vince pitelka on fri 22 nov 02
> Once the iron is reduced in a clay body it is almost impossible to
> reoxidize it - so relying on that to happen in a second firing is simply
> not going to happen.
Ron -
Your posts are informative as usual, but I am wondering about the above. I
have always done a body reduction to bring out the speckles in stoneware
bodies. Years ago I used to go back into neutral or oxidation atmosphere
after body reduction, but I often lost the speckles in part or all of the
kiln. Now, I maintain a partial reduction after body reduction, and I never
loose the speckles. So, I am assuming that oxidation after body reduction
is capable of converting the iron back to the ferric form. Is that just
happening on the surface, where it is most visible? That would certainly
make sense.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://iweb.tntech.edu/wpitelka/
Roger Korn on sun 24 nov 02
I think this would depend on whether the iron particles/speckles are
floating on the surface of the melted glaze and thus available to react
with the kiln atmosphere. I'll usually take my ^10 Cu reds into
reduction at ^016 and back into oxidation at ^10, firing down to ^6 in
oxidation, then shutting the kiln down, with all ports and flues closed.
The reaction does not reverse: i.e., the red doesn't revert to CuO from
reoxidation.
The real question is the reaction rate in a melted glaze, versus that in
a sintering, not yet fused glaze.
Roger
vince pitelka wrote:
>>Once the iron is reduced in a clay body it is almost impossible to
>>reoxidize it - so relying on that to happen in a second firing is simply
>>not going to happen.
>>
>
>Ron -
>Your posts are informative as usual, but I am wondering about the above. I
>have always done a body reduction to bring out the speckles in stoneware
>bodies. Years ago I used to go back into neutral or oxidation atmosphere
>after body reduction, but I often lost the speckles in part or all of the
>kiln. Now, I maintain a partial reduction after body reduction, and I never
>loose the speckles. So, I am assuming that oxidation after body reduction
>is capable of converting the iron back to the ferric form. Is that just
>happening on the surface, where it is most visible? That would certainly
>make sense.
>Best wishes -
>- Vince
>
>Vince Pitelka
>Appalachian Center for Crafts
>Tennessee Technological University
>1560 Craft Center Drive, Smithville TN 37166
>Home - vpitelka@dtccom.net
>615/597-5376
>Work - wpitelka@tntech.edu
>615/597-6801 ext. 111, fax 615/597-6803
>http://iweb.tntech.edu/wpitelka/
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
--
Roger Korn
McKay Creek Ceramics
In AZ: PO Box 463
4215 Culpepper Ranch Rd
Rimrock, AZ 86335
928-567-5699 <-
In OR: PO Box 436
31330 NW Pacific Ave.
North Plains, OR 97133
503-647-5464
Ron Roy on fri 29 nov 02
Hi Vince,
I have been thinking about how to answer this in a concise way. I do think
you are right about the surface iron being able to oxidize enough to not be
a flux enough to melt through the glaze.
I found that if I did not get into reduction fast enough and did not get a
body reduction before the glazes began to seal over was one explanation. If
I did not get into reduction before 1100C (on the pyrometer I was using)
the glazes in the hotter parts of the kiln would prevent body reduction. I
am also sure that some glazes will prevent body reduction at even lower
temperatures - those with high sodium and boron for instance.
I also think it is more likely that the iron at the surface of the body
would possibly be able to be reoxidized but we have to keep in mind - if
that iron is reduced during the bisque firing it starts to flux at quite a
low temperature - Hamer says 900C - from that point on it will combine with
silicates. If we think of a bisque firing say at 04 (1050C) then that
reduced iron has already started to combine - I think that is the reason it
is difficult to reoxidize.
If on the other hand the bisque firing has been fired correctly - with out
reduction - then it is possible that the iron may never have been reduced
before the glaze sealed over.
At any rate - you can see that if it is possible to reoxidize the surface
iron it would be more difficult to reoxidize the inner iron.
Black coring - or bloating - is usually found in the middle of the walls
when it is due to reduced bisque of iron bearing clays so it seems to me
that Hamer is correct in this.
As I said - I needed some time to think this over - if you have more to add
to this I would like to hear it - the more we know about this the better I
think.
RR
>> Once the iron is reduced in a clay body it is almost impossible to
>> reoxidize it - so relying on that to happen in a second firing is simply
>> not going to happen.
>
>Ron -
>Your posts are informative as usual, but I am wondering about the above. I
>have always done a body reduction to bring out the speckles in stoneware
>bodies. Years ago I used to go back into neutral or oxidation atmosphere
>after body reduction, but I often lost the speckles in part or all of the
>kiln. Now, I maintain a partial reduction after body reduction, and I never
>loose the speckles. So, I am assuming that oxidation after body reduction
>is capable of converting the iron back to the ferric form. Is that just
>happening on the surface, where it is most visible? That would certainly
>make sense.
>Best wishes -
>- Vince
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
