328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



sensitive type perhaps unique to glass. The metal dissolves as an 

 oxide in the melt, is frozen in an unstable oxidized state as the glass 

 cools, then is slowly reduced to insoluble metal colloid by polyvalent 

 ions in the glass as the glass is reheated at low red heat. Some of the 

 polyvalent reducing agents are oxides of tin, selenium, antimony, and 

 arsenic. Ultraviolet light promotes the reduction of copper, by 

 producing photoelectrons which then combine with copper ions in the 

 glass. 



This knowledge made it possible to develop photosensitive copper 

 ruby and gold ruby glasses, which remained colorless even on reheat- 

 ing, except in the areas exposed to ultraviolet light through a photo- 

 graphic negative. 



Then it occurred to me that these photographically produced metal 

 crystals, if they were precipitated in a glass supersaturated with so- 

 dium fluoride, might trigger the growth of sodium fluoride crystals 

 and give me the photosensitive opal glass I had failed to produce 

 before. This proved to be true ; not only could sodium fluoride, but 

 several other kinds of crystals be nucleated in three-dimensional pho- 

 tographic patterns. The translucent windows in the north wall of the 

 United Nations Assembly Building are made of photosensitive opal 

 glass with a marble pattern produced in this manner. 



A year or so later, in the early days of television, we were confronted 

 with a different practical problem: how to drill a quarter-million 

 small, precise holes through a glass plate, to make an aperture mask ? 

 On a long-shot chance, I tested all the photosensitive opal glasses and 

 discovered that the crystallized photographic pattern in one of them, a 

 lithium silicate glass that had been shelved as useless, was much more 

 readily dissolved in hydrofluoric acid than was the surrounding clear 

 glass. Before long we had a plate of glass with a hexagonal array of 

 small holes; and this led to the development of chemically machine- 

 able glass. This glass is finding increasing use in the electronics 

 industry because of its capability of being mass-produced in precise 

 complex shapes like those shown in the panels illustrated in plate 2, 

 figure 1. 



AN ACCIDENT LEADS TO GLASS CERAMICS 



Chance, in the form of a runaway furnace, now took a hand. A 

 plate of photosensitive glass that had been irradiated was accidentally 

 heated to several hundred degrees higher than its usual developing 

 temperature. The plate, which we had expected would melt to a pool 

 of glass, altered instead to a hard, strong crystalline ceramic — the first 

 member of a now rapidly growing family of crystalline ceramics 

 made from glass, the Py rocerams. This was not an isolated case ; my 



