devitrification
Devitrification is a glass forgetting that it is a glass. Left in the wrong temperature range, the frozen liquid does what it always secretly wanted to do, its atoms begin to line up into their proper crystals, and the clear, glassy material turns cloudy, milky, or gritty as crystals grow inside it. The word literally means undoing the vitreous, glassy state. It is usually an unwanted defect, a sign that the melt lingered too long where crystals could grow.
The mechanism is the same competition that glassmaking tries to win in reverse. A glass is a metastable supercooled liquid; the crystal is always the lower-energy state. Crystallization needs two things, nuclei to start on and enough atomic mobility to grow, and both are available in the temperature window between the glass transition and the melting point. Cool a melt too slowly through that window, or reheat a finished glass into it (during forming, welding, or bending), and crystals such as cristobalite or the needle-like devitrite nucleate, often at surfaces and interfaces first, and spread. The result is haze, loss of transparency, roughness, and usually a weaker, more crack-prone glass, because the crystal and glass have different thermal expansions and pull apart on cooling.
For most glassmaking, devitrification is the enemy: it is why gobs are formed quickly and why compositions and schedules are chosen to steer around the crystallization nose. But the very same process, when it is nucleated deliberately and controlled with care, is turned into a triumph, the glass-ceramic, in which fine crystals are grown on purpose throughout the glass to make a strong, tough, low-expansion material. So devitrification is not always ruin; it is uncontrolled crystallization, and its disciplined cousin is one of the cleverest tricks in ceramics.
A glassblower who lets a piece dwell too long at dull red heat sees it go cloudy and scummy at the surface, a haze called devitrification or stones, that no amount of reheating fully clears and that makes the glass crack more easily. The cure is to work faster and hotter, keeping the glass out of the crystallization window rather than dawdling in it.
Dwelling in the crystallization window lets crystals nucleate and grow, hazing and weakening the glass.
Devitrification is not melting or dirt; it is crystallization of the glass itself. And it is not always a flaw: the controlled, nucleated version is exactly how glass-ceramics are made deliberately.