What to do when annealing becomes slumping?

[andrew.spencer.2]

So I tried batch annealing in my new homebuilt kiln for the first time this week, usually I just fire it up and garage as I go.
As you can imagine, I was rather less than impressed to find this when I opened the lid several hours later.

The kiln is digitally controlled, but can't ramp up or down on it's own so I need to sit there and gradually increase or decrease the temperature as needed. It always overshoots the target temperature, but never by more that 10^c so I don't know where I'm going wrong. According to the PID, the temp never went much above 500^c for the annealing soak.

That's my thermocouple you can see in the back wall of the kiln, is there any chance that it's too far away from the elements (in the lid), and the beads are getting hotter than the TC is?

[flame n fuse]

that's a shame. Is there a possibility that your temperature calibration is wrong?

[June]

Sorry, I don't know enough about kilns to offer any advise but it is a shame for those lovely beads.

[andrew.spencer.2]

I'm not too sure, I've never had reason to suspect it until now. I've had a look through the manual, and there isn't really a way to calibrate the digital controller. I'm saving up for one of those IR non-contact thermometer gun thingies, that could give me a second opinion on the temp I guess.

[babyshoes]

Oh what a pity! Perhaps next test should be with just a few simple spacer beads in case the same happens again.

[MeadMoon]

My thermocouple sticks out a bit further than that - has it slipped a bit and, if so, can you push or pull it a little further out without causing any damage?

[andrew.spencer.2]

Hmm, that's a good thought. I recessed it in like that on purpose, but not sure why now that I think about it....

[ARBeads]

I think Elaine's hit the nail on the head, the thermocouple needs to be much further in to the chamber, you've created a pocket of air around the end that could be at any temp.
I'd also be inclined to mount the thermocouple a little higher, this would enable you to do larger pieces (fusing) making sure that the thermocouple is above the working piece. The temperature at the work height may be slightly lower than the measured temp, but it's on the safe side.  As with any kiln you need to play to find the perfect temperatures for your requirements.

Hope you manage to get it sorted.

[Lilley]

You could get some cones from a pottery supplier that have known melting temperatures.  Put them in the kiln and fire it keeping an eye on the cones as it reaches the temperature at which the tips of the cones fall over (this does assume you have a peep hole or other means of seeing into the kiln without opening the door and cooling the whole lot to see) but basically you can then use the cones to get an idea of what your thermostat is reading at pre-defined cone temperatures and so then adjust for any difference.  So if cone should melt at say 500C and it does that when your thermostat is telling you it is 490C you know that it is out by 10 degrees at that temperature.  Do the same for a few more temperatures and you will have a better picture of whether, and by how much, you have to compensate for your kiln running hotter than it says it is.  Hope that is making sense.

[Redhotsal]

um.....nice pendants.....? 

Yep, I reckon that the thermocouple needs calibrating. Shame though - good on you for making your own kiln. At least you know it's hot enough to anneal!

[Hamilton Taylor]

Is the PID matched to the thermocouple type?
Also, B&Q have a cheap multimeter which also has a k-type thermocouple - it's not very accurate, but will tell you if you are in the right temp range or not (+/- 15C or so). OK for annealing temps, but don't try to use it for anything hotter...

Sean

[Redhotsal]

The other possibility is that you just drop the temp until you see that the glass isn't slumping. You can check that they are annealed by using a polariser - annealing isn't exactly a precise art - different glasses anneal at different temperatures,  there's even discrepancy between colours in the same manufatured range. - For example - in my experience wheras a clear Effetre glass displays no stress at a certain temperature, an Effetre trans green may still have stress at the same temperature.

But, you can "see" stress in transparent glass and basically if you can eradicate it then you've pretty much annealed the beads. I'm willing to bet there are a lot of folk who have wonderfully posh digitally controlled kilns who think they are doing the right thing but actually are not annealing as they drop the temperature too quickly, or have too low a temperature for the glass, etc. etc.

[Hamilton Taylor]

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