The Effect of Alumina on the Devitrification of a Soda‐lime‐silica Glass 1

The effect of Al 2 O 3 the devitrification temperature of a soda‐lime‐silica glass approximately of the composition Na 2 O.CaO.6SiO 2 has been studied in two series of experiments. In one of these CaO was partially replaced by Al 2 O 3 until the final glass had the composition of the mineral albite, Na 2 O.Al 2 O 3 .6SiO 2 ; and in this series of experiments the melting point of the glass was first lowered, until the glass passed from the tridymite into the Na 2 O.3CaO.6SiO 2 field; then rose slightly to a maximum, fell until the glass entered the field of CaO.SiO 2 , and then rose rapidly until the field of albite was reached. Glasses near albite in composition are practically impossible to crystalliie in the dry way. In the second series of experiments, Al 2 O 3 was added to the same glass, and it was found that the fields of tridymite and Na 2 O.3CaO.6SiO 2 were quickly passed through, with a smaller lowering of melting point than in the preceding series, after which the melting point rapidly increased, with CaO.SiO 2 as primary phase. It is shown that no generalizations can be made as to the effect of alumina on the devitrification of soda‐lime glass. The problem is one of the freezing‐point relations in a four‐component system, and in the absence of definite knowledge of the liquidus temperatures and corresponding stability regions in that four‐component system general statements would probably be misleading. In addition, the devitrification of some five‐component soda‐lime glasses containing both Al 2 O 3 and MgO is discussed.