There are some harmful components in the fused cast AZS block that cannot be removed completely. Those harmful components include glass phase and carbon.

Fused cast AZS refractories are composed of baddeleyite, corundum and a small amount of mullite, glass phase and pores. During the manufacturing process, in order to reduce and eliminate the residual thermal stress, glass phase exists as an absorber of thermal stress. However, glass phase also brings some drawbacks and becomes the weak point of glass furnaces in term of corrosion. During the corrosion process, the glass phase precipitates first and replaced by molten glass which accelerates the corrosion to crystals. At high temperature, glass phase will exude and release bubbles and pollute the glass melt.
 

For fused cast AZS products made by the reducing method, carburizing is serious. The amount of carbon in the bricks can be judged according to the color of the section of refractories. If the color is light gray, the amount of carbon is about 0.03-0.06%; off-white <0.03%; gray 0.08%-0.18%; dark gray 0.15-0.19%. Even in the products made by the oxidizing method, there is some carbon (<0.01%). The carbon exists in the form of carbide mixed with nitrides, sulfides, solid carbon and residual reducing gases. The bricks will release gases when used under oxidizing conditions, which will cause the reduction of the exudation temperature of glass phase. When AZS bricks contain more glass phase (usually about 20%), glass phase will soften and exude above 1100℃. The gases (CO2, SO2) released by the impurities will also accelerate the exudation of glass phase. Without the bonding of glass phase, crystals are peeled off, which is the reason of corrosion of AZS bricks.