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- May 19 2013 01:18 AM
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Posted 19 May 2013Apologies for telling you things you might know already, but this is the way I understand it:
For calibrating we usually need to get some correlation between your controller settings and what results you actually get with cones.
The thermometer/thermocouple is the only sensor that provides feedback to the controller. So the electric controller switches the element on and off to maintain a pre-programmed temperature or rate/ramp (degrees/hr) based only on the temp feedback.
It does not directly measure heat work (as with cones). It can do some calculations, based on temp & time, to calculate the approximate heat work or cone, but I'm not sure if yours does.
To check the kiln you manually calculate the cone results based on what the ramp/rate is during e.g. the last 15mins of approaching the set temperature.
To help with this get a temperature datalogger to measure & log the temperatures you get during firing.
(You will need to insert its probe into the kiln somewhere).
There are tables that tell you what the cone results should be for certain final rates.
e.g. if you approach final temp slowly (or with a soak period) you will get a higher temp cone and if you approach it fast a lower temp cone.
Your calculation should match more or less what you get, unless your temperature probe is wrong. (Or it measures some part of the kiln which is not representative)
As you will know there are other things that can influence your results, e.g. the size of the load (because it will be slower to heat) and the temperature differences between top/bottom, also if you have bung holes open or closed.
I have checked the adverts for your "DynaTrol" controller and it seems to do some calculations for heat-work:
"Electronic Control Matches Pyrometric Cone Performance"
"This feature..[patented].. adjusts the final temperature reached in an Easy-Fire profile based on the speed of the actual firing.
It results in accurate automatic control of heat-work in ceramic firing".
So if this is so good, I wonder why do you not get accurate results ?
Posted 18 May 2013... please tell me exactly what kind of firing speed is safe for 1/4"-1/2" thick earthenware or raku sculpture that has been bisqued, and is being fired with only underglazes or copper carbonate?
I also do earthenware sculptures, often without glaze, with raku clay or groggy clay.
I still want to experiment with how fast you can go, but I can tell you that you can go much faster that the "normal" rates of e.g. 100degrees/hr.
Raku clay is made to withstand being taken out of a raku gas kiln at peak temperature into cold atmosphere, so they can withstand a lot of thermal shock.
They are also made to withstand being fired in a raku gas kiln in a few hours, e.g. I have seen 1000 degrees Celcius in 2hrs (that is if they are bisqued beforehand).
So don't be shy to go fast.
I am much encouraged by all the good news I read here about rapid firing.
Save time, save electricity, save on greenhouse gas emmissions.
Posted 18 May 2013I guess it boils down to how often do you like to change the heating coils and relays in your kiln
I am new to pottery and "magic" of kiln firing but what I have read and told by people with years of experience - faster you heat up, faster you burn out the heating coils/wires. Simple physics.
Hi Mart, you are correct about the switching relays/contactors deteriorating because they switch more often during a slow firing.
However I do not think you waste the elements more during rapid heating.
Most kilns that I know switch the elements hard on & off to control the temperature. More "off" periods for slow heating and more "on" periods for faster heating.
The elements do not go to a higher temperature during fast heating (they still get the same voltage), they just stay on longer. (less "off" periods)
However the added-up on-time will be less during a quick firing because the kiln does not waste time to loose as much heat.
So if the elements deteriorate because they are used more, then a quick firing will actually be better,
Unless there are other factors involved, and if there are I am happy to learn about it.
I think all the electricity you save with rapid firing will also help to buy new elements.
What may make your elements deteriorate more is if the atmosphere in the kiln has moisture or other gasses that add to corrosion/oxidation of the elements.
e.g. if you fire greenware with all bung holes covered from the start.
Posted 17 May 2013I would like to know more myself from other people's experience.
I can offer the following advice:
There are two critical temperatures involving rapid expansion/contraction (Quartz inversion)
at 573⁰C and 220⁰C. Care should be taken going through these temperatures.
The temperature climb up to and around boiling point 100⁰C need to be slower for greenware because if steam develops it can crack/explode
The more grog (particles) in the clay the more resistant it will be to thermal shock. Raku clays are especially good. (Some African people use open wood firing which provide rapid heating with temperature fluctuation, but they use a lot of grog in their clay).
Ware which have varying thickness need slower heating.
Thick pieces need slower heating. Thin pieces can be fired very rapidly.
The dryer the better.
Posted 11 May 2013No
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