1950. ------. [KRCZIL, F.] [Siliceous Material.] Spanish Patent 129,625, no date. Kainer, p. 34.

See German Patents 581,123 (abs. 1952) and 596,093 (abs. 1951), French Patent 748,584 (abs. 1949), and Czechoslovak Patent 60,124 (abs. 1948).