John Baymore at River Bend Pottery

River Bend Pottery   © 1995 - 2011 All Rights reserved

 

 

Unified molecular glaze calculation is often looked on by potters as the ultimate tool for accurate understanding of ceramic glaze behavior. While it certainly is the fundamental basis for truly understanding glaze chemistry, there are a number of variables that the Seger molecular calculation approach doesn't take into account. Hermann Seger's system assumes that there is a completely homogeneous, evenly distributed mixture of the chemistry present. In the real world of glaze batch materials sourcing and melting glass layers onto clay, things aren't always so nice and neat.

 

First, let us take a fired sample of an oxidation glaze fired in the Orton cone nine range which contains no soluble raw materials, and look at it in cross section at a greatly enlarged level. Picture the glaze and clay body sliced in two, and you are looking at the cross-section edge-on in a greatly magnified view. For the purposes of reference, the outer surface layer of glaze is "UP" and the clay body layer is "DOWN".

 

On the outer surface of the glaze (UP), the composition of the glass is pretty much what you would expect from the results of a molecular calculation utilizing Seger's approach. As you move toward the clay body (DOWN) the composition remains pretty constant for a while. Then, as you draw nearer the clay body, the composition starts to change as the effects of the materials in the body start to appear. Crystals developing in the clay body continue to grow into the glaze layer and the glaze erodes the clay surface and begins to dissolve body materials. Bits of feldspar and particles of silica in the body become fluxed by the glaze fluxes and begin to melt. And so on.

 

So the composition of this boundary layer or interface layer is different than that which would be predicted by molecular calculation alone. This change is gradual, not like a distinct line or plane. It is more of a zone of interaction. In some areas of the cross-sectional slice, it can often be hard to tell where the glaze stops and the body begins.

 

As a very broad generalization, this effect is less and less pronounced as the maturing firing temperature of a glaze goes lower. It is also greatly dependent on the exact composition of the clay body itself. This is one reason why glazes can look so different on differing bodies, and why certain glaze defects can occur on some bodies and not on others. A body that has a lot of "glassing materials" will have more of a tendency to let various components bleed into the glaze. A body that is prone to crystalline development will have more of these elements grow into the glaze. A body high in coloring materials will have more of an impact on color development in the lower glaze layers.

 

If the body formulation contains a chemical that has some unique properties, then these properties can then affect the glaze too. For example, if a flameware body contains spodumene, supplying lithium oxide as a flux, that lithium will tend to find it's way into the lower part of the glaze layer and can tend to cause effects which are promoted by lithium in the glaze melt, such as a change in the Coefficient of Thermal Expansion or color rendition with certain colorants. This is true for many materials that could be present in clay bodies including the very common iron oxide (Fe2O3) that those who fire in reduction depend on to tint and variegate their wares.

 

Now, let's change the firing atmosphere of the heating cycle in the prior example to a reducing one and assume that the cooling cycle is in oxidation, as would be found in most typical "reduction" firings.

 

Originally published in "The PotLuck"     May 1998

 

 

More considerations in glaze development

By John Baymore

c 1998 all rights reserved

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