Forest Butera on sat 29 jul 06
Putting a new computer board in my new Paragon 1193 test kiln didn't fix
the erratic firing problem so the next step is to replace the
thermocouple. The first one had turned black. Should they remain silver-
colored? What could have caused it to turn black?
Rogier Donker on sun 30 jul 06
Ah! Forest!
Being inside the firing chamber, thermocouples literally burn up in
time.It is perfectly normal for the tip to turn black. Eventually the
whole tip becomes a charcoal-brittle-black oxidised mass. As it burns
up it also becomes less accurate. Most thermocouples are two twisted
wires, fused into a silvery bead at the business end. Some are not
twisted wires,but rather consist of two wires butted up and welded.
Most hobby kilns have the latter. With each successive firing the
thermocouple becomes a little less accurate. That is why it is very
important to keep a kiln log so you can get to "know" your kiln. You
might want to be interested in my manual, available through the web
site.
BTW: I still have some "junkque" hobby kilns available!
Rogier(always of good courage!)
See us on the web at http://www.donkerstudio.org
Ivor and Olive Lewis on sun 30 jul 06
Dear Forest Butera,=20
You tell us <<...The first one had turned black. Should they remain =
silver-colored? What could have caused it to turn black?...>>
If your original thermocouple was made from Platinum-Platinum Rhodium =
alloy it should not change colour (Types; B, R, T). If it was =
constructed from base metal alloys (Types; E, J, K, N)then it could have =
suffered from Oxidation. There are instances of clay that contains =
Pyrites being fired. This would release gaseous Sulphurous compounds =
into the kiln. If, over a long period these diffused through the ceramic =
sheath then the metal could acquire a black tarnish.
Best regards,=20
Ivor Lewis.
Redhill,
South Australia.
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