kiln "curse" continued

[Barbara Muth]

After the third piece exploded in my kiln this month, I decided that I must be doing something wrong, or my kiln is doing something wrong, or both.

Ran the "know your kiln test". Evenivity confirmed, unevenivity hypothesis ruled out as a cause of death.

My kiln always ran a smidge hot but I wondered if it was getting worse and affecting the annealing process somehow. Fired up a couple of tiles with transparent glass. Each was fired twice. Looked for anneal stress with jury-rigged stross-o-meter using polarized sunglasses (camera store wanted $35 for one 3 inch diameter polarized film!) and saw no haloes at all. Will test temps this weekend using cones from the pottery store.

The three pieces had absolutely no glass in common at all. Only BE tested compatible glass enters my small studio. Here is the interesting thing. All three pieces broke on a slow (200 or 250 degree) ramp up, and all were pieces that had been fired either once or twice before this firing. they all were approximately two layers thick throughout (no bumpies or texture on the glass surface).

Nothing has broken outside of the kiln -- except when I was tile sawing one piece of glass I had made last month that was 3/4 thickness (a regular and thin sheet fused). I attributed the breaks to fast or sloppy tile sawing (I was tired when I got to that piece). None of the other glass fired around the same time broke when tile sawed.

I have had talks with several people including the vendor for my kiln and Bullseye. (I use their glass.) BE is puzzling this out right now.

My hypothesis at this point is that oxidation of the thermocouple has caused the kiln temps to drift higher than before (though I haven't really noticed a diff in full fusing) causing me to have improperly annealed pieces that are under more stress than even my slow ramp up can handle and so they go boom!

Any thoughts?

Has this happened to anyone else?

barbara

P.S. thanks to all of you who offered thoughts, comments and sympathy last week. You have helped me organize my thinking and methods for testing out the cause of this.

[charlie]

it would be a cheap test to just replace the thermocouple, probably cheaper than the glass that you're going through.

perhaps borrowing another one from another kiln would be even easier and cheaper.

[Strega]

I think I would buy some cones, and test the kiln specifically at annealing temperature, because exploding implies stress, and it seems that would have to indicate stress trapped during the anneal. I would try it with the current thermocoupler and then with a borrowed one to find out if the problem was the thermocoupler.
Then I'd light a candle to Pele, the volcano goddess.

After the third piece exploded in my kiln this month, I decided that I must be doing something wrong, or my kiln is doing something wrong, or both.

Ran the "know your kiln test". Evenivity confirmed, unevenivity hypothesis ruled out as a cause of death.

My kiln always ran a smidge hot but I wondered if it was getting worse and affecting the annealing process somehow. Fired up a couple of tiles with transparent glass. Each was fired twice. Looked for anneal stress with jury-rigged stross-o-meter using polarized sunglasses (camera store wanted $35 for one 3 inch diameter polarized film!) and saw no haloes at all. Will test temps this weekend using cones from the pottery store.

The three pieces had absolutely no glass in common at all. Only BE tested compatible glass enters my small studio. Here is the interesting thing. All three pieces broke on a slow (200 or 250 degree) ramp up, and all were pieces that had been fired either once or twice before this firing. they all were approximately two layers thick throughout (no bumpies or texture on the glass surface).

Nothing has broken outside of the kiln -- except when I was tile sawing one piece of glass I had made last month that was 3/4 thickness (a regular and thin sheet fused). I attributed the breaks to fast or sloppy tile sawing (I was tired when I got to that piece). None of the other glass fired around the same time broke when tile sawed.

I have had talks with several people including the vendor for my kiln and Bullseye. (I use their glass.) BE is puzzling this out right now.

My hypothesis at this point is that oxidation of the thermocouple has caused the kiln temps to drift higher than before (though I haven't really noticed a diff in full fusing) causing me to have improperly annealed pieces that are under more stress than even my slow ramp up can handle and so they go boom!

Any thoughts?

Has this happened to anyone else?

barbara

P.S. thanks to all of you who offered thoughts, comments and sympathy last week. You have helped me organize my thinking and methods for testing out the cause of this.

[KellyG]

Barbara, I was following this thread, and wanted to know if you found the problem. Was it the thermocouple? What were the results of your cone tests?

Curious girl

[Brock]

This was a major discussion point at BECon. If memory serves, it was decided that thermal shock was the culprit, possibly exacerbated by incomplete annealing. Izzatso Barbara? Brock

[Bert Weiss]

Barbara

2 words: slump test.

Regardless of what the meters say, you should be anneal soaking somewhere around 100º below slump temp. Even if the meter is off of reality, 100º below should be in the ball park.

If you are way off, I'd replace the thermocouple and do another slump test. Remember the BE glass is the standard to which you are measuring.

[Barbara Muth]

Bert: ran slump test. Also tested with pyrometer.

Brock: it was thermal shock. My annealing is fine. It was my theory that my annealing had been off until Bruce ran a temp test comparing the controller readout of temps with an external pyrometer. At that point I was pretty sure the ramp up was too fast and causing thermal shock -- though the conclusion sis shock me.

Although the kiln is off tempwise on the way up by about 70 degrees, it is fine on the way down and my annealing is fine.

I also did a stress test and saw no stress in the pieces I made for the stress test, even after the third firing.

The conclusion of the glass docs at BECon was that I have been walking a thin line and even though this had worked for me many times before, my ramp up of 200 dph to 1000 was too fast on subsequent (to the first) firings. I was amazed. Partly that has to do with the color contrast in my pieces, but even in pieces with little or no color contrast it is the case.

I have been too busy to test out this theory since I returned. Will get back to the kiln this weekend and let y'all know if slowing the ramp up is making a difference.

Barbara

p.s. Since my "exposition" at BECon, three different people (all quite experienced and somewhat reknown kilnworkers) told me that the same thing had happened to them and that their solution had been to slow down on the ramp up. Made me feel better to know I wasn't alone.

Oh! And Brock did recommend that I chuck my controller.

[Bert Weiss]

Bert: ran slump test. Also tested with pyrometer.


p.s. Since my "exposition" at BECon, three different people (all quite experienced and somewhat reknown kilnworkers) told me that the same thing had happened to them and that their solution had been to slow down on the ramp up. Made me feel better to know I wasn't alone.

Oh! And Brock did recommend that I chuck my controller.

Barbara

Sounds like you have a good handle on it. Assuming that Brock didn't want you to get a better controller, "chuck Brock" LOL

[KellyG]

Thanks for the reply Barbara. Hhhmm, pretty scary though if 200/hr is too aggressive! I've recently had two bowls crack on the slump and now I think I know why.

If you can stand one more question, - - are you firing on mullite shelf or fiber board?

Kelly

[Barbara Muth]

I fire on mullite.