Furnace start-up defects: AZS exudation or corrosion?

Glass defects, such as knots, cords and cat scratches, are often attributed to fused cast AZS refractory. The mechanism behind these defect formations is thought to be either run down of viscous liquid containing high concentrations of Al2O3, Na2O and low concentrations of ZrO2 from superstructure AZS or disruption of the boundary layer on the glass contact AZS. When this viscous liquid does not completely dissolve in the glass melt, glass defects are formed, often containing ZrO2 inclusions. Such defects have been reported both shortly after start-up and during the life of the furnace campaign. This paper describes the difference between AZS exudation and corrosion as it pertains to defects during the beginning start-up of a newly lined soda-lime glass furnace. Exudation and corrosion/exudation tests following the ASTM C-1223-92 exudation procedures and modifications thereof were completed, and analyses of the exudate were compared with that of viscous knot start-up defects.