By Janet Alexander
to control the kiln’s temperature. But, is the controller’s readout accurate? With
the varieties of bronze, copper, and silver metal clays it has become
important to know if your kiln’s temperature’s 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. Pyrometric cones give a temperature
equivalent; they are not simple temperature-measuring devices. According
to a pyrometric cone manufacturer, these cones have over 20 variables
that can affect the cone’s bending. Some of these variables include: cone
composition, particle size of raw materials, type of forming process,
moisture during forming, density of the part, geometry of the part, 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,
a 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 to lower
the temperature of my kiln by a few degrees when setting it at 1650˚F. Now I know why my PMC3 clay has a crystalline
look to it when I fired it at 1650˚F; it was
getting too hot!
|The test kit includes: |
Sensor reader (tester)
K-type thermocouple TP-02A
K-type thermocouple TP-03
Instructions (not well written) Use the TP-02A thermocouple (larger one) for testing your kiln. It has a temperature measuring range of
(-58˚F to 1650˚F).
Install the 9 V battery into the unit.
Insert the plug into the bottom of the sensor reader making sure the plug’s polarity matches with the sensor’s polarity.
Insert the thermocouple into the kiln.
Caution: Don’t insert it past larger ceramic end or the wires will burn!
Place the thermocouple near the kiln’s
Turn on 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 on the sensor reader by sliding the button from the center (off position) to the F (if measuring in Fahrenheit).
The sensor displays its reading.
Allow a minute to stabilize to the temperature.
When finished testing, turn off kiln and sensor reader and allow the thermocouple to cool before touching it.