1. Poulin, M and Lucas J: Mat Res. Bull. 10, 243 1975.
2. Baldwin CM, Almeida RM and Mackenzie JD: J. Non-cryst. Solids 43, 309, 1981.
3. Sun KH: Glass Ind. 27, 552, 1946.
4. Mellor JE: Comprehensive Treatise on Inorganic and Theoretical Olemistry, Vols. VIII and IX. London: Longmans Green & Co., 1929.
5. Winter A: J. Am. Ceram. Soc. 40, 54, 1957.