smelter. Slag has a rarer variant called Rich Slag, which is sometimes produced when Ore blocks are processed in the Induction Smelter.
Regular Slag can be used to craft Clay, Phyto-Gro or Florbs. It can also be smelted to produce Rockwool. Rich Slag can be used to craft Rich Phyto-Gro. Furthermore, it can be used in Induction Smelters to triple the amout of Ingots processed from metal Ore blocks.