1. Blazy, P., French Pat. 2,225,528 (1978).
2. Demarthe, J.M., Gandon, L. and Georgeaux, A., French Pat., 2,271,298 (1974).
3. Demarthe, J.M. and Georgeaux, A., French Pat., 2,373,610 (1976).
4. Mémoires et etudes scientifiques;Van Ceulen;Rev. Métall.,1982