The slag furnace works similarly to the vanilla furnace, but has an extra output slot. It also uses twice the fuel. For most ores, it will produce an ingot with a low (20%) chance of making two instead of one, and also some Stone.
High Al2O3 slag is encountered mainly with Indian blast furnaces because of high Al2O3/SiO2 ratio in the iron ore as well as in the sinter and high ash content in the coke. These slags are having high viscosities. The viscosity of liquid slag is determined primarily by its temperature and the chemical composition.