1. Gold Bulletin, 1971, 4, (2), 30–32
2. U.S. Patent 3,515,571, Donald J. Levy, assigned to Lockheed Aircraft Corp.
3. U.S. Patent 3,176,678, Robert C. Langley, assigned to Engelhard Industries Inc.
4. U.S. Patent 3,176,679, Robert C. Langley, assigned to Engelhard Industries Inc.