1. D. J. Godfrey and P. G. Taylor, ?Special Ceramics 4?, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1968) p. 265.
2. N. L. Parr and E. R. W. May, Proc. Brit. Ceram. Soc. 7 (1967) 81.
3. P. L. Pratt, ?Mechanical Properties of Engineering Ceramics?, edited by W. W. Kriegel and H. Palmour III (Interscience, New York, 1961) p. 507.
4. P. B. Noakes and P. L. Pratt, ?Special Ceramics 5?, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1972) p. 299.
5. R. F. Coe, R. J. Lumby and M. F. Pawson, ibid.? p. 361.