Further Investigations upon the Influence of Boric Oxide on the Rate of Melting of the Batch, and on the Rate of Refining and of Setting of Commercial Glasses of the Soda–Lime–Silica Type
JSGT 1954 V38 T297-T364
A thorough-going investigation, having regard to present-day practice, has been carried out to re-assess the effects on the melting and refining rates of progressively introducing boric oxide as hydrated borax into batches for the production of six different container glasses and of one sheet glass, all of the soda–lime–silica type with variants containing one or more of the constituents AI2O3, MgO, BaO and F. The range of B2O3 in the glasses was 0–2·5%, substituted overall. In all, twenty-nine glasses were studied. The meltings were carried out in platinum–rhodium crucibles (to eliminate corrosion effects) in a town-gas-fired furnace fitted with arrangements for the strict control of the gas and air supplies. Measurements of melting and refining rates (the latter in terms of visible bubbles per 100 c.c.) were carried out at 1400°, 1450° and 1500°, with additional meltings at 1425° and 1475° in special cases. Measurements of the viscosity of all the glasses were carried out over the temperature ranges 525°–700° and 900°–1400°. Chemical composition control was exercised, not only by precise analysis of all the glasses, but also of volatilisation during the drawing of glass fibres and during the high temperature viscosity measurements. The rate of melting of the batches at all temperatures was significantly increased by progressive addition of B2O3, except in the single case of the sheet-glass composition. The rate of refining at all temperatures progressively increased with B2O3 addition in all cases. The rate of increase varied according to the character and amounts of the major constituents of the batches; but even in the presence of the fluxing constituents BaO and F, boric oxide still exercised a marked effect. Coefficients were worked out relating the concentration of B2O3 to the increases in melting and refining rate. The effect of increased temperature alone was also calculated. Increase of B2O3 within the range 0–2·5% progressively lowered the viscosity of all glasses within the range from 1400° down to a point between 6020 and 6250, dependent on the basic composition, below which introduction of B2O3 progressively increased the viscosity. Accordingly, the annealing temperatures were also raised by the addition of B2O3; the Littleton softening point, however, was progressively lowered. From the viscosity data it was shown that a given stage of shaping a glass article could be carried out at a somewhat lower temperature as addition of B2O3 was increased.
R. S. Allison & W. E. S. Turner
