Almost all glasses will crystallize if held at the right temperature for sufficient time. That includes window glass, which will readily crystallize especially if the surface is dirty - even a finger print can aid in crystallization by providing nucleation sites. The only glass I know of which has not been crystallized without going to extreme efforts is vitreous boric oxide.

But just crystallization is not enough to make a glass become a glass-ceramic. Glass-ceramics require controlled crystallization. That means that one wants to form a uniform array of small crystals for the best glass-ceramics. Read the books by McMillan or George Beall (a true expert) or the one put out by Schott Corp. (hard to find, but well worth the effort) for a really through discussion of the formation of glass-ceramics. For a far less detailed discussion (just a chapter, see my Intro to Glass Science and Tech book). I think Varshneya probably also has a good discussion, but I don't have a copy of his book anymore, so I'm not sure how much he discusses this.

Just back from Corning glass museum. As demonstated, the glass-ceramic needs controlled crystallization from glass via addition of nucleaion agents If uncontrollable (spontaneous crystallization , wild glass or ceramic microstructure would be formed instead of glass-ceramic.

Dear Salini,

There are glasses that crystallize easily, but there are also glasses that remain virtually forever amorphous at all temperatures. Everything depends on the composition.

For example, glasses from the system CaO-MgO-FeO-SiO2 (natural obsidian glasses) crystallize very easily, especially if they contain some TiO2. In contrast, the alkali-lime silicate glasses (window glass) hardly crystallize.

Why this is so can be understood by comparing Gibs 'energy (enthalpy + entropy) from crystalline and amorphous states: if Gibs' energy becomes smaller upon crystallization of the glass, then the glass will crystallize; if Gibs energy rises, - the glass remains amorphous.

Sometimes, forming glass-ceramic is preferred thermodynamically, however, from the kinetic point of view, it is too difficult to form the glass-ceramic because of the high activation energy required for the same. As mentioned above, it depends on the chemical composition.