Is glass a liquid or solid?

Glass does not have a crystal structure, it is an amorphous solid. So technically it is not a true solid. Also glass does not sag, it flows, very slowly. Glass panes in the windows of Medieval Cathedrals have been found to be thicker at the bottom than the top. As the age of the windows is known, the rate of flow can be calculated. In the correct chemical environment glass does form crystals, in which case it flakes. This is what causes the iridescent sheen or patina found on ancient glass that has been recovered in an archaeological context.

Very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very Viscous liquid (gasp).

Rap

Glass is in fact a "supercooled" liquid, which is the name given to any substance which exists in liquid state below its normal freezing point. As described above, its liquid behaviour at room temperature is extremely slow hence we can consider it a transparent solid to all intents and purposes. However it will, under the right conditions, crystallise and so solidify, becoming opaque in the process.

Water will also supercool and when flying your aeroplane through a shower of rain at (say) - 10 degrees C, the droplets hitting your wings instantly change to solid ice causing all sorts of problems!

O.C.