1. I. M. Stewart, private communication.
2. S. D. Stookey, British Patent No. 752,243 (1956).
3. I. M. Stewart and G. J. P. Buchi, Trans. Brit. Ceram. Soc. 61 (1962) 615.
4. W. D. Kingery, ?Introduction to Ceramics? (Wiley, 1960), p. 478.
5. R. E. Mould and R. D. Southwick, J. Am. Ceram. Soc. 42 (1959) 542.