Limiting Compositions of Binary Glasses of the Type xR₂O.SiO₂ and of Ternary Glasses of Types xR₂O.yRO.SiO₂ and xR₂O.yR₂O₃.SiO₂ in Relation to Glass Structure

JSGT 1951 V35 T043-T057

This investigation was undertaken to obtain evidence relating to the ways in which various “network-modifiers” are held in silicate glasses. Continuity of structure is essential in any glass, but in an alkali–silica glass the presence of the alkali causes discontinuities in the structure wherever systems such as ≡Si–O–Na or, as is more probable, ≡Si–O– and a free Na⁺ ion occur. The limiting compositions of alkali-silica glasses should thus correspond with Na₂O.SiO₂, K₂O.SiO₂, etc. The introduction of the oxides of divalent and trivalent elements could, on the other hand, provide “bridge” linkages of the form ≡Si–O–Ca–O–Si≡ or ≡Si–O–Al–(Si≡)₂, which would preserve continuity even if the glasses contained the limiting proportions of the alkalis. In the experimental work a glass was regarded as stable if a sample weighing about 10 g remained clear when cast and allowed to cool in air sufficiently slowly to prevent shattering. As judged by this test the limiting compositions of soda–silica and potash–silica glasses were found to be 0·9Na₂O.SiO₂ and K₂O.SiO₂, respectively. The addition of the oxides of divalent and trivalent metals to glasses approximating to the above limiting compositions enabled stable glasses to be made containing RO and R₂O₃ in considerable amounts. It is concluded, therefore, that oxides of the types RO and R₂O₃ act as strong “bridging” links in the glass structure, by the formation of strong ionic or covalent bonds between the metallic atom and the oxygen atoms at the corners of adjoining silica tetrahedra. The introduction of lead oxide enables stable glasses of compositions K₂O.2PbO.SiO₂ to be obtained. This requires either the formation of “double-span” bridges ≡Si–O–Pb–O–Pb–O–Si≡ in the structure, or the formation of PbO₄ tetrahedra. In either case the lead is present in such high proportions that it must play a large part in building up the glass structure.

H. Moore & M. Carey