On the outer skin layer of the glaze there now will be a composition similar to that
predicted from calculations with all of the oxides calculated in their fully oxidized
form. Because of the exposure to air during the cooling cycle, all of the reducible
oxides, such as iron, will be in the most stable fully oxidized state. So on this
surface skin, iron will tend to be in the red state, not the grey or black state.
The exact thickness of this outer oxidized skin will be determined by the gas permeability
of the glass melt to available atmospheric oxygen, and typically is very thin.
Just below this oxidized skin layer, the glaze materials will remain in the reduced
state that was developed during the reducing part of the heating cycle as the glaze
surface sintered and fused over. Therefore, certain elements will act in the melt
different from those on the skin layer and based on what their RO configuration would
predict: ie. any iron present would tend to be a flux acting on silica.
The interface layer we looked at above would still be present as before, but compounds
containing reducible materials in the body will also have been affected by the firing
atmosphere before the glaze sealed the penetration of CO and H gases. This will greatly
affect the bleeding of coloring materials such as reduced iron oxide out of the clay
body into the glaze as the FeO becomes involved in the melting of interface layer
Now, let's add some soluble soda ash into the source glaze batch to supply all of
the Sodium Oxide in this formula, keeping the overall glaze composition exactly constant
chemically. This change dramatically shows the impact of the raw material sourcing
of the oxides on the final melt. Soda ash is completely soluble in room temperature
water in the concentrations typically found in most cone nine glazes. So all of the
Na2O will be in solution, not suspension in the wet raw glaze batch even though it
is evenly distributed into the mixture.
Now, we dip a piece of bisqued pottery into the raw glaze batch and deposit a layer
of glaze powder on the piece. The suspended glaze materials form an evenly distributed
layer of the percentage composition of the glaze batch on the ware....... except
for the soda ash content. The soda content is dissolved in the water and tends to
go with it where it goes. So the soda ions are absorbed into the body wit h the water,
and are not evenly distributed in the glaze powder layer like all the other insoluble
materials. These soda ions that are now in the water inside the pores of the clay
body could affect the melt of the body, however in practice they typically do not
remain inside the clay.
The water used in glazing has to eventually evaporate out of the pot either before
the firing, or during the very early pre- 212F stage of the firing. As the water
migrates out of the clay toward the surface in order to evaporate, the soda ions
go along with it. However they cannot stay with the water as it evaporates off the
surfaces of the piece. So the soda is left behind in and on the outermost surfaces
of the dry glaze layer.
If you were to now scrape off the outermost layer of this glaze and analyze it with
very sensitive equipment, you would find that the content of sodium in this sample
is far greater than that you would find in another sample you have taken from just
below this outermost layer. The soda content of both of these samples would differ
greatly from that indicated through a molecular calculation of the total raw glaze
batch. Each of these areas of what we often think of a one single glaze would have
a completely different molecular formula, and when compared to typical limit formulas,
probably fall into different cone ranges. In this particular case wit the high soda
content, the outermost layer would be the lowest melting glaze.