MODERN GLASS — STOOKEY 327 



glass to induce this channel-type phase separation, leaching out the 

 alkali borate with acid, then heating the porous silica skeleton until it 

 shrinks and consolidates as reconstructed 96 percent silica glass. 



Such structural complexity is compounded by constituents that may 

 interact in oxidation-reduction pairs or precipitate from solution as 

 crystals. Small wonder that scientists disagree on "a" glass structure, 

 and that few scientists are rash enough to explore this untidy jungle — 

 neither truly liquid nor truly crystalline, but related to both. 



Having been one of the foolhardy explorers into this wilderness, 

 and having been invited to elucidate recent developments in glass 

 from a personal point of view, I will take the liberty of emulating 

 Sindbad the Sailor and tell you some of the discoveries in which I have 

 taken part and how they came about. 



GLASSES THAT RESPOND TO LIGHT 



A good starting point is an early foray of mine into glass research, 

 the purpose of which was to investigate and improve upon the "opal" 

 glasses. These did not turn out to be melted gem stones, as I had first 

 guessed, but opaque or translucent white glasses containing colloidal 

 inclusions, usually crystalline, that scatter light. 



Literature survey disclosed, among other things, the recipe of an 

 early German glassmaker calling for ground deer bones to make 

 "bone-ash" opal. Such romantic findings are part of the fascination 

 of research in a medium that has thousands of years of history. 

 (Bone-ash opals, in which calcium phosphate is the insoluble phase, 

 are still manufactured.) 



I was soon struck by the apparent similarity between the behavior 

 of certain opal glasses containing sodium fluoride and that reported 

 for the rare and beautiful gold and copper ruby glasses. All these 

 glasses remain clear when they are first cooled, but develop opacity or 

 color by precipitating colloidal particles when they are reheated. 

 Meanwhile, R. H. Dalton of our laboratory had recently discovered 

 that a copper ruby glass, irradiated with ultraviolet light while in its 

 colorless state, developed a darker red color after reheating. 



Jumping to the erroneous conclusion that, therefore, similar expo- 

 sure of a sodium fluoride glass would result in a photosensitive opal, I 

 found that ultraviolet light had no effect whatsoever on this glass. 



The answer to this puzzle proved to be that the sodium fluoride 

 precipitation results from a simple supersaturation, with crystal 

 nuclei forming at such low temperatures that the glass must be re- 

 heated in order that crystals can grow. The gold and copper rubies, 

 on the other hand, were found to develop their color in a complex 

 sequence of chemical oxidation-reduction reactions, of a temperature- 



672-174 — 63 24 



