Jim Bridgeman on fri 24 aug 07
Hi, I am new to mixing glazes and using a test kiln and have a question
regarding achieving results similar to what I would get in my regular
kiln. After reading the archives on test kilns, I understand that in
order to try to replicate the atmosphere of my regular kiln I need to
program the test kiln to fire similarly. Does that mean that if my
regular (7 cu.ft) kiln takes 8 hours to fire to cone 8 I need to have my
test kiln also take 8 hours to reach cone 8 and that if my regular kiln
takes 16 hours to cool I need to have the test kiln ramp down at a
similar rate?
For example, both kilns have a Bartlett V6-CF controller. If I use the
slow glaze setting at cone 8 with a 15 minute hold for a regular firing
can I just use the same slow glaze settings and hold time on the test
kiln? I am assuming I would need to control the rate of cooling to at
least 1500F, or so, to mimic the natural cooling of the 7 cu.ft kiln.
Thanks in advance for any advice or suggestions. I apologize if I
missed the specifics of mimicking the regular kiln in the archives.
After running a 15pt triaxial test with 3 glazes I already have, I am
looking forward to the whole new adventure of glaze mixing, formulation
and testing.
Jim
John Hesselberth on sun 26 aug 07
On Aug 24, 2007, at 9:02 PM, Jim Bridgeman wrote:
> For example, both kilns have a Bartlett V6-CF controller. If I use
> the
> slow glaze setting at cone 8 with a 15 minute hold for a regular
> firing
> can I just use the same slow glaze settings and hold time on the test
> kiln? I am assuming I would need to control the rate of cooling to at
> least 1500F, or so, to mimic the natural cooling of the 7 cu.ft kiln.
Hi Jim,
Welcome to the wonderful world of mixing and formulating your own
glazes. To match the results between your large and small kilns it is
much more important that you match cooling rates than heating rates.
If you fire to cone 6-8, matching cooling rates down to about
1400-1500 F is very important. Then you can just turn the kilns off.
Why not program a cooling cycle into both kilns so you don't have to
worry about variation in natural cooling rate on the large kiln?
I can't answer whether the slow glaze cycles are the same on both
controllers. Lots of kiln manufacturers use Bartlett controllers and
each may have a slightly different definition of "slow". Usually the
exact ramps are printed someplace in the fine print of the kiln
controller user's guide. What is important in the heating cycle is
that your glazes see the same peak temperature and that the kiln sits
for a few minutes at that peak temperature so the whole kiln evens
out. For this you should NOT trust the thermocouples on your
controller. Use large cones in front of the peep holes and learn to
see them. Thermocouple calibrations between kilns can easily be off
by half a cone or more.
Regards,
John
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
John Hesselberth on tue 28 aug 07
On Aug 28, 2007, at 1:20 AM, Jim Brideman wrote:
> The
> biggest question is that given what I have read regarding how greatly
> cooling at different rates and with different holds can alter the
> characteristics of a glaze, what would be a good starting point for
> cooling
> rates?
Hi Jim,
The one Ron and I recommend for Cone 6 would not be far off. You can
see it at (Question 2):
http://www.masteringglazes.com/Pages/faqframe.html
You might raise the end point for segment 4 to 2000F from 1900F and
turn the kiln off at 1500 instead of 1400F to compensate for the
slightly higher melting/freezing point of cone 8 glazes vs cone 6.
But to get good reprecipitation of crystals you need to go slowly (I
like 125F/hr) through that range where the glaze is still fluid
enough that crystals can form but cool enough they want to crystalize
out.
Jim Brideman on tue 28 aug 07
Thanks for the response. Both controllers are the same with the same
programs and ramp settings, but I do understand that the small test kiln
will naturally cool much quicker than the larger kiln. My intentions are to
program both to heat and cool using the same profile as you recommend. The
biggest question is that given what I have read regarding how greatly
cooling at different rates and with different holds can alter the
characteristics of a glaze, what would be a good starting point for cooling
rates? I understand that there would still be a lot of room for further
exploration, but any thoughts for a starting point for a cooling ramp from
cone 8 would be appreciated. To date I have bisque fired to cone 04 and
glaze fired to cone 8 with a 15min hold at the top(thermocouple is off by
full cone - as you indicated it may be) so actually reaching cone 9, using
stoneware.
Thanks,
Jim
>
>Welcome to the wonderful world of mixing and formulating your own
>glazes. To match the results between your large and small kilns it is
>much more important that you match cooling rates than heating rates.
>If you fire to cone 6-8, matching cooling rates down to about
>1400-1500 F is very important. Then you can just turn the kilns off.
>Why not program a cooling cycle into both kilns so you don't have to
>worry about variation in natural cooling rate on the large kiln?
>
>I can't answer whether the slow glaze cycles are the same on both
>controllers. Lots of kiln manufacturers use Bartlett controllers and
>each may have a slightly different definition of "slow". Usually the
>exact ramps are printed someplace in the fine print of the kiln
>controller user's guide. What is important in the heating cycle is
>that your glazes see the same peak temperature and that the kiln sits
>for a few minutes at that peak temperature so the whole kiln evens
>out. For this you should NOT trust the thermocouples on your
>controller. Use large cones in front of the peep holes and learn to
>see them. Thermocouple calibrations between kilns can easily be off
>by half a cone or more.
>
>Regards,
>
>John
>
>John Hesselberth
>www.frogpondpottery.com
>
>"Man is a tool-using animal....without tools he is nothing, with
>tools he is all" .... Thomas Carlyle
>
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Jim Bridgeman on wed 29 aug 07
John, thank you very much for your reply. I will try your recommedation
as a starting point and go from there. Ron and your book and website are
a wonderful resource and one of the inspirations behind my getting started
in glaze mixing and testing.
Thanks again,
Jim
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