Why does the volume of the lining of the furnace wall of the induction melting furnace for melting cast iron become smaller and smaller?

The induction melting furnace melts pig iron scraps, with many impurities, low temperature of the furnace, and the glaze slag is condensed on the furnace wall, so it is getting smaller and smaller. Increase the temperature of the furnace to 1550℃, and the furnace will be discharged at a high temperature to melt the impurities on the furnace wall. It is too serious and can only be rebuilt.