1. W.W. Anderson and L.W. Rowe, US Patent 2 770 529, November 13, 1956.
2. A.J. Gaskin and A.E. Ringwood, US Patent 2 954 278, September 27, 1960.
3. Columbia-Southern Chemical Corporation, UK Patent 795 164, May 21, 1958.
4. R.G. Becher, Australian Patent 241 110, 1963.