Page "Glass" Paragraph 33

There are three classes of components for oxide glasses: network formers, intermediates, and modifiers.

The network formers ( silicon, boron, germanium ) form a highly cross-linked network of chemical bonds.

The intermediates ( titanium, aluminium, zirconium, beryllium, magnesium, zinc ) can act as both network formers and modifiers, according to the glass composition.

The modifiers ( calcium, lead, lithium, sodium, potassium ) alter the network structure ; they are usually present as ions, compensated by nearby non-bridging oxygen atoms, bound by one covalent bond to the glass network and holding one negative charge to compensate for the positive ion nearby.

Some elements can play multiple roles ; e. g. lead can act both as a network former ( Pb < sup > 4 +</ sup > replacing Si < sup > 4 +</ sup >), or as a modifier.