corundum, synthetic corundum.7, Verneuil, Linde, gems

A.K. Seemann reviews the development of American synthetic crystal production during and after the Second World War, explaining the manufacture, properties, and commercial applications of corundum, spinel, star corundum, and titania. Adapted from a presentation to the American Gem Society’s 1949 conclave, the article describes how wartime demand for instrument bearings stimulated a domestic industry that subsequently developed new crystal forms and gemstone materials. The basic Verneuil process is outlined: finely prepared oxide powder passes through an oxyhydrogen flame, melts, and crystallizes on a progressively lowered support. Control of powder supply, flame conditions, and lowering rate determines the growth and dimensions of the boule. Seemann discusses powder preparation by calcining purified salts, the introduction of coloring agents, and the importance of purity, noting that some contaminants become visible even at concentrations of a few parts per million. Chromium produces ruby, while titanium and iron additions produce blue sapphire; various metal oxides provide colors in synthetic spinel. A major American advance was the growth of long, cylindrical sapphire rods, reducing the labor and material loss involved in converting conventional split boules into small bearings. Studies connected crystallographic orientation with splitting, transverse cracking, and deformation at high temperatures. Near the melting point, rods could be bent or stretched, with material oriented at approximately 45° between the c-axis and growth direction proving easiest to deform. Flame polishing offered an economical means of obtaining smooth surfaces and rounded phonograph-needle tips, whose service life also depended strongly on crystal orientation. High-temperature annealing permitted larger whole boules to be worked without splitting. Additional applications included precision sapphire balls for pen tips and decorative use. Seemann identifies synthetic star ruby and sapphire as a particularly striking development, attributing their asterism to fine needles while withholding the manufacturing method pending patent action. The final section treats titania, synthetic rutile, emphasizing its exceptionally high refractive indices and dispersion alongside its relative softness and cutting difficulties. Produced by a modified Verneuil process, the initially black, opaque, oxygen-deficient crystals become transparent after heating in an oxidizing atmosphere, with a corresponding large increase in electrical resistance. The material’s potential applications therefore extend beyond gems to monitoring furnace atmospheres. For gemstone use, crystal orientation, accurate facet angles, and suitable polishing procedures are essential to avoid a cloudy appearance and excessive rounding of facet junctions. Seemann recommends alumina for final polishing and cautions against overheating during sawing, cutting, or polishing because changes in oxidation state can cause discoloration or fracture. The article presents gemstone synthesis and industrial crystal engineering as closely connected fields, with advances in growth control, orientation, annealing, and finishing supporting both jewelry and technical applications.