James Norton on sat 29 aug 98
I have a new Olympic kiln, 16 inch, two tier, cone 10, with a Dawson
kiln sitter. I glaze fire to cone 6 and use the kiln sitter to shut off
the current. (I know that this is not recommended.) The cone 6 in the
sitter deforms exactly as it should. In order to evaluate the heat
distribution in the kiln, I placed two cone plaques, using cones 4,5,
and 6, one in the MIDDLE of the lowest shelf, and one in the MIDDLE of
the upper shelf. While the cone 6 in the sitter was perfect, the two
cone plaques showed about cone 4. I glaze fire in about 15 hours,
advancing the heat stepwise. This two cone spread seems excessive to
me. Can anyone suggest how this can be minimized?
Jim Norton
jlnorton@execpc.com
John Post on sun 30 aug 98
>While the cone 6 in the sitter was perfect, the two
>cone plaques showed about cone 4....This two cone spread seems excessive to
>me. Can anyone suggest how this can be minimized?
The sitter cones will never match the cone plaque cones. They are
different in size and the one in the sitter is right next to the kiln wall
where the elements are giving off more heat.
Since you know there is a two cone difference between the sitter and the
shelf, you could just use a cone 8 in the sitter to get a cone 6 on the
shelf. This is not always reliable, because loads of different sizes will
heat up differently. This may however work most of the time.
I would put a cone plaque behind each peephole and monitor these near the
ends of each firing. The sitter is really meant as a backup device to
prevent overfirings and not as an accurate measure of heatwork affecting
your ware.
I have an electronic controller on my kiln and still use cone plaques
behind each peephole and on the shelves. I fire the ware until the proper
cones in the plaque have dropped.
I usually set the shut off temp on my controller for 2219 degrees F with a
15 minute soak and a controlled cooldown. Most of the time this hits cone
6. On a recent load that wasn't densely packed this hit cone 8, proving
that the size of the load impacted the final firing temperature.
A two cone difference doesn't seem excessive to me. It's about what I
would expect. If you have glazes with narrow firing ranges or just wish to
have consistent firings you will have to monitor your cone packs more closely.
Cheers,
John Post
potter@c3net.net
Sterling Heights, MI USA
Don Prey on mon 31 aug 98
In a message dated 08/29/98 5:43:52 AM, you wrote:
<me. Can anyone suggest how this can be minimized?>>
Jim, this 2 cone difference is what I would expect from a properly operating
kiln. You need to either use a cone 8 in the sitter or use a pyrometric bar
(not cone) rated at cone 6. I suggest trying the bar.
Don Prey in oregon
Pamela & Evan Kohler-Camp on tue 1 sep 98
Dear Don,
O.K., I admit that I'm still learning, so please excuse what may be a dumb
question. BUT, what's the differance between a cone and a pyrometric bar?
I've seen both but did not think there was a differance. TIA
Pamela in Atlanta, Ga,USA Where 90 degrees is looking pretty good outside.
At 10:32 AM 8/31/1998 EDT, you wrote:
>----------------------------Original message----------------------------
>In a message dated 08/29/98 5:43:52 AM, you wrote:
>
><>me. Can anyone suggest how this can be minimized?>>
>
>Jim, this 2 cone difference is what I would expect from a properly operating
>kiln. You need to either use a cone 8 in the sitter or use a pyrometric bar
>(not cone) rated at cone 6. I suggest trying the bar.
>Don Prey in oregon
>
Don Prey on thu 3 sep 98
Hi Pamela,
Here is what I think I know about cones and bars: first there were cones, in
two sizes, senior and junior. these are intended to be set upright in a clay
pat or holder so that they are leaning over at about a six degree angle.
since they are composed of the same sorts of materials that glazes are made of
they react to heat work (heat plus time) in much the same manner that the
glazes do. so they are meant to give a visual indication of firing progress
and we get to select the end point of the firing (usually with a large cone).
when the kiln setter was invented the junior cones were a natural choice to be
part of a mechanisim that would shut off a kiln at some predeterminded point.
however, the cone is now lying on it's side and being deformed by gravity plus
the weight of the metal rod. the result is that the standing large cone and
the laying down small cone deform at different rates. so they invented the
pyrometric bar which is designed to work in the horizontal position in the
sitter and arranged it so the number associated with the bar is the same as a
large cone number. if you are going to let the kiln sitter terminate your
glaze firings, the bar will give you the most consistant results. put some
large, self standing cone 6's on the shelves with your pots, a number 6 bar in
the sitter and record your results. keeping good records will tell you which
variables to adjust. also, be sure to get ahold of the booklet that Orton
gives away (your cone supplier may have some). They are your best source of
information. and if my information and theirs differ, you had best go with
them.
and if this is just more confusing, let me know....I'll try to do better.
Don Prey in Oregon
Bob Wicks on thu 3 sep 98
In a message dated 9/1/98 10:48:57 AM Eastern Daylight Time,
kclaughed@mindspring.com writes:
<< what's the differance between a cone and a pyrometric bar?
I've seen both but did not think there was a differance. TIA
>>
Don, I'm sure you have an answer to your question by now but I'll add my
comments anyhow. As you know a cone is tapered and small parts melt faster
that large parts. If you push a cone to the point where only the small part
is under the trigger bar, it will bend sooner than if you pushed it to the
point where the cone was thicker. This whole phenomena is corrected by using
a pyrometric bar, which is the same thickness the entire length, therefore
eliminating inconsistent readings. I hope this helps you to understand how
the pyrometric bar works.
Bob
Cameron Harman on fri 4 sep 98
Hi folks!
This is a subject that I have addressed in my free ezine from time
to time. The fact is that electric kilns are by nature not very
uniform. The temperature varies from the outside to the inside
simply because the heat is transferred by radiation only. It is
line of sight.
So, anything that blocks the ware from the heating elements will
have to heat up first then reradiate in order for it to get hot.
since the heaters are all around the load in most electric kilns,
there ill always be some heat coming in from some angle, so the
ware will get a little direct radiation and then will heat up by
conduction as the heat travels from the hot surface to the cooler
surface on the same piece of ware.
This phenomena can also cause cracking of the ware, so that will
limit how fast you can heat a kiln loaded with ware. the more
pieces of ware the longer it will take, generally.
The cone in the kiln sitter will always see more heat than the
cone sitting near a piece of ware on a shelf in the kiln.
Additionally, without some outside controlling factor the top will
always be hotter than the bottom. Today's kiln builders now offer
controls to help solve that problem.
However, it will still be hotter on the outside of the load than
on the inside. It is for this reason that industrial kiln users
have a "soak" built into their firing schedules. As Orton
Foundation will tell you, if you hold the temperature at the point
where the outside cone just starts to move, you can cause it to
bend fully with time (no increase in temperature). At the same
time the interior is given time for the temperature to reach
there. With a long enough soak you can get close to the same heat
treatment in all parts of the kiln.
The kiln sitter will not allow for a soak, so it will always lead
to a difference in heat treatment from outside to inside. in the
case of some ceramic bodies (used mostly by hobbyists) the
temperature difference doesn't matter. However, for some clays it
can be very important and for some glazes it can be very
important.
I hope this helps you.
Cameron Harman
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
wallace myers on sat 5 sep 98
-----Original Message-----
From: Cameron Harman
To: CLAYART@LSV.UKY.EDU
Date: Friday, September 04, 1998 5:41 AM
Subject: Re: kiln heat distribution
>----------------------------Original message----------------------------
>Hi folks!
>
>This is a subject that I have addressed in my free ezine from time
>to time. The fact is that electric kilns are by nature not very
>uniform. The temperature varies from the outside to the inside
>simply because the heat is transferred by radiation only. It is
>line of sight.
>
>So, anything that blocks the ware from the heating elements will
>have to heat up first then reradiate in order for it to get hot.
>since the heaters are all around the load in most electric kilns,
>there ill always be some heat coming in from some angle, so the
>ware will get a little direct radiation and then will heat up by
>conduction as the heat travels from the hot surface to the cooler
>surface on the same piece of ware.
>
>This phenomena can also cause cracking of the ware, so that will
>limit how fast you can heat a kiln loaded with ware. the more
>pieces of ware the longer it will take, generally.
>
>The cone in the kiln sitter will always see more heat than the
>cone sitting near a piece of ware on a shelf in the kiln.
>Additionally, without some outside controlling factor the top will
>always be hotter than the bottom. Today's kiln builders now offer
>controls to help solve that problem.
>
>However, it will still be hotter on the outside of the load than
>on the inside. It is for this reason that industrial kiln users
>have a "soak" built into their firing schedules. As Orton
>Foundation will tell you, if you hold the temperature at the point
>where the outside cone just starts to move, you can cause it to
>bend fully with time (no increase in temperature). At the same
>time the interior is given time for the temperature to reach
>there. With a long enough soak you can get close to the same heat
>treatment in all parts of the kiln.
>
>The kiln sitter will not allow for a soak, so it will always lead
>to a difference in heat treatment from outside to inside. in the
>case of some ceramic bodies (used mostly by hobbyists) the
>temperature difference doesn't matter. However, for some clays it
>can be very important and for some glazes it can be very
>important.
>
>I hope this helps you.
>
>Cameron Harman
>
>--
>**********************************************************
> Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
> e-mail kilns@kilnman.com
> Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
>get your free ezine: http://www.kilnman.com/ezine/ezine.html
> THE place for total kiln and drier support
>**********************************************************
Ron Roy on sat 5 sep 98
Hi Cameron,
I would just like to congratulate you on this - everyone who fires in an
electric kiln will better understand what they are up against after reading
this.
I would also like to add - if you are going to use a sitter - use it only
to tell you when to start watching your cones - when it shuts off a cone or
two below your firing temp. - reset the sitter and manage your firing
manually - and the soaking thing is important.
If you can't afford to sit it yourself - get a controller and program your
firing the way it should be.
RR
>This is a subject that I have addressed in my free ezine from time
>to time. The fact is that electric kilns are by nature not very
>uniform. The temperature varies from the outside to the inside
>simply because the heat is transferred by radiation only. It is
>line of sight.
>
>So, anything that blocks the ware from the heating elements will
>have to heat up first then reradiate in order for it to get hot.
>since the heaters are all around the load in most electric kilns,
>there ill always be some heat coming in from some angle, so the
>ware will get a little direct radiation and then will heat up by
>conduction as the heat travels from the hot surface to the cooler
>surface on the same piece of ware.
>
>This phenomena can also cause cracking of the ware, so that will
>limit how fast you can heat a kiln loaded with ware. the more
>pieces of ware the longer it will take, generally.
>
>The cone in the kiln sitter will always see more heat than the
>cone sitting near a piece of ware on a shelf in the kiln.
>Additionally, without some outside controlling factor the top will
>always be hotter than the bottom. Today's kiln builders now offer
>controls to help solve that problem.
>
>However, it will still be hotter on the outside of the load than
>on the inside. It is for this reason that industrial kiln users
>have a "soak" built into their firing schedules. As Orton
>Foundation will tell you, if you hold the temperature at the point
>where the outside cone just starts to move, you can cause it to
>bend fully with time (no increase in temperature). At the same
>time the interior is given time for the temperature to reach
>there. With a long enough soak you can get close to the same heat
>treatment in all parts of the kiln.
>
>The kiln sitter will not allow for a soak, so it will always lead
>to a difference in heat treatment from outside to inside. in the
>case of some ceramic bodies (used mostly by hobbyists) the
>temperature difference doesn't matter. However, for some clays it
>can be very important and for some glazes it can be very
>important.
>
>I hope this helps you.
>
>Cameron Harman
>
>--
>**********************************************************
> Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
> e-mail kilns@kilnman.com
> Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
>get your free ezine: http://www.kilnman.com/ezine/ezine.html
> THE place for total kiln and drier support
>**********************************************************
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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