Larger ZERODUR® production capacity set to secure jobs and enable reliable delivery

With a state-of-the-art melting tank for ZERODUR® glass-ceramic production, SCHOTT is increasing its production capacities in Mainz and ensuring reliable delivery for its customers. The new second tank, exclusively reserved for ZERODUR®, is intended to meet the long-term high demand for the material and contribute to customers’ success. In addition, the investment will secure jobs at the Mainz site, SCHOTT’s center of expertise on glass-ceramics. The new glass melting tank for the production of ZERODUR® glass-ceramic has now been ceremonially “fired up,” as insiders call the official start to operation.


Fired up: what does that mean?

During firing up, the burner lance of a mobile gas-air burner is inserted into the completely empty tank interior and ignited. There is a special reason for this: before glass can be melted, the residual moisture of the special stone-on-stone refractory material must first be removed with a tempering device and the tank must be brought to an operating temperature of at least 1,000 degrees Celsius. Only then can the actual glass melt begin in the new tank. The pre-mixed raw materials – in the case of ZERODUR®, mainly silicon, aluminum and lithium – are introduced into the pre-heated tank over a period of several days and brought to melting temperatures of more than 1,600 degrees Celsius using a combination of electric and fossil heating.

The new tank at the Mainz site is a so-called discontinuous tank, which means that it is completely filled and emptied at periodic intervals. The molten glass takes a form according to a fixed casting plan, either round or rectangular depending on the final product. The new Mainz tank is also designed for mirror substrates with a diameter of up to 4.25 meters. The two ZERODUR® mirror substrates for the secondary and tertiary mirrors of the Extremely Large Telescope, for which SCHOTT received the order from the European Southern Observatory (ESO) in mid-January, will be cast from this tank.
Fired up at SCHOTT

What happens after casting?

In order to produce a glass-ceramic with the required properties, the so-called raw glass must be subjected to a temperature process in both cooling and ceramizing furnaces. At first, the temperature is slowly increased to such an extent that crystal nucleation begins. After a determined holding time creates the desired number of crystal nuclei, the temperature is further increased to the range of crystal growth. After setting the desired crystallite sizes, blocks are cooled again in a defined manner. The correct ratio of the temperature-increasing glass phase, about 30 percent, and the shrinking crystal phase of about 70 percent with the correct number and size of the crystallites, ensure the combination’s extremely low thermal expansion.

Tank melt diagram

  1. Fusing: The batch consisting of high-purity raw materials that has been placed inside the furnace is heated up inside it. Then, a chemical reaction takes place --> formation of a molten mass.
  2. Purification: Elimination of the bubbles created by chemical reactions.
  3. Homogenizing: Stirring of the bubble-free melt.
  4. Casting & Hot Forming: Liquid glass exits the feeder and is molded into bars, rods or blocks.
Tank melt diagram
Capacity expansion in record time - Thomas Lifka

Capacity expansion in record time

Certified engineer Thomas Lifka, the material scientist responsible for processing at SCHOTT Advanced Optics, on the challenges of constructing the second ZERODUR® glass-ceramic melting tank and perfect teamwork.

Mr. Lifka, you have more than ten years of experience in glass-ceramic production. What is different between current tank construction projects and those in the past?

It used to be that an existing ZERODUR® glass-ceramic tank would be dismantled and rebuilt. This meant that no glass could be melted for a few weeks. In view of the extremely high demand for ZERODUR® glass-ceramic, however – not only for mirror substrates, but also for other applications – it was clear: that this wouldn’t work anymore! That’s why we changed our plans at short notice. Within just three months, the new melting tank, designed in accordance with state-of-the-art technology, was put into operation. This is a capacity expansion in record time!
How was that possible?

In addition to outstanding know-how, carrying out a complex project like this in a short time requires flexibility, especially from other departments involved. We received really great support from the high-tech experts at our Technical Services who did everything possible for us, despite requirements for other tank repairs.

"A thrilling turbo
project with an
awesome team. "

What distinguishes the new melting unit?

We have a state-of-the-art melting tank with the latest features. Internal know-how benefits not only tank construction, but innovative burner technology, a sophisticated process control system as well as measuring and control technology. The project has demonstrated how important it is to have such qualifications and engineering in-house. After all, everything has to be perfect when melting glass: components have to tolerate extremely high temperatures, and the heavy glass-ceramic blocks can’t contain any glass defects or impurities. The highest precision is needed here.

Your final impression?

A thrilling turbo project with an awesome team. When it comes to casting, we all know what parts we play and that we can rely on each other. This and the now-possible simultaneous operation of both tanks offer the best conditions for a large tank yield, satisfied customers and last but not least the lasting success of the ZERODUR® glass-ceramic business.
Fired up at SCHOTT
Christoph Fark - Herman Ditz - Ralf Reschke – Dr. Thomas Westerhoff - Ludwig Dürsch - Christopher Klein