MWednesday is pouring day in Schott AG’s Zerodur glassworks. As on many Wednesdays in the past two years, this morning the employees in the hall on the factory premises in Mainz Neustadt provided a drum-shaped steel mold lined with white mineral wool – about 80 centimeters high and 160 wide. The glass, which will soon begin to solidify, will become six primary mirror segments for the Extremely Large Telescope (ELT).
“We can see it better from above,” says Thomas Westerhoff, head of the Zerodur division at Schott, and leads the visitor up a few steps of a steel staircase. Below us there is a gap of a hand’s breadth, over which the men in protective suits are now placing the mold. The holy of holies of every glassworks is enthroned above us: the melting tank. Two days earlier they had been filled with several tons of a white powder. Composed mostly of quartz, alumina, and lithium carbonate, it is now a 1400-degree mass. In the next moment we get to see them. Shining white and with a consistency reminiscent of honey, it swells out of a tube.
This glass is not for looking through
Soon the form is full and pushed aside. Some of the glowing honey flows briefly through the gap in the floor, then the pipe is closed again. Hanging remains of the melt solidify and glow down to form glass stalactites. One of the men is there with a hammer and knocks it off. Like the excess melt beforehand, the shards disappear into the ground, where a cooling pool of water catches them. The next empty form can be brought. However, it is rectangular, so it has nothing to do with the ELT segments. But other high-tech devices, such as those for semiconductor lithography, also need optical elements made from Zerodur.
Removal of the filled form
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Image: SCHOTT
“It’s technical glass,” emphasizes Westerhoff. In the ELT, its polished and silvered surface will only reflect the light from the depths of the universe. Zerodur is actually not suitable for lenses and other things that should absorb light as little as possible, at least not for all wavelengths. This can be seen in the ELT blank, which was cast on another Wednesday and has since left the so-called cooling furnace, where the freshly filled molds cool down in a controlled manner. The glass cylinder has an appetizing amber hue but is completely transparent. However, the material in front of us is not yet real Zerodur.
This only occurs when the blanks are heated again to almost 800 degrees. Then comes what is known as ceramization: tiny crystals germinate in the midst of the randomly distributed molecules that are typical of glass. In the end, they are just 70 nanometers in size. Glass has become glass-ceramic, and it now has the property that makes it used where the highest level of precision is required: its expansion hardly changes with temperature fluctuations. Because when it heats up, for example, the glass matrix expands, but the carefully dimensioned small crystals contract along one of its axes by exactly the same amount.