Dan Semler on tue 20 mar 07
Hi Dan,
Been thinking about this.
Have you tested the absorption of the body when matured from
oxidation firings ? It would be interesting to see if the reduction
fired body is higher or lower in absorption, as some indication of the
maturation. And another question ? What is the body ? Is it an iron
bearing body or a porcelain or a white stoneware ?
I don't have a good theory, and iron being reduced only leads me to
conclusions that would give a higher expansion body not a lower one
... well unless you formed more glass.... hmmm.... But still bashing
away at it.
Thx
D
Dan Pfeiffer on tue 20 mar 07
I am glad all seemed to have a good time in NCECA, we hope to make it next
year. We found we can't afford to go to both the Alabama clay conference
and NCECA so next year we plan to skip the more local one and try NCECA.
Instead we did a reducing glaze firing that had the result of quite a
number of our pots looks quit good but crazed. The same clay and glaze does
not craze in a oxidizing firing. Any clue what is happening? The end result
looks to be that the clay did not mature to the same extent as in the
oxidizing firing. I started reducing at 1750F for 30 min=92s and than
neutral, more or less, up to ^10+ with more heavy reducing for 30 min's,
than a soak at 1900F for 2 hours.
Dan & Laurel in Elkmont Al
firearts on tue 20 mar 07
Lee,
No it was pretty much the same profile we always use. I did have a
lot of trouble getting a temp rise in reduction so the firing did go a
bit slower but not by much.
In the last few weeks we have done three firings all reduction. The
first one seemed ok, very even reduction and two not as even and in
this last one we really started to notice the crazing. This may be
because I had more dark blue in the kiln that in the first two loads.
You do not see the crazing in the celadon glaze but in one cornbread
baker we tested after cooking same very nice cornbread the glaze is
now very crazed. This using a clay that is very good at oven temps and
we have not had any other problems with it in other bake ware.
Dan
LEE>>>
Is there any other change in your firing schedual besides
reduction?
<<
Lee Love on wed 21 mar 07
Dan,
Is there any other change in your firing schedual besides reduction?
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"To affect the quality of the day, that is the highest of arts." -
Henry David Thoreau
"Let the beauty we love be what we do." - Rumi
Edouard Bastarache Inc. on wed 21 mar 07
Hello David,
data from Plainsman H-440,
a high fire brown stoneware.
See the influence of reduction
at c-10 on the absorption :
Fired Absorption:
Cone 8: 4.0-6.0%
Cone 10: 3.0-4.0
Cone 10R: 2.5-3.5
Hoping this could help in this debate.
Edouard Bastarache
Le Français Volant
The Flying Frenchman
Sorel-Tracy
Quebec
http://www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
www.thepottersshop.blogspot.com
http://www.ceramique.com/cerambooks/rayons/technologie.php
http://www.flickr.com/photos/30058682@N00/
Clayart on thu 22 mar 07
I keeping planning to do the absorption test but never get to it! Next
glaze firing I will be sure to put a slab in to test this, maybe a
blank test tile. Every since I saw how easy there are to make, thanks
to bill van gilder's demo, I now have lots of very nice test tiles. If
I had one in the reduction firing it should had make finding out what
happened much easier. We use a Highwater white stoneware "phoenix", it
does have some iron in it and in reduction makes great spots and can
go toast if very well reduced. Most offend it goes a nice stony gray.
We had a very hard time getting a glaze to not craze because it has a
very low COE, about 67 is my guess.
We refired the lot and that seem to fix the crazing, will need a few
more days to be sure, so it does look like the body just did not
mature. Maybe the problem was the cones just got hotter then the ware
sitting on the shelf? Most were apple baker so they are low bowls.
What if the body had too much reduction? Could carbon buildup do this?
Dan & laurel
>>>
Been thinking about this.
Have you tested the absorption of the body when matured from
oxidation firings ? It would be interesting to see if the reduction
fired body is higher or lower in absorption, as some indication of the
maturation. And another question ? What is the body ? Is it an iron
bearing body or a porcelain or a white stoneware ?
I don't have a good theory, and iron being reduced only leads me to
conclusions that would give a higher expansion body not a lower one
... well unless you formed more glass.... hmmm.... But still bashing
away at it.
Thx
D
<<<<
Dan Semler on thu 22 mar 07
Hi Dan and Laurel,
OK so there are several intersting things in this :
1. Your glazes fit but have little leeway I suspect. The reason I =20
think this is that the switch from redux to ox makes them fit, as does =20
a refire for reduction firing.
2. I'm still not sure about the maturation. The numbers Edouard put =20
out suggest that reduction may mature better, and I'm guessing that's =20
because of the iron. The comparison of absorption tests will help here.
3. I suspect that your glazes a 'hard'-ish if they tolerated a =20
refire. Also the bodies are not overfiring if you had no problems with =20
it.
4. So my guess currently is that you formed more high expansion =20
crystalline phases in the refire - very likely cristobalite. Iron and =20
high temps and the presence of cristobalite all promote cristobalite =20
formation which will help you out with this sort of fit issue. Of =20
course, none of this makes much sense on an ovenware body, which you'd =20
really like/expect low expansion. Dilatometry would help here.
5. The other thing is that the claybody-glaze interface will likely =20
have improved. I don't know much about how this would affect fit though.
So I don't have a good solution for you, unless you fire hotter or =20
longer or both at this point. Certainly, lower maturation is possible =20
but I can't explain it yet. Absorption tests would help. Of course, =20
the slowness of the firing may indicate something interesting. Too =20
many variables !!
Ok, I'll stop now. Let us know how it goes if do do the absoprion =20
tests and whether the refire really did help.
thanx
D
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