A study of refining Part 2 : Mechanisms of refining

GT V02 1961060-070

It is shown that rise of bubbles through the melt is unable to account for the refining behaviour reported in Part I, particularly the disappearance of small seed. Two other nzechanisms whereby seed may be removed are then considered. Coalescence seems to be important only in the earliest stages of melting and attention is then turned to the growth or contraction of bubbles due to gas diffusing across their boundaries. A theoretical expression is derived for a spherical bubble giving the diameter as a function of time, the solubility, diffusivity, nnd saturation of one gas in the melt. The effect of surface tension on the pressure inside the bubble is taken into account but the distortion of the concentration gradient caused by the motion of the boundary is ignored. Numerical calculations for oxygen and carbon dioxide, using assumed solubilities and diffusivities, indicate that diffusion of the former at least is probably sufficiently rapid to be important in refining. Other data supporting this conclusion are cited. It is concluded that two mechanisms governed the refining of the glass beyond the batch-free time, the rise of large seed to the surface of the melt and the dissolving of small seed, but it is not yet possible to give a quantitative account of the observed results. 

Michael Cable*
Department of Glass Technology, University of Sheffield