Fred Parker on sat 3 jun 06
A question for you seasoned glaze gurus:
As everyone who is knowledgeable advises I include cone packs in my glaze
firings in a computer-controlled Skutt 1227. In my last firing I had
three dozen test tiles plus eight larger (21" to 23" or so tall by 6" to
10" radius base) pieces. According to the cone packs I had the greatest
heat difference from bottom to top than ever before -- so much that I
probably slightly underfired the bottom (test tiles) and overfired the top
(large pieces.)
In loading the kiln, I first placed 1" posts to get the first shelf off
the bottom. Next, I placed 6" posts and positioned all of the test tiles
and cone pack #1. Next, I positioned shelves on top of the 6" posts and
loaded the larger pieces. Both shelf levels used two half shelves with a
1/4" gap between them.
In order to place the cone more in the middle of this second level I
positioned two 8" posts together and placed cone pack #2 on top, slightly
off center. I made an effort to evenly distribute the large pieces to
even out heat absorption/radiation over the entire level. Both cone packs
included large ^5, ^6 and ^7 cones set in the same clay.
After firing, ^6 of cone pack #1 (bottom) was at the 2 o'clock position
and ^7 was barely showing signs of bending. Cone 6 of cone pack #2 (top)
was sagging pitifully over the edge of the clay pack, pointing straight
down, and ^7 was noticeably, but only slightly more bent than in pack #1.
All the test tiles on the bottom level look very good. There is minimal
pinholing, but it was a test batch so I would expect that. On the upper
level, however, some of the glazes appear to be somewhat dried out and
overfired.
I fired to 2175F, which is 10F less than my previous firing, where the
cone pack indicated a slight overfire. The cycle was as follows:
100f/hr to 220 hold 30 minutes.
350f/hr to 2,000 no hold.
108f/hr to 2,175 hold for 20 minutes.
-500f/hr to 1,900 hold for 10 minutes.
-125f/hr to 1,400 no hold
shut down.
My concern is, once I test fire glaze samples and get what I want, then
when I glaze a larger piece and it overfires, I might get a fairly crappy
final result in spite of all the systematic preparation. Can anyone
advise? Should I reduce max temp even more? Reduce some of the hold
times? Is this just to be expected with the type kiln I am using?
Many thanks,
Fred Parker
Brian Fistler on sat 3 jun 06
On Sat, 2006-06-03 at 13:44 -0400, Fred Parker wrote:
> A question for you seasoned glaze gurus:
>
> As everyone who is knowledgeable advises I include cone packs in my glaze
> firings in a computer-controlled Skutt 1227. In my last firing I had
> three dozen test tiles plus eight larger (21" to 23" or so tall by 6" to
> 10" radius base) pieces. According to the cone packs I had the greatest
> heat difference from bottom to top than ever before -- so much that I
> probably slightly underfired the bottom (test tiles) and overfired the top
> (large pieces.)
>
> In loading the kiln, I first placed 1" posts to get the first shelf off
> the bottom. Next, I placed 6" posts and positioned all of the test tiles
> and cone pack #1. Next, I positioned shelves on top of the 6" posts and
> loaded the larger pieces. Both shelf levels used two half shelves with a
> 1/4" gap between them.
>
> In order to place the cone more in the middle of this second level I
> positioned two 8" posts together and placed cone pack #2 on top, slightly
> off center. I made an effort to evenly distribute the large pieces to
> even out heat absorption/radiation over the entire level. Both cone packs
> included large ^5, ^6 and ^7 cones set in the same clay.
>
> After firing, ^6 of cone pack #1 (bottom) was at the 2 o'clock position
> and ^7 was barely showing signs of bending. Cone 6 of cone pack #2 (top)
> was sagging pitifully over the edge of the clay pack, pointing straight
> down, and ^7 was noticeably, but only slightly more bent than in pack #1.
>
> All the test tiles on the bottom level look very good. There is minimal
> pinholing, but it was a test batch so I would expect that. On the upper
> level, however, some of the glazes appear to be somewhat dried out and
> overfired.
>
> I fired to 2175F, which is 10F less than my previous firing, where the
> cone pack indicated a slight overfire. The cycle was as follows:
>
> 100f/hr to 220 hold 30 minutes.
> 350f/hr to 2,000 no hold.
> 108f/hr to 2,175 hold for 20 minutes.
> -500f/hr to 1,900 hold for 10 minutes.
> -125f/hr to 1,400 no hold
> shut down.
>
> My concern is, once I test fire glaze samples and get what I want, then
> when I glaze a larger piece and it overfires, I might get a fairly crappy
> final result in spite of all the systematic preparation. Can anyone
> advise? Should I reduce max temp even more? Reduce some of the hold
> times? Is this just to be expected with the type kiln I am using?
>
> Many thanks,
>
> Fred Parker
>
Fred,
The first thing I would check is to make sure all of your elements are
working correctly in the kiln. If I remember correctly, most Skutt
kilns do not use zone-control, but instead use the idea of "balanced
elements" meaning less power in the middle where the kiln is normally
hotter... But if one of your bottom elements aren't firing, or is
getting ready to burn out, it could account for your temperature
variance.
Also, if your 2nd shelf happened to hit right between where the lower
and upper elements are at, it could definitely have "insulated" the
temperature differences between the zones.
Another possibility (one I've done myself, so don't take this as an
insult...) are you absolutely sure your cone6 cone WAS a cone 6 cone on
top? It sounds a little strange that 7 was almost the same on both
levels, 6 was at 2 oclock on the one, but the other was completely
slumped over... Could your "cone 6" on top have possibly been a 5 or
even 4?
It's possible that you don't have as much temperature variation as you
think you did. Also, in my limited experience, unless a glaze is way
over-fired, it's more likely to melt more, running more, and become more
*glossy* than what you are expecting, rather than looking dried out. I
would expect see "dried out" looking glazes more from underfiring than
overfiring. This is also hinted at by the fact you fired 10 degrees
LESS than before... Also, I don't know exactly where your thermal couple
is in the kiln, but I would guess, with your 2nd shelf being about 8"
+/- from the floor of the kiln, that the thermalcouple would have been
in that top zone with your larger "dry looking" pieces.
I'm not saying a computer can't make a mistake, thermal couple could be
off etc... but assuming everything WAS working correctly, and *if* the
thermal couple is in the top zone with the larger pieces, it should have
brought THOSE pieces to the temp you programmed it for. It may be that
the glazes you thought looked over-fired might really have needed that
extra 10 degrees to gloss out properly. I don't know how much
experience you have had previously with the glazes on the larger pieces,
so I can't say for sure on that... You would know better than I would
on that aspect.
Brian
(Personally, if it were me, I'd much rather have made the cone mistake,
than think I had a problem with the kiln... ;)
Ron Roy on sun 4 jun 06
Hi Fred,
In this case - I would have stacked the taller ware first and the tests on top.
Bottoms are usually cooler - so stacking the denser part of the load at the
top would have slowed the top down a bit.
We have to think about the shelves as part of the stack - a bunch of
shelves can really slow a part of the kiln down - because they soak up a
lot of heat
I too am wondering - if the cones said the top was hotter - why would the
glazes be drier?
RR
>As everyone who is knowledgeable advises I include cone packs in my glaze
>firings in a computer-controlled Skutt 1227. In my last firing I had
>three dozen test tiles plus eight larger (21" to 23" or so tall by 6" to
>10" radius base) pieces. According to the cone packs I had the greatest
>heat difference from bottom to top than ever before -- so much that I
>probably slightly underfired the bottom (test tiles) and overfired the top
>(large pieces.)
>
>In loading the kiln, I first placed 1" posts to get the first shelf off
>the bottom. Next, I placed 6" posts and positioned all of the test tiles
>and cone pack #1. Next, I positioned shelves on top of the 6" posts and
>loaded the larger pieces. Both shelf levels used two half shelves with a
>1/4" gap between them.
>
>In order to place the cone more in the middle of this second level I
>positioned two 8" posts together and placed cone pack #2 on top, slightly
>off center. I made an effort to evenly distribute the large pieces to
>even out heat absorption/radiation over the entire level. Both cone packs
>included large ^5, ^6 and ^7 cones set in the same clay.
>
>After firing, ^6 of cone pack #1 (bottom) was at the 2 o'clock position
>and ^7 was barely showing signs of bending. Cone 6 of cone pack #2 (top)
>was sagging pitifully over the edge of the clay pack, pointing straight
>down, and ^7 was noticeably, but only slightly more bent than in pack #1.
>
>All the test tiles on the bottom level look very good. There is minimal
>pinholing, but it was a test batch so I would expect that. On the upper
>level, however, some of the glazes appear to be somewhat dried out and
>overfired.
>
>I fired to 2175F, which is 10F less than my previous firing, where the
>cone pack indicated a slight overfire. The cycle was as follows:
>
>100f/hr to 220 hold 30 minutes.
>350f/hr to 2,000 no hold.
>108f/hr to 2,175 hold for 20 minutes.
>-500f/hr to 1,900 hold for 10 minutes.
>-125f/hr to 1,400 no hold
>shut down.
>
>My concern is, once I test fire glaze samples and get what I want, then
>when I glaze a larger piece and it overfires, I might get a fairly crappy
>final result in spite of all the systematic preparation. Can anyone
>advise? Should I reduce max temp even more? Reduce some of the hold
>times? Is this just to be expected with the type kiln I am using?
>
>Many thanks,
>
>Fred Parker
>
>______________________________________________________________________________
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Fred Parker on sun 4 jun 06
Thanks for your thoughts, Brian.
I doble-checked the cones, and both were, in fact, cone 6. After you
mentioned it, I was kinda hoping I had screwed up there, but no...
Oddly, for my last big glaze firing (which was configured with two full
shelves topped off by a half shelf -- for three layers) the results were
the exact opposite of this time: it was measurably cooler at the top,
warmer in the middle and hottest at the bottom. The only differences were
the 10 degree hotter setting for that firing and the type pieces being
fired: the bottom was test tiles, the middle had medium sized pieces and
the top half shelf had the flat stuff (shallow bowls, larger decorative
tiles etc. By the way, you are right re the "balanced elements" approach
of the Skutt.
I'm thinking I need to pay closer attention to where the shelves align
with the elements. Also, I need to pay attention to the thermocouple,
which I have mostly ignored to date. For this firing I believe it was
just below the level of the upper cone pack, so I suppose it is possible
it is going bad. Not sure what the symptoms of a failing thermocouple
are...
Re the glazes I used, most were new for me, although I had tiles on the
lower level with at least one of them. It was a matte glaze, and a small
tile so it didn't have enough surface area to show the variations I got on
the larger piece, although it seemed to fire to a more uniform (not dried-
out) matte than the larger pieces.
Next time I'll take special care to align the elements with the opening.
Many thanks for your thoughts...
Fred Parker
>The first thing I would check is to make sure all of your elements are
>working correctly in the kiln. If I remember correctly, most Skutt
>kilns do not use zone-control, but instead use the idea of "balanced
>elements" meaning less power in the middle where the kiln is normally
>hotter... But if one of your bottom elements aren't firing, or is
>getting ready to burn out, it could account for your temperature
>variance.
>
>Also, if your 2nd shelf happened to hit right between where the lower
>and upper elements are at, it could definitely have "insulated" the
>temperature differences between the zones.
>
>Another possibility (one I've done myself, so don't take this as an
>insult...) are you absolutely sure your cone6 cone WAS a cone 6 cone on
>top? It sounds a little strange that 7 was almost the same on both
>levels, 6 was at 2 oclock on the one, but the other was completely
>slumped over... Could your "cone 6" on top have possibly been a 5 or
>even 4?
>
>It's possible that you don't have as much temperature variation as you
>think you did. Also, in my limited experience, unless a glaze is way
>over-fired, it's more likely to melt more, running more, and become more
>*glossy* than what you are expecting, rather than looking dried out. I
>would expect see "dried out" looking glazes more from underfiring than
>overfiring. This is also hinted at by the fact you fired 10 degrees
>LESS than before... Also, I don't know exactly where your thermal couple
>is in the kiln, but I would guess, with your 2nd shelf being about 8"
>+/- from the floor of the kiln, that the thermalcouple would have been
>in that top zone with your larger "dry looking" pieces.
>
>I'm not saying a computer can't make a mistake, thermal couple could be
>off etc... but assuming everything WAS working correctly, and *if* the
>thermal couple is in the top zone with the larger pieces, it should have
>brought THOSE pieces to the temp you programmed it for. It may be that
>the glazes you thought looked over-fired might really have needed that
>extra 10 degrees to gloss out properly. I don't know how much
>experience you have had previously with the glazes on the larger pieces,
>so I can't say for sure on that... You would know better than I would
>on that aspect.
>
>Brian
>
>(Personally, if it were me, I'd much rather have made the cone mistake,
>than think I had a problem with the kiln... ;)
>
>__________________________________________________________________________
____
>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.
Brian Fistler on sun 4 jun 06
On Sun, 2006-06-04 at 09:28 -0400, Fred Parker wrote:
[snip]
> I'm thinking I need to pay closer attention to where the shelves align
> with the elements. Also, I need to pay attention to the thermocouple,
> which I have mostly ignored to date. For this firing I believe it was
> just below the level of the upper cone pack, so I suppose it is possible
> it is going bad. Not sure what the symptoms of a failing thermocouple
> are...
>
I can't tell you either... Considering it's a bi-metallic device, I
would think one symptom would be "drift" of the true temperature from
the perceived temperature. (readings are off by X% high or low) I would
think it would be consistently either high or low, but as it fails it
would probably increase in it's error. Either way, I don't think it
would account for the differential you saw.
> Re the glazes I used, most were new for me, although I had tiles on the
> lower level with at least one of them. It was a matte glaze, and a small
> tile so it didn't have enough surface area to show the variations I got on
> the larger piece, although it seemed to fire to a more uniform (not dried-
> out) matte than the larger pieces.
>
You might also consider stopping the fast-ramp at a lower temperature so
your 108 degree/hr ramp lasts longer to allow the heat in the kiln more
time to even out. Maybe start the slower final ramp 100 degrees lower
and see if that helps even it out.
`
> Next time I'll take special care to align the elements with the opening.
> Many thanks for your thoughts...
Brian
William & Susan Schran User on sun 4 jun 06
On 6/4/06 9:28 AM, "Fred Parker" wrote:
> Also, I need to pay attention to the thermocouple,
> which I have mostly ignored to date. For this firing I believe it was
> just below the level of the upper cone pack, so I suppose it is possible
> it is going bad. Not sure what the symptoms of a failing thermocouple
> are...
This is perhaps one of the downsides to a programmable controlled kiln with
one thermocouple - the program is "reading" the heat from only that one area
in the kiln where the thermocouple is.
At school we have two kilns with programmable controllers. Both are L&L with
thermocouples in each section. One kiln is two sections/two thermocouples,
the other three & three. We use these kilns for crystalline glaze firings.
We also have cones at each level for every firing. This system seems to work
very well for us. The cones are very even top to bottom.
Perhaps you should try taller forms top and bottom and shorter forms in the
center of the kiln?
Do you have a venting system on your kiln?
After I installed a venting system on my manual kiln in my studio, I found I
could achieve very even temperatures top to bottom.
-- William "Bill" Schran
Fredericksburg, Virginia
wschran@cox.net
wschran@nvcc.edu
Gay Judson on sun 4 jun 06
I forgot to mention that I do have the vent that I got with my Skutt
and I think it makes a difference in evening out the heat top to
bottom. Also I was told to have at least 1" between thermocouple and
shelf.
Gay
> -----Original Message-----
> From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Fred
> Parker
> Sent: Sunday, June 04, 2006 8:28 AM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: Uneven heat in kiln question
>=20
> Thanks for your thoughts, Brian.
>=20
> I doble-checked the cones, and both were, in fact, cone 6. After
> you
> mentioned it, I was kinda hoping I had screwed up there, but no...
>=20
> Oddly, for my last big glaze firing (which was configured with two
> full
> shelves topped off by a half shelf -- for three layers) the results
> were
> the exact opposite of this time: it was measurably cooler at the
> top,
> warmer in the middle and hottest at the bottom. The only
> differences were
> the 10 degree hotter setting for that firing and the type pieces
> being
> fired: the bottom was test tiles, the middle had medium sized pieces
> and
> the top half shelf had the flat stuff (shallow bowls, larger
> decorative
> tiles etc. By the way, you are right re the "balanced elements"
> approach
> of the Skutt.
>=20
> I'm thinking I need to pay closer attention to where the shelves
> align
> with the elements. Also, I need to pay attention to the
> thermocouple,
> which I have mostly ignored to date. For this firing I believe it
> was
> just below the level of the upper cone pack, so I suppose it is
> possible
> it is going bad. Not sure what the symptoms of a failing
> thermocouple
> are...
>=20
> Re the glazes I used, most were new for me, although I had tiles on
> the
> lower level with at least one of them. It was a matte glaze, and a
> small
> tile so it didn't have enough surface area to show the variations I
> got on
> the larger piece, although it seemed to fire to a more uniform (not
> dried-
> out) matte than the larger pieces.
>=20
> Next time I'll take special care to align the elements with the
> opening.
> Many thanks for your thoughts...
>=20
> Fred Parker
>=20
> >The first thing I would check is to make sure all of your elements
> are
> >working correctly in the kiln. If I remember correctly, most Skutt
> >kilns do not use zone-control, but instead use the idea of
> "balanced
> >elements" meaning less power in the middle where the kiln is
> normally
> >hotter... But if one of your bottom elements aren't firing, or is
> >getting ready to burn out, it could account for your temperature
> >variance.
> >
> >Also, if your 2nd shelf happened to hit right between where the
> lower
> >and upper elements are at, it could definitely have "insulated" the
> >temperature differences between the zones.
> >
> >Another possibility (one I've done myself, so don't take this as an
> >insult...) are you absolutely sure your cone6 cone WAS a cone 6
> cone on
> >top? It sounds a little strange that 7 was almost the same on both
> >levels, 6 was at 2 oclock on the one, but the other was completely
> >slumped over... Could your "cone 6" on top have possibly been a 5
> or
> >even 4?
> >
> >It's possible that you don't have as much temperature variation as
> you
> >think you did. Also, in my limited experience, unless a glaze is
> way
> >over-fired, it's more likely to melt more, running more, and become
> more
> >*glossy* than what you are expecting, rather than looking dried
> out. I
> >would expect see "dried out" looking glazes more from underfiring
> than
> >overfiring. This is also hinted at by the fact you fired 10
> degrees
> >LESS than before... Also, I don't know exactly where your thermal
> couple
> >is in the kiln, but I would guess, with your 2nd shelf being about
> 8"
> >+/- from the floor of the kiln, that the thermalcouple would have
> been
> >in that top zone with your larger "dry looking" pieces.
> >
> >I'm not saying a computer can't make a mistake, thermal couple
> could be
> >off etc... but assuming everything WAS working correctly, and *if*
> the
> >thermal couple is in the top zone with the larger pieces, it should
> have
> >brought THOSE pieces to the temp you programmed it for. It may be
> that
> >the glazes you thought looked over-fired might really have needed
> that
> >extra 10 degrees to gloss out properly. I don't know how much
> >experience you have had previously with the glazes on the larger
> pieces,
> >so I can't say for sure on that... You would know better than I
> would
> >on that aspect.
> >
> >Brian
> >
> >(Personally, if it were me, I'd much rather have made the cone
> mistake,
> >than think I had a problem with the kiln... ;)
> >
> >___________________________________________________________________
> _______
> ____
> >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.
>=20
> ____________________________________________________________________
> __________
> Send postings to clayart@lsv.ceramics.org
>=20
> You may look at the archives for the list or change your
> subscription
> settings from http://www.ceramics.org/clayart/
>=20
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
Fred Parker on mon 5 jun 06
Hi Ron:
I'm not sure how to stack the larger pieces on top. They are around 21"
to 23" tall. The longest posts I can find are 10". Am I missing
something?
Thanks,
Fred
On Sun, 4 Jun 2006 01:16:21 -0500, Ron Roy wrote:
>Hi Fred,
>
>In this case - I would have stacked the taller ware first and the tests
on top.
>
>Bottoms are usually cooler - so stacking the denser part of the load at
the
>top would have slowed the top down a bit.
>
>We have to think about the shelves as part of the stack - a bunch of
>shelves can really slow a part of the kiln down - because they soak up a
>lot of heat
>
>I too am wondering - if the cones said the top was hotter - why would the
>glazes be drier?
>
>RR
Arnold Howard on mon 5 jun 06
From: "Fred Parker"
> As everyone who is knowledgeable advises I include cone
> packs in my glaze
> firings in a computer-controlled Skutt 1227.
> All the test tiles on the bottom level look very good.
> There is minimal
> pinholing, but it was a test batch so I would expect that.
> On the upper
> level, however, some of the glazes appear to be somewhat
> dried out and
> overfired.
Fred, from what I understand, in your first firing the top
section overfired, and in your next firing the top section
underfired. That seems to be the main problem.
As Ron mentioned, shelves add great thermal mass to a kiln.
Thermal mass absorbs energy. Perhaps the inconsistent heat
distribution is due to shelf placement.
Were you near the kiln at the end of both firings? I've
heard of a case where the lid lifted slightly and caused the
top section to underfire. Maybe that happened during the
second firing.
You could also position the witness cones so that you could
see them through the peepholes. Judging the firing by the
cones, you could manually fire the kiln a little longer or
shut it off sooner. In that case, write down the shutoff
temperature, and program that temperature for the next
firing.
Bill mentioned his L&L 3-zone kiln. I am slightly out of
touch with the Bartlett controller, because Paragon switched
to the Orton five years ago. Nevertheless, I am pretty sure
that L&L and Skutt both use the same Bartlett controller.
The Orton and Bartlett controllers are 3-zone. They are
configured at the factory as single-zone, 2-zone, or 3-zone.
If you knew how to configure your controller, you could
convert it to 3-zone. You would also need to rewire the kiln
and add two thermocouples. (The connections for the extra
thermocouples are already on the board.)
But instead of doing that, I would experiment with shelf
placement.
Sincerely,
Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
Ron Roy on wed 7 jun 06
Hi Fred,
Sounds like some fire bricks on end would do it - if you use soft brick at
cone 6 for instance - you could cut them in half and get two two and a
quater by two and a half by nine.
>I'm not sure how to stack the larger pieces on top. They are around 21"
>to 23" tall. The longest posts I can find are 10". Am I missing
>something?
>
>Thanks,
>
>Fred
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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