The history of glass research in Jena

By Sebastian Hollstein

Melting glass in the garden

In the early 19th century, there was only one way to find out the right material composition for optical glasses: trial and error. That’s why Friedrich Körner set up a small glass melting furnace next to his house at Grietgasse 10 shortly after taking on a position as a university mechanic in Jena in 1816. His little project was even funded by Grand Duke Charles Augustus himself.

He wanted to produce flint glass, which was well suited for eyepieces and telescopes due to its optical properties, and which was usually imported from England at the time. He tried out different furnace designs. Körner then produced several hundred kilograms of glass in numerous high-profile experiments, but the material was not up to scratch. Most of it had a coloured cloudiness; Körner was actually pleased with one glass, but it failed the quality control run by Joseph von Fraunhofer, the most respected expert in the field at the time.

In order to advance the experiments, the Grand Duke provided Körner with an assistant, Johann Wolfgang Döbereiner, who was a professor at the University of Jena. The chemist focused primarily on the stoichiometry, i.e., the correct ratio of all components. Their cooperation was a success. For example, the team was able to produce baryte glass in 1828. One journal reported that it was »clearer, harder and specifically heavier than the best crown glass – and had a greater refractive index«.

Calculating optics

Körner laid the foundations for Jena’s glass tradition not only as a scientist, but also as the teacher of someone destined for greatness: In 1834, an 18-year-old Carl Friedrich Zeiss began a four-year apprenticeship with Körner, who was working as an associate professor. Zeiss ultimately opened his own workshop in 1846 and produced his first microscopes just one year later.

Zeiss began to deal intensively with the scientific foundations of mathematics and optics because he wanted to move away from manufacturing lens systems by trial and error, where he would try out different lens combinations, and instead wanted to perform precise calculations. In addition to his own studies, he could fall back on the expertise of a scientist who was 24 years his junior, Ernst Abbe, who had qualified as a professor at the University of Jena in 1863 before working as an associate professor and freelance research assistant at Zeiss’ workshop. In 1870, he became a professor at the University of Jena.

His research – above all his theory of microscopic imaging – ultimately allowed the scientists to achieve their goal: In 1872, Zeiss sold the first microscopes whose optics were based on calculations and whose performance was unmatched by any other product on the market. In order to further advance the production of their equipment, they just had to find an appropriate manufacturer of optical glasses who met their high standards.

»Schott & Co.«

Glass research in Jena is inextricably linked to one name: Friedrich Otto Schott. Born in Witten in 1851, he developed a fascination for the transparent and fragile material at an early age. His grandfather was a glazier, and his father ran a sheet glass factory from 1853. So it comes as no surprise that Otto Schott decided to study chemistry in Aachen. He went on to specialize in glass at Leipzig University before graduating from the University of Jena in 1875, writing his doctoral thesis on the topic of »Contributions to the Theory and Practice of Glass Manufacture«.

However, another much less extensive written testimony was more important in promoting Jena as a location for glass research: On 27 May 1879, Otto Schott wrote a letter to Ernst Abbe to inform him that he had succeeded in producing a glass »containing a significant amount of lithium«. »I assume that the glass in question will have outstanding optical properties in any direction and wanted to take the liberty of asking you whether you would be willing to test its refractive and diffusive properties – or to have them tested by one of your apprentices – to verify whether my assumption is correct. Thanks to your connections to the local Zeiss Optical Institute, it will be easy for you to have the necessary grinding work done on the glass without having to expend any effort yourself«.

Abbe and Zeiss were unable to do much with the material sent by Schott, but they saw great potential in the young glassmaker. They were especially impressed by the fact that Schott had managed to produce melt samples in small crucibles. In 1883, they joined forces to set up a »Glass Technology Research Station« in Jena, which was renamed the »Glass Technology Laboratory, Schott & Co.« one year later – and soon developed into a reliable supplier of optical glasses.

The reinvention of glass

Three years later, in the newly founded company, Otto Schott made a ground-breaking invention that cemented the role to be played by glass in the natural sciences and technical fields. The established types of glass at the time, soda-lime glass and lead glass, were attacked by chemicals and even water. In addition, they often broke due to excessive heat and rapid changes in temperature. A new material was therefore required for experimental investigations in the laboratory – and Schott came up with the solution.

He added boron oxide to the traditional glass component silicon oxide, thus eliminating the weak points of the other types of glass. His borosilicate glass quickly established itself on the market: the company initially used the new material to produce thermometer glass from 1891, and the first laboratory vessels such as beakers and conical flasks were sold two years later. The range was subsequently expanded with the addition of glass cylinders for gas lamps.

The portfolio grew steadily thanks to different variations in glass composition, including household glass – the famous glass from Jena that was produced by Schott from the 1920s. Borosilicate glass still has a wide variety of applications today. It is even used in space telescope optics in the form of highly thin float glass.

Phase separation in glass

The development of electron microscopy offered glass chemists around the world – including Jena – new opportunities for in-depth research into the structure of glass. In 1955, Werner Vogel started using this instrument to carry out an extensive, ground-breaking series of experiments on beryllium fluoride glasses at the Schott & Co. glassworks, in order to investigate the phenomenon of phase separation in glass, which had already been observed by Otto Schott.

After all, contrary to what was commonly assumed, glass components do not necessarily mix to form a homogeneous material, but can form micro-heterogeneous areas during production which, as shown by Vogel through his investigations, take the form of tiny droplets and look cloudy to the naked eye. In the early 1960s, scientists still believed that such separation phenomena were exceptions that could possibly even be traced back to microscopy itself; however, phase separation in glass subsequently emerged as one of the most important branches of research in this field.

Werner Vogel, who researched and taught at the University of Jena from 1969 to 1990, achieved international recognition through his work. His research paved the way for new optical and technical glasses to be developed.