With war brewing in late-1930s Europe, the United States military needed domestic sources for optical glass. New developments like military aircraft — and the airborne surveillance tools and other instruments that went along with it — depended on top-quality optical glass previously available only from European glassmakers.
So Corning, already recognized as an expert in borosilicate glass processing, in 1938 launched a development program for the large-scale melting of optical glass.
The breakthrough project resulted in methods that not only caused a stir in the early 1940s — they also formed a foundation for the way the company still ensures the purity and optical quality of advanced glass today.
Corning scientists knew that melting optical glass in large volumes presented some thorny problems. Each batch had to be as free as possible from striations, seeds, bubbles, or inhomogeneities that would change the uniform refractive index of the glass and compromise optical quality. The bigger the volume, the greater the chances of producing substandard glass.
A team of researchers led by Dr. Charles DeVoe hit on a winning combination.
First, they developed a way to direct an electric current through the glass bath, melting the various ingredients evenly from the inside out and improving upon gas burners, in which some parts of the batch could boil while other parts were relatively unheated. It also enabled a “continuous melting” process, a big step up from the previous method of melting one giant clay pot at a time.
Just as important would be a new design for mixing the glass ingredients as they melted.
And a proper stirring method, especially with a mixture as demanding as molten optical glass, proved a remarkably difficult challenge.
Vigorous, random stirring — such as a cook might use for mixing a cake batter or sauce — would create bubbles or other imperfections in the molten glass mixture. It could also leave streaks of different compositions within the glass mass, rendering the entire batch unusable.
DeVoe and his team painstakingly experimented with every aspect of stirring. They used high-viscosity oils to mimic the molten glass mixture, and added colorants to study the behavior of fluids as they combined in the melting tank.
They intently observed and recorded every conceivable aspect of the process. Different blade shapes and angles created different flow patterns within the mixture. Changing the speed and other seemingly small variables could mean the difference between a homogeneous mixture and one that was wholly unusable.
The stirring was so crucial to the success of the project — and such a valuable piece of intellectual property — that the top-secret experiments were kept carefully segregated from other research operations. The final optimized stirrer design was quickly protected by a U.S. patent awarded to DeVoe in 1944, and Corning successfully began high-volume production of optical glass the following year.
More than seven decades later, stirring remains a crucial part of the melting and production processes of today’s advanced glasses.
For example, the proprietary fusion process that forms today’s super-slim advanced flat glasses — Corning® EAGLE XG®, Corning® Gorilla® Glass, Corning® Willow® Glass, Corning Lotus™ Glass, and Corning Iris™ Glass — is successful in large measure because of the precisely mixed glass composition that flows into the forming mechanisms.
Corning scientists and engineers have continued to build on DeVoe’s stirring methods over the years, updating it to suit the new glass compositions but still drawing on the basic concepts he introduced in that proprietary design.
Dozens of contemporary patents still reference the DeVoe innovation. And he remains an enduring icon of Corning’s deep understanding of glass melting and forming.
