The gasses pass into and through the fusing layer of glaze producing a productive stirring action which helps to make the glaze more uniform or homogeneous. The boiling of gasses in the glaze is very slow in the early stages when the viscosity of the fusing glaze is greater, but as the temperate elevates and as the glaze becomes more fluid the activity is increased and vigorous. As time and temperature continue to work on the clay and glaze, the active dislocation of gas from the mixtures gives way to smooth, liquefied, but viscous, glass. We have reached the maturation temperature for the glaze. Viscosity We need to keep in mind that the viscosity of the melt at high temperatures is an exceedingly important attribute.
Viscosity also depends upon the nature of the materials used in the composition of the glaze.
The rate and uniformity of the fusion process depends on those glazes which begin to fuse early and encompass, permeate, and dissolve the more refractory raw glaze materials. Of course the uniformity within and the quality of thickness in a mature glaze depends upon the viscosity of that mixture at maturity. Because of this potential problem, most clay artists prefer to 'soak' the kiln before it is turned off and allowed to cool. The additional time in the elevated temperature helps to let the last of the gasses move on out of the liquid and the surface tension can help it regain its uniform surface. In drastic cases the viscous glaze may create crawling as the gas bubbles are liberated and the surface pulls the glaze out and away from where the bubbles moved though the molten mix exposing the raw clay body beneath.