[Clayart] 5 hour firing

Vince Pitelka vpitelka at dtccom.net
Sun Dec 20 20:11:26 EST 2015

Sumi wrote:
"So iron oxide becomes iron in the glaze, copper carbonate becomes copper in the glaze, etc. Copper carb is green, copper is red, so when glazes colored with copper carb are fired in oxidation (an electric kiln) they come out green, when fired in reduction they come out red."

Hi Sumi - 
For the sake of good information, lets clarify a bit.  Once you get above red heat everything in the clay and glazes turns to oxides.  Iron is still iron oxide in the glaze, but in oxidation it is red iron oxide, and then in reduction it becomes black iron oxide, and anywhere it is locked within the glass it remains black iron oxide, while where the glaze is thin or on the claybody it reoxidizes to red iron.

Regarding copper, the carbonate turns to oxide above red heat, but it is a question of whether it turns to red copper oxide or black copper oxide.  Red copper oxide is obviously red, but it is difficult to achieve in copper reds, and then it happens when you least expect it on some overlaps with oribe glazes and other such copper-containing green glazes.  Black copper oxide is green in reduction when diluted in a glassy matrix.  That's the color in oribe glazes, Pinnell strontium matt, other such green glazes. 
- Vince 

Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka at dtccom.net   

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