Chemically strengthened glass

Chemically strengthened glass emerged because thin glass cannot be thermally tempered, yet the applications demanding thin glass were multiplying. Steven Kistler's 1962 ion-exchange process solved a problem that wouldn't find its killer application for another 45 years.

Thermal tempering works by cooling glass surfaces faster than the interior, creating compressive stress that resists breakage. But as glass gets thinner, establishing a meaningful temperature difference between surface and core becomes impossible. Thin glass and thermal tempering are fundamentally incompatible. For decades, this seemed an acceptable limitation—most applications requiring strength could accommodate thick glass.

Corning's 'Project Muscle' began investigating alternatives in 1960. S. Donald Stookey started researching ion exchange by June of that year, and the concept was discussed at a Florence symposium in September 1961. The breakthrough came in 1962 when Kistler, working independently alongside Paul Henri Acloque and Jean Paul Tochon at Saint-Gobain, demonstrated that ion exchange could triple the compressive strength of glass surfaces.

The process exploits the size difference between sodium and potassium ions. Glass is submerged in a molten potassium salt bath at approximately 400°C. Smaller sodium ions migrate out of the glass surface while larger potassium ions from the bath replace them. These oversized potassium ions crowd together, and when the glass cools, this crowding creates intense compressive stress—the same protective layer that thermal tempering produces, but achievable in glass far too thin for thermal methods.

Corning researchers found that adding aluminum and zirconium oxides improved the effect further. By the mid-1960s, they had developed 'Chemcor,' a chemically strengthened glass marketed for applications where strength-to-weight ratios mattered: phone booths, prison windows, eyeglasses, automobile windshields, and aviation glazing. In 1968, approximately 100 Dodge Dart and Plymouth Barracuda racing cars used Chemcor windshields where minimizing weight was essential.

Chemcor remained a specialty product through the early 1990s, used in commercial and industrial applications but never achieving mass-market scale. The glass industry had solved the thin-glass-strength problem, but no mass market demanded thin, strong glass.

That changed when Apple approached Corning in 2006 to develop a display screen for the iPhone. The smartphone required glass that was simultaneously thin (for weight and touch sensitivity), strong (to survive being carried in pockets), and optically clear (for display quality). Corning revived its decades-old ion-exchange expertise, though Gorilla Glass became a different formulation than Chemcor.

By October 2017, Gorilla Glass was used in approximately five billion devices worldwide. The strengthening technique that Kistler developed to solve an obscure materials science problem had become essential to the smartphone revolution—but only after waiting four decades for the application that needed it to emerge.