float glass disaster

rosanna gusler · Post by **rosanna gusler** » Fri Jan 16, 2004 7:46 am

i think that a slump firing in the middle 1200f range is what you need. hold for an hour or so. then cool to annealing slloowwwly. that way whatever un even ness there is will be minimized. allow the glass to stiffen up evenly. since you went to a full fuse this is happening on cool down. do you still have your first warped piece? it would be interesting to look at it for stress. any chance that trapped air is doing this? this is so weird. rosanna

charlie holden · Post by **charlie holden** » Fri Jan 16, 2004 11:21 am

Annah James wrote:
OK. I did change the firing schedule. As I was loading the kiln this time I thought that the edges of the glass were too close to the sides, so I only did two pieces instead of three and I centered them in the space. The top elements are 18" above. Since the sides that are "down" are the sides that have the elements closest to them, I think that the side elements in the kiln either need to be baffled, or I need to set up the thermocouples so that one is reading more in the center of the space - and program the controller so that the top elements are firing more than the side elements. I think that your take is right on - that it's uneven heating.

Do you think I could flatten these pieces out by doing a slump firing where I hold it longer at a lower temp, and baffle the side elements? Charlie, I REALLY appreciate you taking the time with this...it's always hard to learn something new when I'm working on a commission piece....oy

annah

If you can't raise the shelf, then baffle the side elements and refire. Make sure that the thermocouple is to the inside of the baffles -- in other words, the thermocouple should extend past any nearby baffle in towards the middle of the kiln. It doesn't have to be in the exact center of your kiln, you just don't want it to be trapped between the baffle and the elements.

You will need to go as high as your last firing, or soak for a substantially longer time. At this stage I would probably put in a soak around 1400-1475 for a very long time. (Actually, use the temperature that Bert recommends for complete slumps, whatever that is.) Then I would look into the kiln every ten minutes during the top soak, until I could say for sure that the pieces had flattened out enough. Then I would push the controller over to the annealing cycle. I would do this again the next time you do these pieces. That way you have a solid program that you worked out using your own kiln that you know works that you can make minor adjustments to.

I'm guessing at this point that you are firing on ceramic board laid on the floor of your kiln, and that your kiln has a base with shallow walls, maybe 4 inches deep, and one element on each long wall of the base. Bert often recommends setting fiber board on the floor, but Bert doesn't have side elements in his kiln, only ceiling elements. The type of kiln I'm guessing you have is built this way because they assume you are going to be using mullite kiln shelves raised off the bed of your kiln, and that you will need to get some heat under the shelves for even heating.

I'm going to also guess that the edge of your glass is 3 inches away from the side elements. That means that it is 6 times closer to your side elements than your ceiling elements. So, from physics 101, the edges of your glass are getting roughly 36 times (6 squared) more intense IR than the center of your glass. That's why they are slumping and the center isn't.

ch

Annah James · Post by **Annah James** » Fri Jan 16, 2004 12:01 pm

Charlie - you were close when you made the assumption about the kiln design - the only difference is that I am not firing on boards - I laid out a fiber blanket right on the bricks and am firing on that. I wonder if the bottom is cooling too fast. Other than that, your assumptions were pretty close.

So the question is - take it up to full fuse again or up to annealing and hold?

The other thing is that I never look in the kiln while it's hot. I just don't do it...over the years I have caused more problems by opening it up, so I am a little wary about lifiting the lid while it's hot.

Whas sounds the most realistic to me is to take these steps:
1. baffle the side elements
2. move the thermocouple to the middle zone
3. refire up to full fuse, and hold it longer
4. cool it slower, so that it does not warp
5. do the anneal and the rest of the cooldown the same as before

thanks all of you...

a

charlie holden · Post by **charlie holden** » Sat Jan 17, 2004 10:43 am

If you're going to keep firing on the floor of your kiln you might as well disconnect the side elements. They are more of a problem than a help in that arrangement.

I think everybody should be able to look into their kiln and should do so often, every time they make a new type of piece and until they have their schedule nailed. It's the best way to learn what's going on. Aren't there any peep holes in your kiln? You shouldn't have to lift the bell. You can have some peep holes cut if you want.

As far as your list 1-5, I agree with everything but 4. Cool down as fast as you can to avoid devit, especially with float. The glass won't warp during cool down.

ch

Annah James · Post by **Annah James** » Sat Jan 17, 2004 12:40 pm

Charlie:

Well, we'll see what happens tonight when I open it up. I did go a little slower on the cooldown, just in case the bottom of the piece is cooling faster than the top.

My tech guy moved the thermocouples yesterday so that they are more evenly placed, with one in the center down low near the glass.

He also reprogrammed the kiln so that the side elements don't come on as much - there's an offset option. I'll be doing a test firing to test the differential between the heat on the top and the bottom of the kiln so we can set the offset more accurately.

thx!
annah

Annah James · Post by **Annah James** » Sat Jan 24, 2004 5:21 pm

Well, well. I haven't been online, because I have been dealing with what showed up with the firings...turns out it wasn't the firing schedules that was making my pieces curve. No, no. The support structure on the bottom of the kiln (1/4" thick I-beam steel) warped. Yep. warped up 3/8" in two places in the kiln. Plus, I found that the sheet steel that's under the fire brick was moving as well at full - fuse temps. It never occured to me that it could be a structural problem with the kiln itself...I feel sort of naive.

So I had the kiln guy over and he built a turnbuckle sort of arrangement out of heavy chain and pulled the I-beam down. Then he tack-welded the sheet steel to the bottom of the framework. I just ran the first firing since all of that has been done, and it does not look like it moved at all. I am considering whether or not to set my firebricks on edge to give more insulation between them and the sheet steel underneath. Any ideas about that?

whew.
Annah

Don Burt · Post by **Don Burt** » Sat Jan 24, 2004 5:53 pm

Brock wrote:
and If I want it can get to 1000 from ambient in 20 minutes.

I've been hoping to hear some numbers like that.

Good for magnets, not much else, at that speed. Brock

Stained glass, enamel tests, salad croutons

Brock · Post by **Brock** » Sat Jan 24, 2004 6:29 pm

Don Burt wrote:
Brock wrote:
and If I want it can get to 1000 from ambient in 20 minutes.

I've been hoping to hear some numbers like that.

Good for magnets, not much else, at that speed. Brock

Stained glass, enamel tests, salad croutons

Pizza. Shrimp on slate. Really. Heat it to at least 700, pull it out of the kiln and fry marinated shrimp on it.

I don't know if my kiln would work that fast for stained glass Don, (I'm assuming you meant enamelled pieces of glass for use in a SG window?). There is something other than just raw speed going on in those gas fired railway kilns that you can heat up so rapidly. I think the shape of the roof, the barrel vault, somehow evens out the heat. Same thing in the Hoaf. A very uniform heat source(s) eliminates the possibility of thermal shock. Brock

Kevin Midgley · Post by **Kevin Midgley** » Sun Jan 25, 2004 2:07 am

Who designed the kiln? Sounds like they needed some more knowledge of the expansion of materials. Welding everything together on the bottom of the kiln will not allow for expansion to take place. Watch out for new problems. Kevin in Tofino

charlie holden · Post by **charlie holden** » Sun Jan 25, 2004 4:57 pm

Annah James wrote:Well, well. I haven't been online, because I have been dealing with what showed up with the firings...turns out it wasn't the firing schedules that was making my pieces curve. No, no. The support structure on the bottom of the kiln (1/4" thick I-beam steel) warped. Yep. warped up 3/8" in two places in the kiln. Plus, I found that the sheet steel that's under the fire brick was moving as well at full - fuse temps. It never occured to me that it could be a structural problem with the kiln itself...I feel sort of naive.

So I had the kiln guy over and he built a turnbuckle sort of arrangement out of heavy chain and pulled the I-beam down. Then he tack-welded the sheet steel to the bottom of the framework. I just ran the first firing since all of that has been done, and it does not look like it moved at all. I am considering whether or not to set my firebricks on edge to give more insulation between them and the sheet steel underneath. Any ideas about that?

whew.
Annah

Wow, that's scary. More insulation should help protect the I-beams. This seems to be another example of unintended results from changing the way things have always been done. (Firing on the floor instead of on a raised shelf.)

You didn't say how thick the sheet steel is. I built my klins with 1/4 inch steel. Steel will always warp towards the heat. (This has been a big issue in the studies of why the World Trade Center collapsed.)

Any new kilns I build will have steel grating instead. The grating is more than 1/2 inch thick and designed to be rigid, because it is most often used on stair treads, walkways and drains. It may stay cooler overall since it has room to breathe. The grating would allow some air circulation above the support beams as well.

ch

charlie holden · Post by **charlie holden** » Sun Jan 25, 2004 5:04 pm

Brock wrote: Pizza. Shrimp on slate. Really. Heat it to at least 700, pull it out of the kiln and fry marinated shrimp on it.

Try this Brock: Shrimp on rock salt. Pack rock salt about an inch deep into a pan. Smooth top surface. Heat and cook, flipping shrimp once. Has to be rock salt, not kosher or fine salt.

We do it on the stove top in a cast iron skillet. Cooks slowly but tastes great. I don't know why it wouldn't work in and out of a kiln like your slate.

ch

Annah James · Post by **Annah James** » Sun Jan 25, 2004 10:18 pm

Charlie:

The steel is all 1/4". I talked to the builder the other day about using grating under the fire brick (though he may have been thinking about the expanded steel stuff - not as heavy as you are talking about) - he thought it might move too and then rasp the bottom of the fire bricks off.

re: kiln furniture - I do pieces that are upwards of 30 x 60, where would I GET kiln furniture that size? (and where do you get the second mortgage to pay for it??) hahaha! I was under the impression that firing on the 'floor' of the large kilns was a normal thing. Am I wrong about this?

Seems to me that turning the bricks on edge is a smart thing to do - I'll go for it when I get the payment for the commission I am working on (oh, cash flow...)

Any other thoughts?
Annah