Tommy Humphries on fri 19 oct 01
Witness cones should be the large cones, either the regular, or the free
standing will do.
The large cones are just way easier to see looking through the peepholes.
Tommy
----- Original Message -----
From: "Deborah"
To:
Sent: Friday, October 19, 2001 2:51 PM
Subject: Pyrometers, cones & temperature
> Thanks for the answers, my next question is are these witness cones
> something diff. than the ones that I buy for my kiln sitter or can I make
a
> cone pack with kiln sitter cones?
>
> Deborah
> Estevan, Sk
>
>
____________________________________________________________________________
__
> 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.
>
Deborah on fri 19 oct 01
Hello again!!! Another question from the darkside of ignorance. First I
would like to thank you all for your past prompt attention to my
questions. Each answer has been invaluable to me in my quest to be a
ceramic artist and functional potter. Anyway here I am once again, hat in
hand humbly putting a duh question to you all.
I use an electric kiln with a kiln sitter and go to cone 6 with the tapered
orton cones. In the past some of my firings have been slightly off and so
in my need to understand what is going on inside my kiln I borrowed a
friend's pryometer and stuck it in my top peep hole during a bisque firing
to cone 04. The kiln sitter popped about 1850 degrees F and so with the
next firing to 04 I let it be in there until 1940 degrees F. The bisqued
clay M390 a terracotta from Plainsman seemed a richer colour in the one I
took the temp up to 1940 degrees F. Anyway I continued on satisfied that I
had fired to cone 04 the second time due to the visual temp. reading on the
pryometer and did a glaze fire to cone 6. The kiln sitter popped or shut
off at about 2050 degrees F and I reset it and tried to bring it up to 2332
degrees F. After about 2.5 hrs the pryometer showed little change in temp.
and so I gave up thinking I was going to underfire.
I now have some very blistered/cratered shiney (suppose to be matte) bowls
and the clay is not a rich brown colour but a washed out brown. I know
that I have over fired, but my question is why????? When the temp of the
kiln never seemed to reach cone 6...
duh Deborah in Estevan, SK
the pottery wasteland
Deborah on fri 19 oct 01
Sorry, correction on the cone 6 temp that I was trying to reach was 2232
degrees F.
Deborah
Estevan, SK
Cindy Strnad on fri 19 oct 01
Deborah,
The pyrometer was affected by the air coming in
from the peep hole.
They pyrometer may not have been accurate even if
it weren't affected by this air--you don't know
unless you test it with shelf cones.
Time equals heat work, so you can overfire even if
you never reach your target temperature.
Kiln sitter cones tend to bend at about a cone
lower than shelf cones. Place your shelf cones
about 6 inches away from the peep holes. Use them
to calibrate your kiln sitter and use them from
then on to keep tabs on your kiln sitter. They're
invaluable.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Leslie St. Clair on fri 19 oct 01
Hi Deborah,
Witness cones are the best way to tell if
you've reached temperature. Make a cone pack with
large cones 5,6 and 7, and put these on the bottom,
middle and top shelves. This will tell you what
temperature you hit in different areas of the kiln.
Pyrometers are good for a reference, but they
measure only one spot in the kiln, and measure only
temperature, not time, which is also a factor. Also,
in my kiln, the top is a cold spot so a pyrometer
reading there would be way off.
It also sounds as if you should put a higher cone
in your kiln sitter. Most electric kilns require a C7
in the sitter to get to C6, or a 03 to get to 04.
I hope this helps.
Leslie St. Clair
Ft. Mitchell, KY
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Tommy Humphries on fri 19 oct 01
Several points to answer in your question...Here goes
1. Pyrometers can LIE! Never trust a pyrometer over your cones. Use them
only as a guide till you know how your kiln fires.
2. You stuck the thermocouple through the top peephole. You should only
use a hole that is just large enough for the thermocouple to fit through. A
larger hole will allow air to infiltrate, possibly causing a low temperature
reading on the pyrometer. Many peephole plugs have a small diameter hole in
them for just this purpose.
3. Cones do not measure heat, but the effects of heat. I always explain
this as being like cooking biscuits in the oven. Set the temperature to 500
and the biscuits will be nice and brown in only a couple of minutes, but
sticky and undone in the center. Set the temperature to 350, and in a bit
longer time the biscuits are the same brown on the outside, but nice and
done on the inside too.
Cones tell you when your clay is done all the way through. Many kilns could
reach final temperature in 2 hours if you let them...but the pots would not
be fired correctly.
4. When using a kiln sitter always use large cones on the shelves of the
kiln as witness cones. If the witness cones say that the sitter is shutting
down at the wrong time, you can adjust the sitter to shut down properly. I
never let a load of pots fire without at least one cone pack somewhere in
the kiln. This is just an early warning device to tell me when something is
going amiss.
Tommy
----- Original Message -----
From: "Deborah"
To:
Sent: Friday, October 19, 2001 9:46 AM
Subject: Pyrometers, cones & temperature
> Hello again!!! Another question from the darkside of ignorance. First I
> would like to thank you all for your past prompt attention to my
> questions. Each answer has been invaluable to me in my quest to be a
> ceramic artist and functional potter. Anyway here I am once again, hat in
> hand humbly putting a duh question to you all.
>
> I use an electric kiln with a kiln sitter and go to cone 6 with the
tapered
> orton cones. In the past some of my firings have been slightly off and so
> in my need to understand what is going on inside my kiln I borrowed a
> friend's pryometer and stuck it in my top peep hole during a bisque firing
> to cone 04. The kiln sitter popped about 1850 degrees F and so with the
> next firing to 04 I let it be in there until 1940 degrees F. The bisqued
> clay M390 a terracotta from Plainsman seemed a richer colour in the one I
> took the temp up to 1940 degrees F. Anyway I continued on satisfied that
I
> had fired to cone 04 the second time due to the visual temp. reading on
the
> pryometer and did a glaze fire to cone 6. The kiln sitter popped or shut
> off at about 2050 degrees F and I reset it and tried to bring it up to
2332
> degrees F. After about 2.5 hrs the pryometer showed little change in
temp.
> and so I gave up thinking I was going to underfire.
>
> I now have some very blistered/cratered shiney (suppose to be matte) bowls
> and the clay is not a rich brown colour but a washed out brown. I know
> that I have over fired, but my question is why????? When the temp of the
> kiln never seemed to reach cone 6...
>
> duh Deborah in Estevan, SK
> the pottery wasteland
>
>
____________________________________________________________________________
__
> 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.
>
Frederich, Tim on fri 19 oct 01
Deborah,
There are several things to consider about your firing question.
Pyrometers may not be reading the actual temperature in the kiln. The
thermocouple may be off and the meter may not be that accurate. The higher
the temperature in the kiln, the more inaccurate the reading may be.
The only true way to actually know when the proper "heatwork" has taken
place with the ware is to have witness cones on the shelf next to it. The
cone in the kilnsitter may not be giving you a true value due to the
variances such as not being set properly, worn sensing rod or a hot spot if
it is too close to the element.
The heating rate during the final part of the firing will also affect the
amount of "heatwork" that the ware recieves. A slower firing will have a
lower end point equivalent temperature than a fast firing. This is why cone
charts have different columns of equivalent temperatures for different
heating rates.
Always trust your witness cones and calibrate your temperature readings to
the cone if you want to use the pyrometer as a guide.
Best regards,
Tim Frederich, Orton Ceramic Foundation
-----Original Message-----
From: Deborah [mailto:cornerstone4@SK.SYMPATICO.CA]
Sent: Friday, October 19, 2001 10:46 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Pyrometers, cones & temperature
Hello again!!! Another question from the darkside of ignorance. First I
would like to thank you all for your past prompt attention to my
questions. Each answer has been invaluable to me in my quest to be a
ceramic artist and functional potter. Anyway here I am once again, hat in
hand humbly putting a duh question to you all.
I use an electric kiln with a kiln sitter and go to cone 6 with the tapered
orton cones. In the past some of my firings have been slightly off and so
in my need to understand what is going on inside my kiln I borrowed a
friend's pryometer and stuck it in my top peep hole during a bisque firing
to cone 04. The kiln sitter popped about 1850 degrees F and so with the
next firing to 04 I let it be in there until 1940 degrees F. The bisqued
clay M390 a terracotta from Plainsman seemed a richer colour in the one I
took the temp up to 1940 degrees F. Anyway I continued on satisfied that I
had fired to cone 04 the second time due to the visual temp. reading on the
pryometer and did a glaze fire to cone 6. The kiln sitter popped or shut
off at about 2050 degrees F and I reset it and tried to bring it up to 2332
degrees F. After about 2.5 hrs the pryometer showed little change in temp.
and so I gave up thinking I was going to underfire.
I now have some very blistered/cratered shiney (suppose to be matte) bowls
and the clay is not a rich brown colour but a washed out brown. I know
that I have over fired, but my question is why????? When the temp of the
kiln never seemed to reach cone 6...
duh Deborah in Estevan, SK
the pottery wasteland
____________________________________________________________________________
__
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.
Deborah on fri 19 oct 01
Thanks for the answers, my next question is are these witness cones
something diff. than the ones that I buy for my kiln sitter or can I make a
cone pack with kiln sitter cones?
Deborah
Estevan, Sk
FireRight on fri 19 oct 01
Hi Deborah ~
Pyrometric cones are not temperature measuring devices. If you
examine the "cone charts" (
http://www.fireright.com/kiln/docs/general/conchart.html )
you'll see that the temperatures listed are specified with respect to the
rate of temperature change, nominally 270-degF/Hr. When the temperature rate
of change is not regulated, the temperature at which the cone matures varies
widely ... higher for faster rates of change and lower when the kiln is
"slower".
Practical electric kilns are not usually able to produce rates of
change as high as 270-degF/Hr once the temperature in the kiln reaches 1600
to 1800 degrees, because the skin temperature of the elements is usually
only about 2450-degF. As the temperature in the kiln approaches the
temperature of the elements themselves, the kiln begins to loose its ability
to increase the temperature. At some point the heat loss equals the
remaining heat input capability, and the kiln "peaks out" ... stalling at
some maximum, as you experienced.
The ramification of this as far as cones are concerned is that at
high operating temperatures the temperature at which the cone bends will
always be less than what is shown on cone charts because the rate of change
will always be something less than 270-degF/Hr. How much less depends upon
how fast the temperature is increasing.
Temperature comparisons are always likely to provide divergent
results unless conducted under close laboratory conditions. For the case in
point, you should know that potentially wide levels of "uncertainty" exist
for practical pyrometers and thermocouple type temperature sensors.
The practical analog-type "kiln pyrometer" that has been sold as a
kiln accessory for many years has "uncertainties" amounting to hundreds of
degrees on the high end. Unless the unit you are using originally cost
someone $150 or more, it cannot be relied upon for anything other than
measuring temperature rates of change (which was the original intent.)
Today's digital electronic type indicators are inherently more accurate
because they're not affected by the mechanical errors inherent with magnetic
meter movements, and are not sensitive to sensor resistance, but ...
... while type K thermocouple temperature sensors are accurate to
within a few degrees with respect to their mV/Deg curve, practical kiln
sensors are usually made of heavy 8-gauge wire, which gives them a very slow
response time. This is often aggravated by twisting the ends before welding
them together to form the element, and leaving a large blob at the weld. The
result is that as long as the temperature being measure is changing, the
measurement shown on the pyrometer will not be correct ... representing what
was going on several minutes ago, rather than right now.
Finally, electric kilns are pure convection/radiation heating
systems and may therefore have significant temperature gradients in the
firing chamber ... depending on the design of the kiln and upon how you've
loaded it. The only fair comparison would be to locate the tip of the
thermocouple temperature sensor right at the cone plaque or KilnSitter cone
supports.
The "bottom line" is probably that kiln firing is an art, rather
than a science ... if for no other reason than that fact that the equipment
is too rudimentary to support close control over firing parameters. I'm sure
that most will agree that consistent success comes from finding out
empirically what works for you, then sticking to that.
Gene Warner
FireRight
Frederich, Tim on mon 22 oct 01
Deborah,
You can use small cones as witness cones if you remember that they will
start to deform at a higher temperature than the larger witness cone at the
same heating rate. Gravity plays a role in the bending of the cone as it
starts to soften and glass is formed, thus the larger cone will fall a
little faster than the small cone. Small cones are also harder to see in
the atmosphere inside the kiln.
Best regards,
Tim Frederich
-----Original Message-----
From: Deborah [mailto:cornerstone4@SK.SYMPATICO.CA]
Sent: Friday, October 19, 2001 3:51 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Pyrometers, cones & temperature
Thanks for the answers, my next question is are these witness cones
something diff. than the ones that I buy for my kiln sitter or can I make a
cone pack with kiln sitter cones?
Deborah
Estevan, Sk
____________________________________________________________________________
__
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.
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