THE EFFECT OF BORIC OXIDE ON THE DEVITRIFICATION OF THE SODA-LIME-SILICA GLASSES. THE QUATERNARY SYSTEM,Na2O-CaO-B2O3-SiO21

Commercial glasses of the soda-lime-silica types do not consist solely of these three oxides, but in addition contain significant amounts of other constituents which arc introduced incidentally as impurities or deliberately for some beneficial influence. Such minor additions may have either a favorable or unfavorable influence on the tendency of the glass to devitrify, and an investigation into the effect of such minor components is desirable. Previous results with MgO and with A1₂O₃ have shown that a systematic study of a four-component system is necessary to the understanding of its devitrification relations, and that arbitrary and fragmentary excursions into such a system give little insight into the general effect of the minor constituent and may be misinterpreted to erroneous conclusions as to the effect of this constituent on glasses of slightly different composition. Accordingly, in taking up the study of the effect of B₂O₃ on the devitrification of the soda-lime-silica glasses, it was added to 21 glasses in and near the field of devitrite, Na₂O·3CaO·6SiO₂, in the ternary system, Na₂O–CaO-SiO₂, in amounts up to about 5%. In all cases the liquidus temperature was lowered, and the tendency toward devitrification reduced. Both of these effects are desirable and add to the thermal stability of the glass. With glasses containing 10% CaO, initially in the field of tridymite or so near to it that a small addition of B203 brings the resulting glass into the tridymite field, this initial lowering is replaced by an increase in the liquidus temperature which passed through a maximum at from 4 to 6% B₂O₃. With all other glasses the lowering continues, and the rate of decrease is greatest with glasses originally in the field of Na₂O·2CaO·3SiO₂. An exploration of the quaternary system showed that no new compounds are formed in any mixture that can be obtained by adding B203 in amounts up to 50% to any mixture in or near the devitrite field. It also indicated the probability that a narrow band of immiscible liquids is formed, extending across the ternary system, CaO-B₂O₃–SiO₂, and including the mineral danburite, CaO·B₂O·2SiO₂, but probably not extending far into the quaternary system. The fields of wollastonite and of Na₂O·2CaO·3SiO₂ were found to sweep out regions in the quaternary system extending over the square, Na₂O·SiO₂-Na₂O·B₂O₃-CaO-B₂O₃-CaOSO₂, and the field of Na₂O·CaO·SiO₂ was found to extend over the middle of this square, and is probably adjacent to the fields of wollastonite and of Na₂O· 2CaO·3SiO₂. A compound is formed between Na₂O·B₂O₃ and CaO·B₂O₃. There is included a general survey of the literature relative to the effect of B203 on the properties of glass.