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PRESENTATION

by Mario Moretti
Definition
Glass is a solid amorphous material formed by the progressive solidification of a viscous liquid, obtained by the fusion of crystal minerals.
Nature
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.
Composition
Glass is composed of an homogenous mixture of oxides in variable proportions, which are distinct in moulders and modifiers of the vitreous lattice. The principal lattice moulders (for this reason they are also called glazers) are silica (SiO2) and boric acid (B2O3), but numerous other tri or tetravalent oxides (phosphorous, germanium etc.) have these characteristics.
The modifiers can be categorised as smelted (alkaline oxides, principally sodium and potassium) and stabilizers (calcium, magnesium or barium etc. alkaline-earth oxides).
Oxides of some elements like lead can be both moulders and modifiers of the lattice. Silicate glass, which is the biggest group, is made up of a lattice in which silicone and oxygen atoms are connected by strong covalent chemical bonds. Other elements interrupt the lattice continuity (for this reason they are called modifiers) thereby creating ionic type connections.
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