Testing Your Kiln's Temperature

by Janet Alexander

 Now that most kilns have a computerized controller it’s become easier to control the kiln’s temperature. But, is the controller’s readout accurate? With so many varieties of bronze, copper, and silver metal clays, it has become important to know if your kiln’s temperature reading is correct. Otherwise, you may have problems with the clay not sintering correctly or getting too hot and melting. Additionally, the kiln should be tested for hot spots and cooler spots. Every kiln is different.





Testing can be done by using a pyrometer (which is how I used to adjust my kiln’s temperature before there were controllers) or by using kiln pyrometric cones. The pyrometric cones are supposed to bend when heated to a specific temperature for a certain amount of time. That means that pyrometric cones give a temperature equivalent; they are not simple temperature-measuring devices. According to a pyrometric cone manufacturer, cones have over 20 variables that can affect the cone’s bending, including the cone's composition, particle size of raw materials, type of forming process, moisture during forming, density of the cone, geometry of the cone, setting height, and angle and the heating rate. Atmosphere also affects bending behavior. Wow, that’s a lot of variables!  So, let’s look at testing with a pyrometer or a kiln test kit.

I used the kiln test kit sold by the PMC Connection. According to experts, the average kiln controller is accurate to ±10°F (±5.5°C) so keep this in mind while testing. Additionally, run the test several times but move the thermocouple to different areas of your kiln. Place it towards the back, near the front, off to each side, and etc. Test at different temperatures. I conducted tests at 1110˚F, 1200˚F, 1290˚F, 1470˚F, 1560˚F, and 1650˚F. I found that both readouts were within a few degrees of each other until the temperature got up to 1650˚F. Then they were off by 10˚F! So, I added another thermocouple from my casting kiln.  All three read different degrees but were within the accuracy range.

Now I know why my PMC3 clay had a crystalline look to it when I fired it at 1650˚F; it was getting too hot! So, now I lower the temperature of my kiln by a few degrees when setting it at 1650˚F.

The Test Kit Includes:

• Sensor  reader (tester)
• 9V Battery 
• K-type thermocouple TP-02A  
• K-type thermocouple TP-03 
• Case 

Use the TP-02A thermocouple (larger one) for testing your kiln. It has a temperature measuring range of (-58˚F to 1650˚F).



The Steps

1.  Install the 9 V battery into the unit’s back.

2.  Insert the plug into the bottom of the sensor reader making sure the plug’s polarity matches with the sensor’s polarity. 

3.  Insert the thermocouple into the kiln.

Caution: Don’t insert it past larger ceramic end or the wires will burn!

4.  Place the thermocouple near the kiln’s thermocouple for the first run of testing.

5.  Turn on the kiln and set it to hold at the test temperature for at least 15 minutes.

The sensor reader can read in Fahrenheit (F) or Centigrade (C). Turn it on by sliding the button from the center (off position) to the F (if measuring in Fahrenheit).

6.  The sensor displays its reading. Allow it a minute to stabilize to the temperature. Test at various temperatures. I found that my kiln was accurate until it reached over 1350 (F).

7.  When finished testing, turn off kiln and sensor reader and allow the thermocouple to cool before touching it.

Kiln Maintenance For Metal Clay Artists

There are two types of kilns, brick and ceramic muffle. Mine is a muffle. Muffle kilns heat up faster than the brick type and can easily be moved from classroom to classroom without damaging the kiln. Its best to not move the brick type kiln because, movement can cause the bricks to become loose since they are held together with mortar.

Over time the muffle can develop cracks. I've been told that the cracks shown here in my kiln were caused by opening the door while the kiln is very hot. I'm guilty of this, due to opening the door while enameling. These types of cracks are not harmful. As long as the muffle isn't breaking apart in pieces, and the elements are still secure inside the muffle. As the muffle heats up it expands and the cracks close.

You should occasionally inspect your kiln by checking the thermocouple in the back of the kiln making sure it sticks out from the back of the kiln about 1/2". It can sometimes inadvertently get pushed into the muffle. The thermocouple measures the kiln's temperature and sends this information to the kiln's computer. If it is pushed into the muffle it can't accurately measure the temperature inside the kiln. 

If you are having problems with melting your metal clay while firing, then you should test the thermocouple for accuracy with a kiln tester. Here is a link to my instructions on testing the thermocouple: Testing Kiln.  

Sometimes the door's latch needs adjusting. The latch should lightly catch so that it doesn't shake the kiln. The door doesn't need to close tightly against the front of the kiln. There needs to be some space for the door's material to expand when it heats. If you work with enamels, you don't want the door to shake the kiln when closing otherwise you may find your enamel has fallen off your piece! On my kiln the door latch adjusts by twisting a screw in or out on the latch.  

It’s also good to check the kiln's plug wires making sure they are not cracked or damaged.  If they are seek advice from a kiln repair company.