John Baymore at River Bend Pottery

River Bend Pottery   © 1995 - 2011 All Rights reserved

 

 

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 mentioned here.

 

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.

 

Originally published in "The PotLuck"     May 1998

 

 

More considerations in glaze development

By John Baymore

c 1998 all rights reserved

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