synthetic corundum.7, gems

This digest, based on a report in the January 1947 issue of Watchmaker, Jeweller & Silversmith, summarizes German wartime production of synthetic sapphire and spinel, giving particulars of manufacturing capacity, operating conditions, product quality, and an adaptation for industrial use. Two factories are identified: the I.G. Farben plant at Bitterfeld and the Wiedes Karbid Werke at Freyung. Both employed the basic Verneuil process to produce boules. The Freyung installation had 204 burners and a stated capacity of 60,000 carats daily, while the Bitterfeld factory had approximately 300 burners and a somewhat greater capacity. At the time described, aquamarine-colored synthetic spinel accounted for 90 percent of the Wiedes plant’s production. The operating data indicate boule weights of 150–200 carats, growth periods of four to five hours, and cooling periods of half an hour to one hour in the furnace. Powder tapping occurred at a frequency of 60–80 times per minute. Hydrogen consumption was reported as four cubic meters per hour, with a hydrogen-to-oxygen ratio stated as 2:3, and overall powder utilization of 60 percent. These figures accompany a breakdown of production quality: under normal operating conditions, 60 percent of the boules were classified as first quality, 20 percent as second quality, and 20 percent as third quality. The report therefore records both the scale of manufacture and the distribution of its output among commercial grades. Particular attention is given to wartime work by Wilhelm Epler, collaborating with Albert Maucher and Drescher Kaden. A serious shortage of diamond powder prompted them to develop a method for hardening the surfaces of synthetic spinel jewel bearings after the bearings had been fashioned. The resulting bearings could be worked with carborundum powder, whereas corundum required diamond powder. The digest presents this as the most interesting wartime development in the German synthetic industry. It closes by stating that no evidence had been found that the German factories had developed synthetic corundum or spinel in rod form comparable to material being manufactured in the United States.