Affects of Chemistry on Annealing Point
The change in the transition temperature is affected by the rate of cooling; it is also affected by the chemistry - or composition - of the glass. The transition temperature in silicates (glass of various compositions) is related to the energy required to break and re-form covalent bonds in an amorphous (or random network) lattice of the tetrahedra form of the glass molecules.
A covalent bond is one that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is what covalent bonding refers to.
The transition temperature is influenced by the chemistry of the glass. For example, addition of elements such as Boron, Sodium, Potassium or Calcium to a silica glass helps in breaking up the network structure, thus reducing the transition temperature and the melting temperature. Alternatively, Phosphorus helps to reinforce an ordered lattice, and thus increases the transition temperature.
The modifiers commonly used in glass-making are: sodium oxide, potassium oxide, lithium oxide, calcium oxide, magnesium oxide, and Lead oxide. Although there are over 2,000 known additives to glass. The minerals used to colour the glass seem to have minor affects upon the glass composition as they generally are in a colloidal suspension without forming bonds to the silica atoms.
If an oxide, such as sodium oxide, is added to silica glass, a bond in the network is broken and the relatively mobile sodium ion becomes a part of the structure. With increase in the amount of modifier, the average number of oxygen-silicon bonds forming bridges between silicon atoms decreases. The principal effect of a modifier is to lower the melting and working temperature by decreasing the viscosity. An excess of modifier can make the structural units in the melt sufficiently simple and mobile that devitrification (crystallization) occurs in preference to the formation of a glass. The skills of the glass makers lie in the balance of factors relating to the transition and working temperatures, and the maintaining the resistance to devitrification.