Paul B on wed 30 nov 05
On the last firing i did, which was the first one with a new burner system,
there was one part of the kiln that seemed have gotten a heavier reduction
than the rest, as the bases of the pots had a much darker look than normal;
and some of the pots in that area had blisters. Some were very small and
mostly healed over but there were a few bad ones that did not show up at
all in the parts of the kiln where reduction was lighter. I start reduction
when the hottest part of the kiln gets to ^012, but at that point in the
firing there are still parts that are much cooler, perhaps ^014 like at the
top. I have wondered if this is too early and plan to start later next time.
Is it likely that this kind of situation could be the source of blistering?
thanks,
Paul
Ron Roy on thu 1 dec 05
Either the clay is reduced or not - I don't think the problem is the
reduction - perhaps the clay body - if you have a recipe I can check it out
- compare it to other cone 10 reduction bodies and perhaps tell if is a
body problem.
The other possibility is an improper bisque firing - if the organics are
not burned out properly and there is enough iron in the clay you can get
over fluxing.
Glad to check the clay out for you.
RR
>On the last firing i did, which was the first one with a new burner system,
>there was one part of the kiln that seemed have gotten a heavier reduction
>than the rest, as the bases of the pots had a much darker look than normal;
>and some of the pots in that area had blisters. Some were very small and
>mostly healed over but there were a few bad ones that did not show up at
>all in the parts of the kiln where reduction was lighter. I start reduction
>when the hottest part of the kiln gets to ^012, but at that point in the
>firing there are still parts that are much cooler, perhaps ^014 like at the
>top. I have wondered if this is too early and plan to start later next time.
>Is it likely that this kind of situation could be the source of blistering?
>thanks,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Paul B on thu 1 dec 05
Ron,
the clay body is:
epk: 30
hawthorne bond 35 mesh fireclay: 30
OM-4 ball clay: 15
goldart: 15
G200: 10
Although the OM-4 and goldart are both known to vary a lot, i have tried
substiting them out with no apparant effects. Blisters on some pots but not
on others, some firings not many but others more, sometimes more in one
part of the kiln but not in another part.
I fire this body to cone 10/11 and, at least accoring to my tests, it is
close to 2% absorption.
Please let me know what your thoughts are.
thanks,
Paul
Ivor and Olive Lewis on fri 2 dec 05
Dear Paul,
Not much evidence to go on here to make a valid judgement but I would =
make an assumption that some organic material, though carburised, still =
remains in your clay at Cone 014, even at Cone 012. I would also assume =
that your clay contains some Iron minerals. Reduction of these by Carbon =
will release gas and as this evolves it will blister your maturing =
glaze.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Sam Kelly on sat 3 dec 05
First off how many burners on your new system?
If more the one are they connected to the same pressure regulator?
How do you calculate reduction, by eye at the spy hole or an Oxygen probe?
Sam
Ron Roy on sat 3 dec 05
HI Paul,
When I compared it 3 other cone 10 reduction clay bodies from Tuckers I
find it short of alumina - which is what I use mostly to gauge
refractoriness.
At least part of the problem has to be that clay body - from where I sit anyway.
I do approve of the amount of spar and I do agree - no silica is a good
idea - if you do need some temper to cut down shrinkage - consider some
Kyanite - 100M - or some 30 M grog.
I also advise you to sieve through 30 mesh - at very least the Hawthorn -
and buy the 40 mesh Hawthorn.
EPK - 45.0
Hawthorn - 25.0
OM#4 - 10.0
Gold Art 10.0
G200 - 10.0
Total - 100.0
You should also be aware that if the organics are not being burned out in
the bisque firing - that can result in overfired clay - but I tend to think
if it's happening in one part of the kiln and not others then it's probably
the body.
If the bisque firing was the problem you would find it happening more where
the clay is thicker - around bottoms as well because they take longer to
get the organics burned out.
If you are bisquing in a gas kiln make sure each burner is oxidizing - you
can check this with some darker clay buttons around the bisque kiln.
Remember - if a bisque kiln is reducing - nothing will flame inside it -
because there will be no free Oxygen - use a stick to find out.
I test absorbency by weighing dry - right out of the kiln, into boiling
water for two hours - cool with cold water - and weigh right away. Don't
try to weigh hot bars - the water is evaporating to fast.
You should be making a couple of test bars from each batch of clay you make
1" by 1/2 " by 6" long. They will give you the information you need to make
adjustments - especially if you are doing the same with your raw materials.
Hope this all makes sense - if not let me know - RR
>Ron,
>the clay body is:
>
>epk: 30
>hawthorne bond 35 mesh fireclay: 30
>OM-4 ball clay: 15
>goldart: 15
>G200: 10
>
>
>
>Although the OM-4 and goldart are both known to vary a lot, i have tried
>substiting them out with no apparant effects. Blisters on some pots but not
>on others, some firings not many but others more, sometimes more in one
>part of the kiln but not in another part.
>I fire this body to cone 10/11 and, at least accoring to my tests, it is
>close to 2% absorption.
>Please let me know what your thoughts are.
>thanks,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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