The state transition: solid <--> liquid in a crystalline solid occurs instantaneously at a very precise temperature ( Tf fusion). In glass this occurs gradually and continuously for a progressive viscosity variation over temperatures in which the vitreous transition Tg temperature is identified. For this reason, the 'technical history' (duration and intensity of the fusion, duration and cooling thermal curve) has a notable influence of the physical and chemical qualities of the resulting glass.
Depending on its composition and its thermal history, glass can be transparent, translucent or opaque, colourless or coloured. At room temperature, it is very hard (5-6 hardness on the Mohs scale) and fragile, it is not porous, it has a strong shininess characteristic, it notably refracts light rays, it expands only slightly with heat, of which it is a bad conductor; it does not dissolve in water and acids, even if concentrated, except for hydrofluoric acid, even if disposes of - in small amounts, and particularly when hot - modifying ions from its surface. It does dissolve in basic solutions. It does not burn, does not calcine; under the effects of high heat it goes through various stages of viscosity, when white incandescence it is fluid, when red it is soft and doughy. It is in this last viscosity state that glass can be shaped.