There are two problems with sunlight.
The first is that some pieces of glass may crack or split in sunlight because they had stresses within which the sunlight makes worse. I have never had this happen to any of my glass, and I like to keep some pieces in the sunlight. But I know that some glassmakers now apply a sticker (Caithness was one, I think) which advises not to put the item in full sunlight. Bit disappointing though because that's one of the great features of glass - how it looks with sun on it.
The second kind of damage that can be caused by sunlight is that some kinds of glass will change colour from excessive exposure to sunlight. This applies mostly to clear glass, not coloured. The most common change is for clear glass to turn pale purple. Some people like it that way, and actually collect "Sun changed glass". You can get the same effect more quickly by irradiating the glass, and this does upset some glass collectors a lot.
The following quote is from the Glass Encyclopedia:
Arsenic was used in glass-making before the second world war (pre 1940) and to a lesser extent after that date. When irradiated or exposed to sunlight for a long time, this glass turns yellow. The effect can be seen in the headlamps of very old cars. Manganese was widely used before about 1930 to decolorise impurities from clear pressed glass. Some glassworks went on using manganese after that time and some may still use it today. When irradiated or exposed to the sun for many years, antique glass with manganese turns deep purple.
There are other chemicals which have been used in old glass which change the colour of the glass when exposed to strong sunlight or irradiation. Particularly vulnerable to these color changes are depression era glassware, sulphide marbles, Czech perfumes bottles, cut glass and early American pressed and blown glass.
From:
http://www.glassencyclopedia.com/irradiatedglass.html
Topic: Is sunlight causing damage to glass objects? (Read 1994 times)
