devitrification

Lighting. A change in the crystalline structure of glass (such as the quartz envelope of a lamp) causing it to weaken. More at http://en.wikipedia.org/wiki/Devitrification . Also the primary reason given for why touching T-H lamps is bad: "Devitrification is the term used. The quartz often turns opalescent white or crystalline white in the area of skin contact."

(In college Stage Lighting 201, I was taught that "Devitrification is the phenomenon whereby Xenon lamps explode for no apparent reason." Although over-simplified and inaccurate, I got 100% on the exam.)