Thursday, May 17, 2012

PMC melting in my kiln

This makes no sense to me.

Several years ago, when I fired my first ever batch of silver metal clay, I went with the “optimum” recommended firing schedule of 1650 for two hours.

The things that came out of the kiln after that firing were sad, burnt, melted pieces of silver.

Sort of put me off working with silver metal clay for awhile. A long while.

The next couple of times I tried silver, I used a reduced heat.

When I’d mention to people my “silver melting” episode, they’d look at me kind of like, “You just didn’t know what you were doing. Silver doesn’t melt at 1650 degrees.”

So last night, I decided to be brave. After all, it’d been YEARS since that episode. I read… from about six different sources… that 1650 was optimal and safe since silver melts at 1763.

Please note, my kiln (Jen Ken’s AF3P 11/4.5 Glass Fusing & Bead Annealer Kiln) maxes out at 1700 (supposedly). 1700 is the top temperature of that kiln… so what the heck is going on?

Programmed the kiln for a full ramp to 1650 with two hours of holding time. I knew something wasn’t going right when it took 3½ hours to reach 1650. And sure enough, when I removed my test bead from the kiln, it was just a melted blob of silver. AAAAAhhhh!!! What is WRONG with my kiln?! How can my silver be melting in a kiln whose top temperature is below silver’s melting point.

Well, obviously my kiln not only goes up to (and possibly over) 1763, but it does so while lying to me and telling me it’s holding at 1650.

Here’s last night’s bead, before….



And after…



Here’s the other side of it… you can see where I took a burnisher to it (in the center) just to confirm this ugly white lump was indeed pure silver.



Apparently this is weighing heavily on my mind… had a restless night of kiln dreams (with a cat thrown in for good measure… not in the kiln, just in the dream).

So first thing this morning, I pull on some sweats and head outside with one more test bead. I program the kiln for a full ramp to 1550 with a 45 minute holding time (this is just a test, not a bead I want to keep).

And I sit there… waiting and watching. So far so good… it took the kiln 54 minutes to ramp up to 1550.

Fingers super crossed that last night was a fluke (for the supremely long ramping time) and that I’ll find a good temperature to fire my future silver at. And yes, for the record, you MAY end a sentence with a preposition… it’s like lane-splitting (when a motorcycle goes between two cars in adjacent lanes)… there is really no rule/law against it; it’s just assumed it’s wrong/illegal.

The 45 minutes went by quickly… and out of the kiln comes a perfectly decent looking test bead. White, not misshapen. Yay! And hallelujah!

Which leaves me to wonder (‘cause I can’t think of anything else) that maybe my kiln freaks out when it’s pushed near its limit.

I’ll probably do some more testing until I find the perfect “place”.

One other theory could be that my kiln runs about 100 degrees hotter than it says it is. So maybe firing at 1550 is really firing at 1650… and firing at 1650 is really firing at 1750.

I’m sure there’s a way to test the interior temp of a kiln (last time I asked someone they said, “That’s why you have a pyrometer in your kiln.” Not helpful.)

3 comments:

  1. I borrowed one of these handheld digital pyrometer from my friend & found out my new kiln was firing about 15-20 degrees hot. She got it because her kiln was under-firing by 150 degrees! Test firing is definitely a good idea, for some good info on test firing & troubleshooting I suggest Hadar's instructions at artinsilver.com.

    http://www.riogrande.com/Product/Hand-Held-Digital-Pyrometer-with-Thermocouple/703099?pos=1

    Hope that helps!

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  2. Thanks Kim! I did end up getting an external pyrometer and my testing showed that my kiln did indeed run hot. Around 200 degrees hotter at the lower temps and around 80-100 degrees hotter at the higher temps.

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  3. Test firing is always a good idea. Glad you got it figured out! Crazy that it runs that much hotter than it's "max".

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