Glass disease
Glass disease is the name used for a type of deterioration in old glass, especially medieval and early modern stained glass, in which the surface becomes pitted, cloudy or crusted. Small beads of moisture appear on the surface, which has led to it being called weeping.
It is caused by the chemistry of the glass. Medieval glass often contained a high proportion of potash and a low proportion of lime, which made it less stable than modern soda-lime glass. Moisture slowly leaches the alkali out, leaving a hydrated layer that breaks down into a network of fine cracks, called crizzling.
Conservators protect affected windows by controlling moisture, adding an external protective glazing layer, and carefully cleaning and stabilising the surface.