Trabka, J Christian on thu 27 jul 06
Mel,
A theory on Iron, bloating, and glazes.
When bisquing the pot does not bloat.
When glaze firing the pot does bloat.
Theory:
Iron oxide is not changed in the bisque. As the glaze heats it
eventually begins to melt and forms a seal. At certain temperatures iron
oxide wants to get rid of some of the oxygen which we see in a color
change. If a seal forms (due to a partially melted glaze or the
interaction of the iron/clay) the oxygen gas has no where to go and we
see it as bloating.
Speculation:
A slower ramp up may allow the gas (oxygen) to escape before a seal is
formed (outside of the pot hotter than the inside of the pot).
Chris
Ivor and Olive Lewis on fri 28 jul 06
Dear Christian,
An interesting theory <<...Iron oxide is not changed in the bisque. As =
the glaze heats it
eventually begins to melt and forms a seal. At certain temperatures iron =
oxide wants to get rid of some of the oxygen which we see in a =
color....>> which when you look at the melting point temperatures for =
the various Iron Oxides does not stand up. They melt well above =
stoneware firing temperatures. The decomposition temperature is also =
well over 1300 deg Celsius though Iron oxide is reduced to Iron by =
Carbon and Carbon monoxide as low as 650 deg C..
What is more probable is that there are solid state reactions between =
Iron Oxides and Silica that form Iron Silicate. This has a melting point =
in the 1150-1200 deg C region. I consider this to be the root of the =
idea that Iron Oxides are vigorous fluxing agents. We should remember =
that Silica sand has been used as a flux in the smelting and forge =
welding of Iron implements for some 3000 years.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Tom at Hutchtel.net on mon 31 jul 06
From: "Ivor and Olive Lewis"
Subject: Iron and Bloating
> What is more probable is that there are solid state reactions between =
> Iron Oxides and Silica that form Iron Silicate. This has a melting point =
> in the 1150-1200 deg C region. I consider this to be the root of the =
> idea that Iron Oxides are vigorous fluxing agents. We should remember =
> that Silica sand has been used as a flux in the smelting and forge =
> welding of Iron implements for some 3000 years.
Interesting theory, Ivor. I would like to see someone collect the crumby
stuff that is the center of most bloats I
ve seen and have it analyzed. Might shed some light on this. We've been
lucky in not having any for some time.
Tom Wirt
Ivor and Olive Lewis on fri 4 aug 06
Dear Tom ,
I cannot ever recall having this fault assault any of my recent work so =
I would not have a specimen. Nor do I have access to a Lab where I could =
do the investigation.=20
Of one thing I am certain. Solid State Reactions may play a greater part =
in the changing of clay, rocks, minerals and other compounds into =
ceramic fabric and glaze coating than is currently realised. It is a =
factor that seems to be ignored by some who give us instructions. I do =
not recall reading of "Solid State Reactions" in any of the Pottery =
books I have.
With regards to Iron Silicate, the "Fayalite" field on the =
FeO-Fe2O3-SiO2 Phase diagram is enclosed by the 1200 deg C Isotherm, =
well below the melting point of any of the contributing oxides.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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