Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, -T, the liquid volume,v], and the seed density,psand upon a set of materials constants: the reduced crystal–liquid interfacial tension, α the fraction,f, of acceptor sites in the crystal surface, and the reduced glass temperature,Trg. The glass-forming tendency will be greater the larger are -TandTrgand the smaller arev].ps, andf. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass temperature. There are at least some glass formers in every category of material, according to bond type, i.e. covalent, ionic, metallic, van der Waals or hydrogen. However, it is not established whether or not every substance can be put into a glass form.