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Borosilicate glass has been around for over 100 years. It was first formulated in Germany by Otto Schott in 1893 with the trade name Duran® that still exists today. In 1915 Corning Glass Works in the United States developed a similar material which was patented in 1915 under the famous trade name Pyrex® (Code 7740).
The whole idea behind making these borosilicate glasses was to have a durable material with a low enough coefficient of thermal expansion to survive rapid changes in temperature without cracking or breaking. In general the material consists of 70% pure silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide and 1% calcium oxide. The high silica content is the reason for the low expansion coefficient of the glass. The other constituents that have been added to the silica help reduce the melting temperature of the glass. For example pure fused silica Si02 has a softening point of 1600°C while typical borosilicate glasses are closer to 800°C. Because borosilicate glasses have such low working temperatures, this allows for an easier opportunity to manipulate i.e. melt and fuse the glass compared to fused silica and other zero expansion glasses.
This ability to cast the glass over a reasonable temperature range coupled with a low thermal expansion is the reason why the material was chosen as the primary mirror substrate in the Mount Palomar 200” and Lick Observatory 120” Shane telescopes.
In recent years, there has been a resurgence in the use of borosilicate glass with the introduction of light weight spin-cast technology developed by the University of Arizona’s Steward Observatory Mirror Lab. They use a variant of the glass known as E-6 manufactured in Japan by the Ohara Corporation. In fact these spin-cast primary mirrors are used in the largest and some of the largest telescopes including: the Large Binocular Telescope (2 x 8.4 meter); Large Synoptic Survey Telescope (8.4 meters); Magellan I and II (6.5 meters); MMT (6.5 meters) and more.
We at Hextek Corporation use Schott Borofloat 33® and Duran® borosilicate glasses for the production of light weight mirror blanks. Though we can cast substrates, the low working temperatures of the glass allows the use of our Gas-Fusion process to fabricate a sandwich structured mirror blank. Our substrates have been finished into mirrors and are currently being used as primaries, secondaries, tertiaries and instrument optics for large telescopes including the Large Binocular Telescope, MMT, Sloan Digital Sky Survey, Astrophysical Research Consortium 3.5 meter telescope, Keck I, Subaru, Lowell Observatory and others.