Sunday, 15 February 2015

Looks Like the Kiln is Overfiring

Recently I purchased some large cones used to check the temperature the kiln is firing at.  Because I have a kiln that uses a kiln sitter and doesn't have a temperature controller or a thermocouple, I am relying heavily on the little mini cone that bends when it reaches a certain temperature to turn off the kiln.

I've had my suspicions that my old sitter kiln may be firing higher than what I wanted it too. Some of my glazes had been starting to run a little too much, it never used to be too much of an issue.

I had the opportunity to purchase a used portable thermometer that could be used to measure temperatures up to 1300degrees Celcius.  While it needed the end replaced, it was still useable and appeared to have a reasonably accurate temperature.

Firstly the end is placed in the kiln peephole while the kiln was firing and after a fairly short wait it comes up with the temperature at the end of the wire.
(just a demo to show how it works, kiln already emptied)
The kiln had been loaded and had been on for 8 hours already.  When I put the probe in it came right up to 1094degrees Celcius.  Because this was a bisque firing I only wanted it to go to cone 04 (approx. 1000degrees Celcius)  Already it was 100 degrees over according to this thermometer.   The kiln turned off shortly after.
This is a pic of the little 04 cone that bends when it reaches the correct temperature.  When it bends the middle bar falls down when triggers the cut  off switch. The number of the cone correlates to the temperature it bends at.
I thought with end probe being a little burnt off that perhaps it wasn't that accurate anyway.
However when I unloaded the kiln I found that the two large o4 cones that had been sitting on the top and bottom shelf had well and truly fallen and were resting on the kiln shelf.  This had confirmed the over firing.
 A closer look at the fallen 04 cone that was on the top shelf

Normally the cone wouldn't bend right over to touch the shelf.  Unfortunately I don't have any 03 cones to see just how far the temperature really did get.

The pots from this load all looked a little duller in colour than the normal pinky bisque shade.  Just slightly though.

It will be interesting to see how well they absorb the glaze. I'm also wondering just how long this has been happening as usually I don't put cones on the shelves to check.
The large crescent that I talked about in the last blog was bisque fired with this load as well.

The top overhang did drop quite a bit, I had a feeling it might.  It will be interesting to see how much more it drops in the final glaze firing where the temperature is another 260-80 degrees above the bisque.

The lesson I've learned is to regularly check the firing temperatures with cones on the shelves.  Not too important with the glaze firing as most of my clays/glazes will withstand a little higher temperature without any negative affects, but the bisque firing temperature can affect how much glaze can be absorbed as the clay has already started to vitrify and is not so porous.

Things would be much easier if I had a new kiln with a temperature controller and a thermocouple, then I'd know exactly at a glance what was going on.  It looks like I'll have to sell a heap of bonsai pots to afford one of those!