Devitrification (Devit) on lens surfaces

[Struan Gray]

I once knew someone who ran an art glass studio that went bust when persistent low-temperature devitrification ruined their productivity. The (porous, impossible to sanitise) lining of the kiln harboured crystalline dust which nucleated devitrification in new pieces at lower temperatures than the process normally occurs.

But even that was a few hundred degrees celcius.

I agree with Harold that you are probably seeing something related to the 'glass sickness' which affects some kinds of vintage glass. It's a combination of migration of metal salts from the bulk to the surface of the glass, and changes in the way dangling bonds in the glass structure are terminated - which can affect the physical structure, making it porous. Had you been using a polar solvent - or an alkaline one - I would have been sure that's what you are seeing, but I'm no expert in what xylene does and does not dissolve when it comes to glass components.

One of the early historical glassworks in Sweden is particularly susceptible to these effects because of the mix of ingredients they put into their melt. There has been quite a bit of work done on finding out exactly what the chemical changes are which take place (mostly potassium migration) and whether there is any cure for sick glass (executive summary: no). Anecdotal evidence from friends in the trade is that healthy-looking glass objects can suddenly develop symptoms if held at elevated temperatures for long periods (hours to days). Sound familiar? There's a useful summary (in Swedish) here:

http://www.raa.se/publicerat/rapp2011_19.pdf

Note that this is indeed related to early attempts to grow a bloom for anti-reflection purposes (or the way glass goes cloudy after many trips through the dishwasher), but it also involves substantial changes to the structure and porosity of the surface layer of glass which make it physically fragile.

[Harold_4074]

Organic chemistry is not my forte, but it is possible that xylene penetration into the damaged glass resulted in some reaction (polymerization, for example) that created stress. It is also possible that absorbed moisture was displaced by the xylene, leading to enough change in stress state to either create or reveal fractures. Your comment that one case involved a hot-air treatement would support this latter conjecture.

The lens with the near-edge damage supports the condensation/corrosion theory; exposed to cyclic temperature changes near the dew point, the metal barrel would cause the edge glass to lag behind rising air temperatures, whereas the main part of the lens could track quickly enough to stay above the dew point. (At certain time of year, the sledgehammer, axes and picks in my garden shed get rusty, whereas the shovels and rakes next to them do not. It took me a while to figure this out...)