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| Photo credit: Kiln Frog |
Sometimes it is difficult to replace a kiln shelf back into the kiln with work on it. This is normally because the thermocouple sticks out from the wall of the kiln. Questions have been asked if the thermocouple can be below the shelf to make it easier to place. Others have asked about reducing the distance into the kiln that the thermocouple projects into the kiln.
These are both bad ideas.
Thermocouple Under the Shelf
It is not a good idea to have thermocouple
under the shelf because it will then measuring
the air temperature under the shelf. The air temperature under the shelf
can be as much as 100ºC/180ºF lower than above the shelf where the work is
being fired. This will cause an overfire
on the way up. Setting the top
temperature for 790ºC/1454ºF may give an actual air temperature of up to 890ºC/1634ºF!
On the way down at annealing temperature,
the air temperature below the shelf is hotter than the air temperature above. It
might be annealing at 582º/1080ºF. This
will result in improper annealing at too high a temperature. The cool
will start too early. The time at the
appropriate annealing temperature will be too little. And the cool finish at too high a temperature.
It would be a disaster of a firing. Don't do it!
Reduce the Distance into the Kiln
Another
suggestion is to reduce the distance the thermocouple is into the kiln. This produces inaccurate readings too.
If the
thermocouple is not fully inserted, it records a lower temperature than when fully
inserted. I know this from bitter experience. This results in the pieces being
overfired. But also in inadequate annealing,
just as when the thermocouple is under the shelf.
The
placement of the thermocouple is critical to the reading of the air
temperatures in the kiln. The
thermocouple should not be moved unless absolutely necessary. If it is moved, it must be checked to be in
the same location as originally placed, because if it is not replaced exactly the temperature readings will be different than
previously.
