1. G. Hetherington,J. Brit. Ceram. Soc. 3 (1966) 595.
2. H. Scholze,Glass Ind. 47 (1966) 546, 622, 670.
3. J. C. Neerman andF. R. Bryan,Anal. Chem. 31 (1959) 532.
4. M. Helzel,Amer. Ceram. Soc. Bull. 48 (1969) 287.
5. J. B. Taylor, private communication.