Reconsideration is given to the allegedly anomalous thermal expansion of alkali borate glasses, and to explanations for this behaviour which have been advanced. Several issues (eg differences in the thermal expansion data found by different investigators, theoretical expressions for the thermal expansion, borate glass structural information) are addressed, which have a direct bearing upon this phenomenon. Explanations for the borate glass anomaly offered by previous investigators have been in terms of the changes in structural features of binary borate glasses as the alkali concentration is increased. It may be observed, however, that much ambiguity and contradictory evidence exists regarding these structural changes. It is concluded that the thermal expansion behaviour of alkali borate glasses will not be fully understood until a quantitative relationship between thermal expansion and structural features is derived.
