A part of a soldier block, placed in a float glass furnace near the hot spot area, was
investigated to learn about the changes in the microstructure during the production of the block,
during the use for glass melting and after the shut down of the furnace and the cooling of the block.
Beside the three phases after the production (baddeleyite, corundum, vitreous phase) during use as a
soldier block mullite and secondary corundum as well as secondary zirconia may occure. Cooling
down the used block after the furnace campaign the beginning of the crystallisation of feldspars
may be seen.