Glass and crystal are two materials that are obtaine by means of a melting process, that is to say, by heating a mass of special components until it melts and then cooling it so as to obtain a vitreous state.
Not all substances, cooled from their liquid state , can become glass. In fact, most elements and compounds, after melting and cooling, return to their original state (e.g. metals).

The commonest substance that can exist in the vitreous state are : silica, boric oxide and phosphoric oxide. These substances are called in fact VITRIFIERS and they have the lion's share of the composition necessary to obtain glass. Glass formed, for example, just from silica would be difficult to make.

As already mentioned, the material must be melted and, given that the melting point of silica is 1700°C, it would be necessary to have special furnaces to reach such an elevated temperature; furthermore, this kind of glass would be very difficult to work.

When manufacturing normal glass, other materials are therefore added to the silica, especially compounds of sodium and potassium, to lower its melting point. These substances are called FLUXING AGENTS.

Glass made from silica, sodium and potassium is very unstable as it is soluble in water and can therefore be easily attacked by atmospheric agents (e.g. common glass).
Therefore, in order to make Crystal, other substances, clled STABILISERS, must be added to the compositions of common glass.