John Baymore at River Bend Pottery

River Bend Pottery   © 1995 - 2011 All Rights reserved

 

 

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 silica material.

 

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.

 

Originally published in "The PotLuck"     May 1998

 

 

More considerations in glaze development

By John Baymore

c 1998 all rights reserved

MORE