Hilary Kerrod on mon 27 nov 06
Vince wrote: ' As you well know, in reduction red or
ferric iron reduces to black or ferrous iron, which is a powerful flux.
This happens even in oxidation once you reach adequate temperature. In
reduction, iron is capable of reacting with silica at remarkably low
temperatures, and is a primary cause of carbon coring when there is early
reduction or inadequate air circulation in a bisque firing.'
Does this have a bearing on Greek black figure firing, allowing the black
(terra sig?) areas to flux AND be black while the rest is red?
Hilary Kerrod in N Z
Vince Pitelka on tue 28 nov 06
I wrote:
"in reduction red or ferric iron reduces to black or ferrous iron, which is
a powerful flux. This happens even in oxidation once you reach adequate
temperature. In reduction, iron is capable of reacting with silica at
remarkably low temperatures, and is a primary cause of carbon coring when
there is early reduction or inadequate air circulation in a bisque firing."
Hilary Kerrod wrote:
"Does this have a bearing on Greek black figure firing, allowing the black
(terra sig?) areas to flux AND be black while the rest is red?"
Hilary -
No, because both the red and the black terra sig have the same amount of
iron in them. There is some disagreement as to the true tecniques of Attic
painted pottery, but the best one I have heard is that the black terra sig
contains a higher concentration of alkaline flux. Thus when the kiln is
placed in reduction as it approaches maturity, everything turns black (as
the iron reduces from red ferric to black ferrous oxide), but as the kiln
cools, the black slip has vitrified, and therefore does not reoxidize to
red, whereas the more porous, less-vitrified red slip reoxidizes. If the
kiln is overfired, everything vitrifies and the whole pot is black. I've
seen examples of that in museums - the ones that were overfired, where you
can barely make out the outline of the decoration in the black surface. If
the ware was underfired, it all reoxidized to red, but then of course it
could be refired.
Hope that all makes sense -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
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