The Thermal Expansion of Magnesia-containing Glasses

JSGT_1920_V04_T115_T120 (6 pages)

The question of the thermal expansibility of magnesia-containing glasses is a matter of considerable interest and importance. It is commonly supposed that magnesia exerts a supremely beneficial effect on glass by reducing its coefficient of expansion, and for this reason, mainly, has been added to glasses, for chemical, illuminating, and heat-resisting purposes. The chief ground for this belief undoubtedly lies in the results of Schott, who, calculated" the factors representing the contribution which each oxide made to the expansion of a glass. Whereas the factor for the increase of the cubical coefficient of expansion due to the addition of each 1% of silica was 0·8×10^-7, that for boric oxide and for magnesia were both only 0·1×10^-7, a result which, if correct, would make magnesia of very great value as a glass constituent. A glass composed of silica, boric oxide, and magnesia would thus possess an exceedingly small thermal expansion.

S. English & W. E. S. Turner