Could My Kiln Be Getting Hotter?

PolychromeBeads · #1 2019-01-08, 11:58am

I have a Glass Hive Regular Guy, bought in Nov. 2010. I replaced the coil in July 2014 and it has been running like a champ ever since.

I have a very conservative annealing schedule: garage at 920, anneal at 940 - but lately all my striking glass has come out over-struck. Is my kiln starting to run hotter? Does this happen? Or is there some sort of maintenance of which I am unaware?

Aimee

echeveria · #2 2019-01-08, 1:54pm

You should call Pam and Mike and ask for their advice. They would probably know better than anyone. I have been told the temperature probes wear out over time, but I am not sure of how they fail.

PolychromeBeads · #3 2019-01-08, 2:34pm

Yeah, I just thought someone might have had experience with this issue. My schedule is crazy this week and the phone keeps ringing with Mom problems

- but I'll get to it!

Aimee

kansassky · #4 2019-01-09, 9:15am

You can easily calibrate your kiln.

Take one of those round oven thermometers.

Set your kiln to 400 degrees.
Put your oven thermometer inside.
Wait for the kiln to come up to temp.
Check the oven thermometer reading.
Do they match?

FWIW, I found my kiln was 40-45 degrees hotter than what the pyrometer said.

I made adjustments in my program.
I now anneal at 900. (Instead of 940.)
No more overstriking.

PolychromeBeads · #5 2019-01-09, 11:36am

Dix, that is incredibly clever! Thanks!!

Aimee

Speedslug · #6 2019-01-09, 3:27pm

Three other ideas come to my mind ....

The first is batch dependent from the manufacturer ... the size of the tumblers they use to mix the raw ingredients together can be pretty big particularly if it is a very popular glass ... so the amount of the various chemicals added to the basic glass recipe can be from ounces to pounds.

As a result the mix will vary somewhat and when you finally get your hands on it a temperature change of 15 degrees f one way or the other could easily change the way your current batch will strike in the kiln.

That is the one that I would be putting my money on if I were a betting person.

The next is the one that Kathy mentioned; Temperature probes are made of two kinds of metal that produce a voltge change when heated and if you are using them a lot one would expect them to change a little bit over time ... again it only takes 10 or 20 degrees one way or the other.

Yet another possibility is the control system that the temperature probe feeds into. This is less likely to be the source of this change but it is possible.
A little bit of corrosion on a wire from the probe can change the signal it puts out, a small bug that got through the screen could have died in a wrong place and changed the signal going into a control chip changing the heating signal but not the displayed signal.

Then there are the relays or SCRs (silicone controlled rectifiers) feeding power to the heating elements that handle a LOT of power / current / heat and some changes with age should be expected with them as well.

When we are using temps of 940 degrees a change of 10 degrees is only a little over 1%.

As a technician type myself, I am surprised that we don't hear of over striking more often.

As for the batch mix ratios I think that colors like Coral having such wide variations is a really good example of just how sensitive the ingredient quantity and quality can be when trying to get one particular color out of a glass melt from one month to the next over decades.

PolychromeBeads · #7 2019-01-09, 4:11pm

Thanks for the info Phill, but I think I have found my problem.

Tested my kiln using the method suggested by Dix, and holy crap! It looks like it is running 50 degrees hotter than it says. That would certainly cause all my striking glass to overstrike.

I will lower my temps, and start working on why the kiln is running hot. Phill may be right that the probe could be worn - I'll have to see what I can find on that as a next step.

Thanks for all the help!

Aimee