Non-metallic impurities agglomerate and deposit on furnace walls. In the course of time, they change to corundum which adheres fixedly to the furnace wall. These built-ups attack the furnace walls and reduce the capacity of the furnace. If they are not removed, they infiltrate small surface irregularities and cracks and destroy the walls due to which the isolation effect is reduced. Schaefer’s Wall Cleaner Flux softens the corundum built-ups on the wall. After the treatment these built-ups can be pushed off by means of a tool.