1. N. L. Parr, G. F. Martin andE. R. W. May, ?Special Ceramics?, edited by P. Popper (Haywood and Co., London 1960) p. 102.
2. A. F. McLean, Proceedings of the 2nd Army Materials Technology Conference on Ceramics for High Performance Applications, edited by J. J. Burke, A. E. Gorum and R. N. Katz (Brook Hill, Chestnut Hill, Mass, 1974) p. 9.
3. R. J. Bratton andA. N. Holden,ibid. (1974) p. 37.
4. D. J. Godfrey, Ceramics for High Performance Applications Conference, Newport, RI, USA (1977) to be published.
5. D. J. Godfrey andM. W. Lindley,Proc. Brit. Ceram. Soc. 22 (1973) 229.