Study of the Volatilisation and Vapour Tension at High Temperature of an Alkali–Lead Oxide–Silica Glass
JSGT 1932 V16 T219-T239
An investigation has been made of the volatilisation which occurs when a glass of percentage composition SiO2 55·57, PbO 29·83, Al2O3 (+Fe2O3) 1·29, CaO 0·22, Na2O 5·18, K2O 7·60 was heated in a controlled manner in an electric furnace at temperatures from 900° to 1400° for periods of time up to 200 h. Lead oxide was far and away the most volatile of the oxides present, loss of alkaline oxides being appreciable only above 1200°. At 1400° from a layer of the glass 3 mm deep, all the lead oxide waslost by isothermal distillation in less than 40 h. From the still (that is, unstirred) surface of such a glass melted in a commercial pot of 30 inches internal diameter it is calculated that 3 1/3 ounces of lead oxide would be lost in the first hour at 1400° and 1 ½ ounces at 1300°. It was shown that a straight line graph was obtained on plotting log (rate of initial loss) against 1/T°(Abs), thus showing a relationship similar to that between vapour tension and temperature. The simple relationship x/a−x=kt, where x was the amount volatilised at time t and A the original amount of the volatile constituent in the glass, held satisfactorily up to the time of half loss. The area exposed, the depth, and the rate of diffusion in the glass all influence the rate of loss from the surface. A long period experiment established that a definite concentration gradient is set up in a crucible deep relative to the diameter. Assuming this concentration gradient to be a straight line, the general approximate equation connecting rate of volatilisation with time and depth of glass (h), namely, log(A−2x/A)=−2kt/h was deduced. Finally, vapour tension determinations of the molten glass were carried out in a specially-designed apparatus at temperatures between 1100° and 1350°, the values ranging from 0·16 mm Hg at the former to 3·90 mm Hg at the latter. It was shown that the relationship between vapour tension and temperature was precisely the same as between rate of volatilisation and temperature. The heat of volatilisation of lead oxide was calculated to be approximately 60,000 calories.
Eric Preston & W. E. S. Turner
