Ian Currie on wed 25 jul 01
Greetings
Just a little in reply to the person who had a similar experience to Tim
with a rutile giving unreliable tan/blue colours. (Sorry, I inadvertently
lost the post in my daily deleting frenzy.)
One thing I have become aware of is how uneven temperature is in many
commercial electric kilns, and how unaware of this many users are. I have
made this discovery during the workshops that I conduct around Australia and
the US, where we nearly always have an overnight firing in a computer
controlled electric kiln.
A number of problems arise, and confusion, because many have stopped using
cones, or use only one pack.
It would seem that a two cone difference between bottom shelf (say, 3 inches
above the floor) and top shelf about 6 inches below the lid is normal.
Depending on the size of the kiln and number of tiles I may fire several
shelves in the bottom, and several in the top, with nothing stacked in the
middle. I do this because I want a difference of firing temperature between
two identical groups of grid-tiles. Sometimes the kiln is full, but I
always put at least one set of cones on the bottom shelf and one on the top,
and a difference of 2 cones is normal. This is in spite of a 1 hour soak at
top temperature. I am really pleased with the result, as it gives me two
"cone numbers" in one firing, but imagine this might be a problem with some
glazes if the potter is assuming even firing.
It is hard diagnosing a problem through the key-hole view of an e-mail
message, but I suspect that the temperature (more accurately cone
number/heat work) is to blame.
Almost without exception people are surprised to find this degree of
variation within their own kiln. A first thing to try is to put several
sets of cones in your firing. At least 2 sets, as described above... with
identical samples at each location i.e. same glaze, same clay, same glaze
thickness. If you are still getting the tan/blue variation on different
samples at the same cone, on the same body, then we have to look further.
But I suspect this may be the culprit. If however the blue appears in
samples fired higher than the tan result, then everything is as it should
be... order prevails.... the laws of the universe are in place...
If not... we need more detail to proceed.
Regards
Ian
http://ian.currie.to/
Marvpots@AOL.COM on wed 25 jul 01
Dear Ian:
Thanks for your comments.
My kiln is a gas (propane) fired kiln; I fired to cone 10 in reduction;
cone packs top and bottom reacted almost identically.
My best suspicion is that despite pyrometer readings (I have one in the upper
right back wall and one in the middle of the front) I exceeded cone 10 since
cone 11 went down as well and that that might have affected the result.
Other than that, I can find nothing in the recipe; many firings in the past
using the same glaze gave good variagated blue shiny results contrasted with
the tan/matte results this time.
Again, yhour comments are always welcomed and respected.
Marvin Flowerman
Giles on wed 25 jul 01
Have you bought new rutile recently? A couple of years ago when our
distributor started getting rutile from Australia instead of Africa (I
think) there was a definite difference in our results with some, but not all
of our rutile glazes.
Kristen Giles
mgiles@onramp
----- Original Message -----
> Other than that, I can find nothing in the recipe; many firings in the
past
> using the same glaze gave good variagated blue shiny results contrasted
with
> the tan/matte results this time.
>
> Again, yhour comments are always welcomed and respected.
>
> Marvin Flowerman
Ian Currie on thu 26 jul 01
Hi Marvin
Are you saying that you had cone packs in this firing, or are you refering
to a test you did some time before?
If you are saying that it apparently overfired, and a rutile blue has gone
tan, then this is not what I would expect. Is there any way the glaze mix
might have changed?
Is there any zinc oxide in the recipe?
Regards
Ian
http://ian.currie.to/
-----Original Message-----
From: Marvpots@AOL.COM
To: CLAYART@LSV.CERAMICS.ORG
Date: Thursday, 26 July 2001 4:16
Subject: Re: Rutile again and Uneven Kilns
Dear Ian:
Thanks for your comments.
My kiln is a gas (propane) fired kiln; I fired to cone 10 in reduction;
cone packs top and bottom reacted almost identically.
My best suspicion is that despite pyrometer readings (I have one in the
upper
right back wall and one in the middle of the front) I exceeded cone 10 since
cone 11 went down as well and that that might have affected the result.
Other than that, I can find nothing in the recipe; many firings in the past
using the same glaze gave good variagated blue shiny results contrasted with
the tan/matte results this time.
Again, yhour comments are always welcomed and respected.
Marvin Flowerman
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Earl Brunner on thu 26 jul 01
I have refired good Floating Blue and had it turn out this way (dead brown).
Ian Currie wrote:
> Hi Marvin
>
> Are you saying that you had cone packs in this firing, or are you refering
> to a test you did some time before?
>
> If you are saying that it apparently overfired, and a rutile blue has gone
> tan, then this is not what I would expect. Is there any way the glaze mix
> might have changed?
>
> Is there any zinc oxide in the recipe?
>
> Regards
>
> Ian
> http://ian.currie.to/
>
> -----Original Message-----
> From: Marvpots@AOL.COM
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Thursday, 26 July 2001 4:16
> Subject: Re: Rutile again and Uneven Kilns
>
>
> Dear Ian:
> Thanks for your comments.
> My kiln is a gas (propane) fired kiln; I fired to cone 10 in reduction;
> cone packs top and bottom reacted almost identically.
> My best suspicion is that despite pyrometer readings (I have one in the
> upper
> right back wall and one in the middle of the front) I exceeded cone 10 since
> cone 11 went down as well and that that might have affected the result.
> Other than that, I can find nothing in the recipe; many firings in the past
> using the same glaze gave good variagated blue shiny results contrasted with
> the tan/matte results this time.
>
> Again, yhour comments are always welcomed and respected.
>
> Marvin Flowerman
>
> ____________________________________________________________________________
> __
> Send postings to clayart@lsv.ceramics.org
>
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>
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> melpots@pclink.com.
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
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--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
Marvpots@AOL.COM on thu 26 jul 01
Dear Ian:
No, this was not a new mixture.
No, there is no zinc oxide in the recipe which consists of Dolomite, custer,
Whiting, EPK Kaolin, Flint and 8% rutile.
The next time I fire my gas kiln I will set up some experiments to determine
the effect of kiln placement and I'll post the results.
Again, many thanks for your interest.
All the best.
Marvin
Ian Currie on tue 31 jul 01
Hi Marvin and also Tim
Been trying to get back to your rutile blue problem... have just found a
very good reply from Tom Wirt/Betsy Price dated July 27 which you have no
doubt read. This seems to give good advice. The only thing I would add to
it is (wait for it!!!!) the usefulness of the grid method in these
intractable cases.
I know I go on about it, but this is what I know! And in many many cases
it finds an answer. Notice I said "an". This highlights the focus of the
method, which is not to focus on just one glaze (or analysis/Seger formula),
but a spread. With the multitude of affective variables at play in the
ceramic cauldron there are usually many solutions to a specific problem.
There are many compensating variables in recipe/clay/firing that move the
phenomena (both desirable and not) around a multi-dimentional matrix. Two
of the more effective compensating variables (and also the source of many
analytical variations in our materials, and therefore the actual source of
many problems) are alumina and silica, and these two are varied in any
standard grid as laid out at my web page.
http://ian.currie.to/
See in particular the Calculation Page where one can produce a grid of 35
recipes that systematically vary alumina and silica over a wide range.
Having said that, alumina/silica variation is only part of the solution (but
an important one, usually omitted). But the rest you probably know
already... varying the flux set, varying the raw material source of the
fluxes, varying the amount of rutile, and also changing firing and clay
body.... some of which has been addressed in the Tom Wirt/Betsy Price post.
Not sure if I can help any more from this distance!! Sorting a difficult
problem like this by e-mail is like trying to perform keyhole surgery
wearing welding goggles. On the other hand, I know EXACTLY why there was
just now a percussive "thump" from my bisque kiln, and why there are bits of
clay spread around the setting! One understands the meaning of "mixed
feelings".
The grid method is explained at my web site and also in great detail in my
most recent book: "Revealing Glazes - Using the Grid Method", illustrated
and for sale at the same web site.
Thankyou
Ian
http://ian.currie.to/
-----Original Message-----
From: Marvpots@AOL.COM
To: CLAYART@LSV.CERAMICS.ORG
Date: Friday, 27 July 2001 2:51
Subject: Re: Rutile again and Uneven Kilns
Dear Ian:
No, this was not a new mixture.
No, there is no zinc oxide in the recipe which consists of Dolomite, custer,
Whiting, EPK Kaolin, Flint and 8% rutile.
The next time I fire my gas kiln I will set up some experiments to determine
the effect of kiln placement and I'll post the results.
Again, many thanks for your interest.
All the best.
Marvin
____________________________________________________________________________
__
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/
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