7.1 Summary
7.1.2 Glaze Composition Effect on Bubble Evolution
The glaze surface tension and thermal expansion decreased and the viscosity increased as PbO was substituted for Na2O.
Substitutions of CaO for Na2O in a leadless glaze increased the surface tension and lowered the thermal expansion. The viscosity initially lowered with increases in CaO:Na2O within a RO:R2O range from 0.2:0.8 to 0.5:0.5, however, the viscosity increased as RO:R2O was raised from 0.5:0.5 to 0.6:0.4.
Overall, measured changes in surface tension due to varying composition
correlated well to predictions by Kucuk’s model, although there was a bias of 10mN/m or less. These results are consistent with the observation discussed in Chapter 4.
In general, bubble volume correlated well with surface tension and viscosity values. The volume tended to decrease as RO:R2O was raised by substituting PbO for Na2O and tended to increase as RO: R2O was raised by substituting CaO for Na2O. The bubble volume tended to increase with temperature in all cases except in the leaded glaze with extremely high level of Na2O (20 wt%). In this case, the volume was very high at low temperature (950oC) and progressively lowered as the temperature was increased to 1150oC. The high Na2O level caused a very low viscosity and may have resulted in higher diffusion and increased volatilization.
Sources of gas for bubble formation include de-hydroxylation of the clay near the
temperature, the water trapped in the glaze, and atmospheric N2 and O2 trapped between frit particles.
Two statistical models were developed to predict the surface tension-viscosity- bubble volume relationship. The model predict that that lowering surface tension is beneficial for bubble removal, however, the bubble volume tends to decrease with the continuing increase of surface tension. Decreasing viscosity generally lead to an increase of bubble volume. The models predict “ideal” conditions for lowering bubble volume are surface tension values at the lowest end of the range tested (below 260mN/m) and a viscosity range of approximately 102 – 102.5 Pa•s. based on our observation and a study sited in Parmelee,6 the ideal viscosity for glaze was close to 102 Pa•s.
7.1.3 Substrate Effect on Bubble Evolution
The bubble number and size on porcelain and porcelain glass substrate are higher than those on an alumina substrate. The pores in the substrate are a new source of gas, which possibly results in a second growth of bubbles. The reaction at the interface facilitates the bubble nucleation and residing at the interface. More bubbles resided on the interface of glaze-porcelain substrate, and bubbles in a glass substrate themselves grew and migrated towards the interface.
7.2 Suggestions for Future Work
Bubble gas composition influences the mass transfer across the surface.
Therefore, it is very important to know the gas composition, which could be analyze by Raman spectrum. The effect of the porcelain and the porcelain glass substrates on the bubble nucleation needs further investigation.
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