Paul Borian on tue 24 oct 06
Does anyone have experience using black iron with bone ash in reduction?
Does it yield the kind of reds if used in the right base glaze that red
iron does?
I was using a combination of red and black iron (mostly just because i had
some black iron sitting around i wanted to use up) in my slip glaze and
when i added bone ash it started showing reds, but was never sure what
influence the black iron had. Now i am out of the black iron i just wanted
to use up and just using red iron but i think the glaze was nicer with the
black so i am buying more soon.
thanks,
Paul
Tom Buck on tue 24 oct 06
Paul Borian:
Unless you fire unusually fast, black iron oxide (either FeO or
FeO.Fe2O3 aka magnetite) will change mostly to Fe2O3 by 1800+ oF and then
will convert largely back to FeO under strongly reducing conditions and
thusly serve as a "flux oxide" aka "melter" to make the base glaze more
fluid/runny. Some Iron Oxide Red can be formed at high cones by "striking"
the ware, that is, "oxidizing conditions".
Calcined animal bones will yield a material composed mainly of phosphates
of calcium (bulk of material) and magnesium, small amounts of calcium
fluoride and calcium carbonate (aka whiting). Today, potters seldom obtain
"bone ash" from calcined bones. Instead, the chemical tricalcium
phosphate, made from fertilizer plant phosphoric acid, is the material we
receive as "bone ash".
Since tricalcium phosphate, as such, doesn't melt until well above
Cone 10, the main way it gets incorporated into the alumino-silica matrix
(or main glass) is as a second glass formed from CaO and other flux oxides
present in the base glaze recipe. If the recipe contains low levels of
Bone Ash, the phosphate glass will like disperse in the Al/Si Liquidus; at
high levels two glass phases likely will occur, and thus change the
surface characteristics of the fired/cooled pot.
Commerical potteries use Iron Oxides for brown colour solely; they
avoid orange/red iron oxide crystallines (see M. Bailey's book) because
the resultant glazes are fickle, take extra effort/time to achieve. For
studio potters, the red crystalline effect comes from finding the right
"Bone Ash" and the right "Iron Oxide" and the right firing program.
Maybe you'll get lucky.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
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