Fast-cooling when painting and firing ?

[Buttercup]

After an over-firing disaster I have to redo several pieces and wonder how quickly I can cool the kiln and remove one lot of glass before putting in another. (I'm down to one small kiln until I narrow down the problem with the bigger kiln.)

I'm firing enamels and stainers onto 3 mil art glass. I read somewhere on the 'net, but can't remember where, that the lid can be cracked at the top temp, 1080F-1150 F , and the temp. dropped, but can't remember how low, before shutting it again, or left very slightly cracked open till at room temp.

Normally I fire to temp, soak, then turn the kiln off leaving the door/lid shut until it's at room temp. but that takes hours, which is normally OK. I'd like to speed this up if possible.

Can anyone help with this, please? Jen

[Valerie Adams]

In a pinch, I've removed single layer, 3mm powder printed or enamel painted glass at about 400°, setting it aside and insulating it while it cools.

Yesterday though, I was drying a sheet of 3mm with my heat gun. I walked away and heard it break in half. So, hot glass outside the kiln is obviously risky.

I don't crash cool my kilns, but I've heard that others do to around 1000°.

[Morganica]

Best way to find out is to try it. I don't like doing that because it's hard on the kiln and can lose glass if too much cooler air starts swirling around in there...but if you've got no other choice try it first with a load you care least about. Then when you get the routine down, move to your more precious pieces.

[Buttercup]

Thanks very much Valerie.

Is the reluctance to crash cool because it is hard on the elements? I've never done it, just had a vague idea it wasn't good practise.?

Makes one nervous just thinking about it! I guess a top loader is easier to cool as the lid is a fair way from the glass. My small kiln, the only one working right now, is a ceramics kiln with a front door and side elements so there's no way to leave it cracked open that won't expose the glass to a draught.

Thanks for the input, Jen.

[Buttercup]

Thanks Cynthia. Think I'll take the conservative approach with the pieces I have ready to go and I certainly don't want to stress my kilns. They seem to have fragile sensibilities. Normally speed isn't an issue as I don't do production.
Thanks again, Jen

[Valerie Adams]

I don't do it for the same reasons Cynthia mentioned, plus, I'm not usually in that much of a hurry.

[Buttercup]

Thanks Valerie. I'm usually not in such a hurry but do need to make up some time. I'll probably just get up at a silly hour and swap over the glass and give the wallabies a good laugh as I make my way to the workshop in hot-summer night attire and boots. Jen

[williamslaybaugh]

I don't crash my kiln anymore, but when I did I would balance the temperature off at around 1000F like Valerie said, than program my cooling cycle.
I don't think that crashing a small kiln at the upper temperatures is going to save you that much time though. It's the lower range temperatures that take most of the time to cycle through. Although small kiln is a relative term.

For a 3mm thick piece, Bullseye recommends 85 minutes from 960F to room temp. When I went on a tour of a window glass factory they went from molten furnace glass to room temp. in 8 minutes. I know there like comparing apples and oranges but as long as your cooling rate is consistent there's a lot of wiggle room in an art glass schedule. You might want to try propping your kiln open and use some thin strips of 1in fiber blanket as baffles to allow more heat to escape without allowing a draft.

[Bert Weiss]

You could try removing glass around 500ºF and placing it between layers of fiber blanket. When I studied with Albinus Elskus, he would take glass out, around 300 and place it between thick layers of newspaper. Once the glass is cooler than 700, you are just needing to prevent thermal shock, as the internal stresses are as relieved as they will ever be, by your anneal cycle (whatever that was).

I don't recommend crashing the kiln, but you can simply shut it off and wait for the glass to cool at whatever rate the kiln allows, while closed up. 3mm glass anneals quite quickly.

[Buttercup]

Thank you for that suggestion, William. Using blanket to protect the glass from draughts would probably allow safer cooling with the door cracked open a tiny bit. I suppose you'd set that up before firing as it would be a bit tricky after?

The small kiln takes several hours to come down to room temp. as I normally do it, just by shutting it off. I'd love to be able to go from 960 to room temp. in 85 minutes if it didn't damage the kiln or the glass.

Thanks Bert, but I'm not that brave! I'd be afraid of the pile of newspaper igniting and the bucket of water thrown over it would no doubt crack the glass anyway. It's daylight now so I'm off to check on it.

Thanks again for the help. Jen

[Don Burt]

Putting a fan directed at the outside of the kiln might speed it up a little. You speculated about a solution: open the kiln and quickly toss a bat of fiber on top of the glass and close the door until evenivity is restored in a few minutes, then crack the door with less direct draft impact to the glass on the shelf, and presumably an overall greater rate of cooling. I can cheerfully recommend this, but I have no idea if it will work but I've always wondered if it would work, and it would be great if you incured the risk and proved it true. Remember to take video in case something catastrophic or funny happens.

[Buttercup]

Hmmmm. There's a rollup door right beside the kiln which I've always taken care not to open if there's a breeze when the kiln is firing. The small kiln takes forever to come down naturally. Must be uber-insulated. Last night it took 10 hrs to come down to 150℉ from a high of 1080 when it was fired last night. Opening the rollup door during the cooldown might speed it up.

As for the video record of an attempted batt fling in a 1000℉ confined space....(WGB Olympic event?) ...I'd probably end up chucking the camera in as my eyebrows shrivelled. I do appreciate your encouragement and support, Don, but deferring to your experience I'll let you go first and eagerly await your results.

[Barry Kaiser]

For the Newbies, remember that size does count.
The above discussion is very conservative since it is oriented toward larger pieces.
If you are fusing pieces below 2x2 inches (pendant sized), the rules change. You have much more flexibility.
I open my kiln wide at top temp, and crash to 900. Close the kiln and crash back to 900 after the temp rebounds up to about 1100.
I then close the kiln and wait for the temp to drop naturally to about 800. Crack the kiln about 1 inch and when the temp gets to 500, I open wide.
Shelf, with pieces on it, are removed at 400 and placed on a tile surface.

Barry

[Peter Angel]

The ideal solution is to have 2 kilns!

One kiln is heating up while the other is cooling down.

[Buttercup]

Thanks, Barry but I'm painting and firing much larger pieces.

Peter, I do have two kilns (three actually) one not yet wired in. Of the two that are operational they are taking it in turns to have hissy fits. Just replaced the thermocouple on the small one and it's behaving well again. Have just replaced the fibre board in the lid and reattached the elements in the mid-sized kiln after a fry-up. Tried again and had another fry-up so won't be using it till the thermocouple in it is replaced, too. It's old anyway so is probably overdue.

I redesigned the glass so the pieces will fit in the small kiln. Can just get 12.5" pieces in it. Jen