How Do I Know If My Metal Clay Kiln Is Running Hot And How To Fix It? by Holly Gage
Updated: Oct 29, 2020
I knew something was awry when a couple of batches of Metal Clay pieces had pitting and vermiculite stuck all over it. I knew the signs, my kiln was running too hot. You know the little tube with metal sticking out of the middle in the back of your kiln, that's a thermocouple. The thermocouple must not touch anything so it can read the temperature in the kiln chamber properly. It is the thermocouple that is the key to the whole mystery.
I called up the manufacturer where I bought the kiln and he told me how to determine exactly how hot my kiln was running, and what he told me blew my mind.
Here is the secret to knowing if your kiln is accurate!
Kiln tech support asked me to turn on the kiln and look at the reading on the controller panel. (This kiln says 66ºF) The reading on the panel should be the reading of the ambient temperature of the room.
If the temperature on the panel equals the actual temperature in the room then...
wait for it.... your kiln is firing at the accurate temperature. What, that's it??!!! YES!
What if your Kiln is not firing at the accurate temperature?
One thing you now know for sure is your kiln ambient temperature and your room temperature are different. Let's take a look at the problems that can come from the inaccuracy of your kiln and troubleshoot how this affects your Metal Clay sintering. Before you begin, take clear notes and records of what is happening and what you are doing. These will be invaluable if you need to talk to technical support or if you need to keep track if you altered something on your kiln.
Signs your kiln is under-firing?
There are several signs your Precious Metal Clay piece has underfired in the kiln.
Breakage is often the first sign. If you are firing at the correct temperature and for the length of time suggested, and your pieces are still brittle and break easily, chances are your Metal Clay kiln is not hot enough. Note: The white appearance of Silver Metal Clay on the inside is NOT an indicator, all fired Silver Metal Clay looks like that.
Shrinkage is also an indicator. Shrinkage rates are estimated on the package of your Metal Clay. If your piece has not shrunk within the range of suggested temperatures and is under those estimated temperatures, then it could be underfired. Taking before firing and after firing measurements will help you to determine the shrinkage.
Metal Clay can usually bend a bit. I am known for saying I am not a big fan of extreme bending, (in general, I don't think Metal Clay is made for dramatic bends), but subtle bending such as 10- 15 degrees shouldn't break a piece.
Signs your kiln is over-firing?
Here are the signs your Precious Metal Clay piece has overfired in the kiln.
In the first picture, you can see the vermiculite sticking to the Fired Metal Clay Jewelry indicating the kiln is too hot.
Also, the black marks and divots are pitting from a kiln overshooting its target temperature.
In the second picture, the piece started to melt, you can see that by the bubbles that formed on the surface. This looks a bit different than bubbles forming when the piece isn't completely dry when fired. The appearance is fewer bubbles and usually bigger in size.
Lastly, full melting where your piece becomes an unrecognizable blob stuck on your kiln shelf.
How to fix a Metal Clay kiln that is overshooting the kiln temperature.
One way to compensate for a kiln over firing is to adjust your firing temperature. Let's say for example your kiln is over-firing by 20ºF.
The ambient temperature display on the controller panel reads 46ºF.
The room temperature, however, is 66ºF.
This means your kiln is going to fire 20ºF more than it should.
Now, you want to fire a load of Fine Silver Metal Clay to 1650ºF, so to compensate, you will adjust your kiln to 1630ºF.
Learn more about your Kiln in my blog post, "Kiln Fring, Sintering, and Silver Metal Clay by Holly Gage
The second option is to Calibrate your Controller using a Thermocouple Offset
Arnold Howard of Howard Kilns is a Paragon Kiln distributor. He was so kind as to provide information on How to “Calibrate” a Paragon Sentry Xpress 3-Key Controller, Paragon Sentry Xpress 4.0, Paragon Sentry Xpress 5.0, and the Paragon SC2.
Some people wish for a way to improve the accuracy of their controller because perhaps they can tell that their silver clay is under- or over-fired.
There is a way to “calibrate” your controller using a little-known feature called Thermocouple Offset. First, look at the plastic overlay of your controller for the version number. Unfortunately, the Sentry Xpress versions 2.0 and 3.0 do not have Thermocouple Offset. It was introduced in version 4.0 and vastly improved in 5.0.
How to “Calibrate” the Sentry Xpress 4.0 with Thermocouple Offset
You can adjust the controller to fire up to 20°F (11°C) hotter or cooler than the factory setting.
The instructions for the Sentry Xpress 4.0 and 5.0 apply to the SC-2 unless the kiln has an earlier controller such as the 3.0. In that case, the kiln doesn't have Thermocouple Offset, because it was added to the 3-key controller starting in version 4.0.
From the [IdLE] display, press the Down Arrow key. After rate, temperature, hold, etc., [Strt] will appear.
While [Strt] is displayed, press the Up Arrow key.
[TCOS] will appear. Press the arrow keys to change the controller temperature.
Press the START key to return to the [Strt] display.
To fire the controller, press START again. [-On-] will appear. Or to return to [IdLE], press START two more times.
How to “Calibrate” the Sentry Xpress 5.0 with Thermocouple Offset
You can change the temperature setting of your controller up to 45°F (25°C) hotter or cooler than the factory setting.
1) From the [IDLE] display, press the Up Arrow repeatedly until [TCOS] appears. Note: With each Up Arrow key press, you will see a series of options or adjustments. When you see [EXIT], press the Up Arrow again to see more options. When [OPT2] appears, press Start/Stop. That will take you to more options.
When you see [TCOS] press the Start/Stop key. The current Thermocouple Offset adjustment number will appear. 0 indicates the factory thermocouple setting.
Using the arrow keys, select a new adjustment number. Then press the Start/Stop key. [IDLE] will appear. "H" followed by a number in Thermocouple Offset means the kiln will fire hotter by that many degrees. "C" followed by a number means the kiln will fire cooler by that many degrees.
The Third way to deal with the problem is by Upgrading Your Controller
Not all kilns have been mentioned here, but if you want to upgrade your controller, call your manufacturer. The price of a thermocouple varies according to kiln type, but it looks like with a little advice from your supplier and possibly Youtube you could install it yourself within minutes.
My last bit of advice
Don't do your testing on your masterpiece. I want to cry for the person who announces using their masterpiece they took hours to make only for something to go wrong. Make small test pieces. I usually make a simple charm or earring, that way if my tests go well I have a small treasure, but if they don't go well, no worries. I choose to avoid using regular old test strips because it seems like such a waste if all goes as planned.
Thank you Arnold Howard for your expertise on the topic. Any questions concerning the thermocouple offset function can be addressed to him.
Arnold Howard, Distributor
Howard Kilns https://paragonweb.com e-mail: firstname.lastname@example.org
Disclaimer: Holly Gage is not a professional kiln expert, just a gal who has used kilns for 19 years. Please refer to the manufacturer of your kiln if you are unsure of what you are doing. Holly Gage nor Arnold Howard can be held responsible for actions leading to injury or kiln damage.
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