1. Belding J. H. and Letzgus E. A.: US Patent 3 950 504, 13 April 1976.
2. Kingery W. D., Bowen H. K. and Uhlmann D. R. ‘Introduction to ceramics’, 656–667; 1976, New York, John Wiley & Sons.
3. Cranmer D. C. ‘Introduction to glasses and ceramics’, Vol. 4, 18; 1991, Materials Park, OH, ASM International.