How many pairs of Stookey lenses has your practice dispensed this week? Being glass, the answer is probably not many, as most lenses are resin these days; but for the principle of photochromic lenses we have the pioneering glass scientist, Stanley Donald Stookey, who died in November 2014 aged 99, to thank. And if you’re reading this on your tablet or mobile – well, you have Stookey (and some frozen chickens) to thank again.
Born in the small village of Hay Springs, Nebraska, in 1915, to two teachers, he took a master’s degree in chemistry before gaining a doctorate in physical chemistry in 1940 from Massachusetts Institute of Technology. He had two job offers, from Corning Glass Works and Nabisco, the baking and food company. Not wanting to go into baking, he opted for glass research which, ironically, involved baking glass at high temperatures.
A fascination with the process of ‘nucleation’ ensued, as a way of producing crystalline structures within glass by the addition of tiny quantities of elements or compounds to the molten material, that would act as nuclei for the formation of crystals as the glass cooled. The amount and rate of cooling could control the crystallisation process. By this method he found he could add copper or gold to produce ruby glass; and sodium fluoride to make milky-white opal glass. Stookey initially concentrated on the latter, investigating its little-known chemistry in the hope that some useful applications could be found for it. By 1950 he had found a way to make it photosensitive, and registered the first of his 60 US patents for a form of this ‘Fotoform’ glass that, in daylight, resembles marble. A testament to the invention is the north face of the UN headquarters in New York which is clad in this material.
The ruby glass, too, could be made photosensitive, in this case to ultraviolet light, providing a method whereby photographs could be ‘implanted’ in the glass through its exposure to UV. Applications included an idea for copper-infused ruby glass coins to be produced with Abraham Lincoln’s portrait suspended within, as a way of reducing the reliance on scarce copper resources in wartime America. The US Treasury seriously considered the possibility, but decided it was too expensive; as also proved the case for the spies’ spectacles that would reveal secret messages on exposure to the right kind of light.
An accident with some Fotoform glass produced a serendipitous interlude in Stookey’s photosensitive researches, resulting in his discovery of a completely novel class of material and a huge windfall for Corning. The incident occurred in 1952, when Stookey had placed a sample of his photosensitive glass in a furnace, to be heated to 600ºC. On returning some time later he noticed that the temperature gauge had become stuck at 900ºC. Expecting to find the interior of the furnace ruined by a molten mess, he was instead surprised to find that his sample was still intact. In retrieving it with a pair of tongs, he dropped the sample and was stunned when, rather than shattering, it bounced. ‘It sounded like a piece of steel bouncing,’ Stookey recalled, ‘so I figured something different must have happened.’ Indeed it had: Stookey had inadvertently created the first synthetic glass-ceramic.
The lithium silicate that had been transformed accidentally into a new milky-white material proved to be lighter than aluminium but harder than high-carbon steel and immensely stronger than ordinary soda-lime glass. Corning realised that this made it perfect for cooking with, patenting it in 1960 as Pyroceram and marketing it as CorningWare. The white dishes decorated with blue cornflowers became ubiquitous in US kitchens, earning Corning a fortune. But there were also other applications. Pyroceram’s strength and lightness made it desirable for use in chemistry laboratories and microwave ovens, while NASA used the same manufacturing process for its shuttles’ glass-ceramic nuts and bolts; and transparency to radar made it the ideal material for missile nose-cones.
Photochromic developments
Meanwhile Stookey turned his attention to the properties and possible applications of reversible photochromatic glass, ie materials that change colour when exposed to light but return to their original transparency when that light is removed. Together with his colleague, William H Armistead, he patented such a material in 1962 (US Patent 3,208,860: Phototropic material and article made therefrom).
In a paper published in Science (‘Photochromic Silicate Glasses Sensitized by Silver Halides’, W H Armistead and S D Stookey, Vol 144, No 3615, pp 150-154, 10 April 1964), they pointed out that ‘hundreds of photochromic organic and inorganic substances’ have been described. But for them to have useful applications they need to exhibit an important property, hitherto not found in any existing material: ‘True reversibility of the colour change – that is, freedom from fatigue with repeated light-and-dark cycling.’
Armistead and Stookey achieved their aim by adding quantities of silver halide crystals to borosilicate glass. In essence, exposure to light would cause a chemical reaction to occur similar to that in traditional photographic film; except that this reaction is reversible. Only tiny amounts of these crystals were required: less than 0.1 per cent by volume, comprising crystals less than 0.1 microns across (about 100 times thinner than a human hair). Corning first marketed a glass photochromic lens using this technology in 1968, under the Photogray brand. Photobrown, Photogray Extra and Photobrown Extra followed. In the UK, Pilkington introduced their grey and brown Reactolite and Reactolite Rapide versions in the 1970s.
Project Muscle
The success of CorningWare had helped to fund another area of research, named Project Muscle, which was aimed at developing other ways of strengthening glass. It was found that adding aluminium oxide to a glass mixture before it was submerged in a reinforcing hot potassium salt bath resulted in a remarkably strong, durable glass. It could be bent and twisted extraordinarily before fracturing and could withstand 100,000 pounds of pressure per square inch (compared with 7,000psi for standard glass). The Corning scientists tested the material’s limits by dropping samples from the top of their nine-storey building and by bombarding it with the aforementioned frozen chickens. Although it was marketed in 1962 as ‘Chemcor’ and was used in a small way for spectacle lenses and some other applications, it was really a material ahead of its time.
The advent of mobile phones, in particular Motorola’s flip-top Razr V3 model of 2005 that featured a glass, rather than a plastic, screen, got Corning thinking about their old Chemcor samples. Then, in 2007, Apple came calling. Their requirement was for a huge amount of then non-existent chemically toughened – and optically excellent – 1.3mm thick glass for their new iPhone. But the Chemcor samples were 4mm thick. A huge effort to reformulate Chemcor and devise a manufacturing process for the new material resulted in the production of the first ‘Gorilla Glass’ by May of that year. Most devices you touch or swipe now are likely to involve interaction with Gorilla, probably in its 20 per cent stronger second or, even better third, version.
Stookey, who was still consulting at Corning aged 97, received the National Medal of Technology in 1987 from President Reagan for his research into glass ceramics, photosensitive glass and photochromic glass. In 2010 he was inducted into the National Inventors’ Hall of Fame. Stookey recalled that when he began working at Corning, ‘glass chemistry research had barely started. My main objective was to be a pioneer, discover new things, produce things that had never been seen before.’ Or, indeed, seen through a glass darkly.
David Baker is an independent optometrist