So in this case, the outermost layer is beginning to melt at a much lower cone than
the rest of the glaze due to the high soda content there. This outer layer is, like
the interface layer, not distinct, but is a zone. This outer layer will react with
the kiln atmospheric conditions as you would expect for a glaze of that particular
cone range. So the surface layer can become gas impermeable to CO and H much lower
then the rest of the lower reaches of the glaze. And the surface layer can do things
like trap free carbon particles in the rapidly melting surface. This differing composition
can affect the reactions of the outermost surface on cooling conditions also. Many
of the cone nine-ten Shino glazes are a good example of this phenomena, and can appear
as quite shiny when looked at in the kiln at very low temperatures. Only the very
outermost layer is shiny, the rest of the glaze is still far from melted.
I know of no real way to accurately predict all of this or take it into account in
a calculated formula. If you know you have soluble materials, you know that this
type of action will happen. The exact extent can only be established by testing,
as far as I know. No available software has yet cracked the sourcing issues just
It is complicated to predict these kinds of behaviors in any exacting way. For example,
to do this you need to know how much of a slightly soluble material actually will
go into solution. That amount depends on variables such as the particle size of the
raw material, if any milling of the batch is being done, the PH of the water used
to suspend the batch, the time the batch spends mixed in the water, and the effect
of other glaze components or suspenders on the soluble material. Whew!
In the case of my own Shino glaze which contains a high level of soluble soda compounds,
I have done extensive testing and also know the formula of the batch with the soluble
soda content completely removed, and with it all concentrated into one quarter of
the total batch. This information helps me develop firing cycles, but it is really
only an approximation utilized to assist my general understanding. It is not scientifically
exact. Nothing takes the place of real world testing work.
So as you approach getting a better understanding of glazes through studying molecular
calculation, keep in mind that you have some added variables that affect the final
outcomes. Molecular calculation is a crucial tool to technical understanding, but
you also need to understand a bit about the raw materials themselves and the action
of the fire. Glaze does not exist in isolation. All things for you to consider in
glaze development and in tracking down defects.