Heating/annealing TOO SLOW! Options? Alter the kiln?

[Mark Selleck]

I'm currently slumping some carved bowls in 1/2" float, 16" diameter. Once I get past 700 on the down cycle, the kiln loses temperature "naturally" very slowly, about 30 deg/hr, rather than the suggested 200/hr rate. The first time I did this I decided to crack the top and see how that influenced the cooling rate. It doubled it, to about 60/hr. This is a 30 x 60 x 14 coffin kiln, and when say I cracked the top I mean not much, about 1/8" on the leading edge, away from the hinges. I also loosened the bottom vent plugs slightly, so there could be a very slow influx of cool air from the bottom leading edge. The first two bowls came out fine, and passed the visual "no stress test" when viewed through polarized film. I'm wondering if I should be "happy" at this point, or if I dare open the plugs a little more, to increase air flow.

Second option: I'm considering altering the kiln. I'm wondering if it would be worth ripping out the firebrick side insulation, and replacing it with fiber. The kiln currently has fiber insulation in the top, brick on the bottom and sides. Top elements rheostat switch, side elements controlled. It is 1 year old, Bartlett controller. Am I crazy? I already know that, but suggestions/other opinions would be appreciated.

[Bert Weiss]

I'm currently slumping some carved bowls in 1/2" float, 16" diameter. Once I get past 700 on the down cycle, the kiln loses temperature "naturally" very slowly, about 30 deg/hr, rather than the suggested 200/hr rate. The first time I did this I decided to crack the top and see how that influenced the cooling rate. It doubled it, to about 60/hr. This is a 30 x 60 x 14 coffin kiln, and when say I cracked the top I mean not much, about 1/8" on the leading edge, away from the hinges. I also loosened the bottom vent plugs slightly, so there could be a very slow influx of cool air from the bottom leading edge. The first two bowls came out fine, and passed the visual "no stress test" when viewed through polarized film. I'm wondering if I should be "happy" at this point, or if I dare open the plugs a little more, to increase air flow.

Second option: I'm considering altering the kiln. I'm wondering if it would be worth ripping out the firebrick side insulation, and replacing it with fiber. The kiln currently has fiber insulation in the top, brick on the bottom and sides. Top elements rheostat switch, side elements controlled. It is 1 year old, Bartlett controller. Am I crazy? I already know that, but suggestions/other opinions would be appreciated.

Mark

I'll comment on cracking the kiln. There is a lot you can get away with when you are firing float glass that is uniform in thickness. I was firing 3 pieces of 10mm about 12" x 84", kiln carved, and I could crack the kiln a little below 500º and never had a problem. I wouldn't recommend this when firing BE in multicolors. If the glass is annealed properly and survives the cool down there will be no residual stress caused by the cool down rate. Cracking the kiln around the strain point can cause stress that won't relieve itself unless heated back above 1000.

I couldn't advise you to tear your kiln apart, without knowing more about how it would go back together. I think that brick sided kilns are very good as annealers for blowers who open the kiln to place work in while it is hot. For fusers it is not at all helpful to have brick sides.

I use a brick floor because it is practical as a way to get a flat floor that won't warp. I cover my brick floor with lots of blanket and boards.

I know of a few strategies to cool down a kiln faster. I tried putting a stainless steel tube with holes along it througn my kiln. It caused glass to crack on heatup and it would heal at temp, but leave a killer scar. I tore it out. Henry Halem, who advised me to try that, has a system where he blows air through the pipe with a certain pressure. He essentially is creating a convection oven, which evens out the heat throughout the kiln.

The best system I've seen is quartz tubes with the elements coiled around the outside of the tube. The inside of the tubes are terminated outside the kiln in a chamber with a fan. At the point where the kiln has ceased to cool fast enough, the fan comes on and helps remove heat from the kiln. The EK Miller kilns made in Portland OR have this design.

[Cynthia]

http://glassnotes.com/

You can see Henry Halem's design for the convection thing at the site listed above. I wouldn't retrofit your kiln. It's a good kiln isn't it? Just figure out new ways to safely get the cooling rate to speed up.

I have a fiber kiln of the same dimensions as yours. It gives up heat slowly too, and I do vent to cool it more quickly. I manually vent at 500F to about 1/2" with kiln furniture in the same fashion you describe. I don't have plugs to open up...but if I did, I'd pull them. Then at 200F I open the lid to 3" or so with more props. I am not advocating that this is foolproof, or that it would work in your kiln the same as it does in mine. I just know I can get away with it in my kiln. I also have a plate mounted vent that seems to help speed up the cooling process, but not enough if I am waiting to load up the next set of pieces.

I don't do production...so speed isn't such an issue...but if I don't vent, a schedule that should take a day can take two. Nice big kilns have their downsides...fiber or brick. Don't mess with a good kiln (so says Cyndy), just dink with ways to carefully cool it faster.

[John]

Mark, my kiln also drops very slowly, It will drop from mid 700's cel. to 600's pretty quick, but from there onwards it can take some 16 hours to about 150 deg cel.If I don't pull the vents. I have 6 vent plugs, 4 in the roof 2 at either end. (this is a top hat) I pull the the top 4 out at 150 the othe 2 side ones at 100 deg cel. I am also starting to push it a little bit by bit to lower my cool down time. I am yet to have one break. Touch wood. Annealing is great I can cut the glass and it breaks just perfect. I have a custom kiln that has blanket walls so I can't comment on doing a refit. But I would try to keep the kiln as is and push the boundries of the glass demons.


John

[Bert Weiss]

Mark

It just hit me what you can do. You can pin blanket to the bricks. The blanket can be rigidized. Board would also work. No need to tear it apart, just isolate the bricks from the heat.

[Geri Comstock]

The answer to this question depends on how deep your kiln is...

My kiln is 18" deep and the shelves are about 4 inches from the bottom. I start venting my kiln at about 650 on the way down if I need to cool it faster that it cools naturally. I use a small kiln post for this at one corner. As it gets closer to room temp, I open the lid a bit more to the point where it's open the width of one kiln brick.

In my kiln, with a piece that's uniform in thickness and only 2 or 3 layers thick, I can open the kiln all the way at 250f and not crack the piece. It's because it's so deep that the glass doesn't have a chance to get thermal shocked.

This took me some time to work out for my kiln back in the days when I was trying to do 3 or 4 firings a day, every day. For background information, my kiln is 24" wide and 46" long inside. I gradually pushed the limits over time to see just how much venting I could get away with before things cracked from thermal shock.

I'd recommend waiting until your kiln gets below 700F (depending on where your thermocouple is located) before you start venting. By then the glass should be below the strain point.

Good luck with this -

Geri

[Lynne Chappell]

Good point Geri about the shape of the kiln in respect to venting. My top loading kiln can be propped open a little quite safely. In the Clamshell kiln, because the lid opening is level with the glass, the glass catches a chill quite easily. I don't prop the lid open until it is down to 250F and then only 1/2". I can take out the peep hole plug earlier, but that doesn't seem to speed it up very much.

[Brian and Jenny Blanthorn]

I'm currently slumping some carved bowls in 1/2" float, 16" diameter. Once I get past 700 on the down cycle, the kiln loses temperature "naturally" very slowly, about 30 deg/hr, rather than the suggested 200/hr rate. The first time I did this I decided to crack the top and see how that influenced the cooling rate. It doubled it, to about 60/hr. This is a 30 x 60 x 14 coffin kiln, and when say I cracked the top I mean not much, about 1/8" on the leading edge, away from the hinges. I also loosened the bottom vent plugs slightly, so there could be a very slow influx of cool air from the bottom leading edge. The first two bowls came out fine, and passed the visual "no stress test" when viewed through polarized film. I'm wondering if I should be "happy" at this point, or if I dare open the plugs a little more, to increase air flow.

Second option: I'm considering altering the kiln. I'm wondering if it would be worth ripping out the firebrick side insulation, and replacing it with fiber. The kiln currently has fiber insulation in the top, brick on the bottom and sides. Top elements rheostat switch, side elements controlled. It is 1 year old, Bartlett controller. Am I crazy? I already know that, but suggestions/other opinions would be appreciated.

Be awair that thee is some risk 2 what I am suggesting

But never open bottom / side top vents at once

One only

IE all top / all bottom

Then swap em round this is due 2 thermal shock if U can pre heat the incomming air 2 the bottom then that will B OK

Say taking a pipe from the top of the kiln then in2 the bottom

With a fan

Another thought following on from Berts idea is set some stainless pipes in2 the brick n cover this with the fiber then blow cold air throgh it

And or use a hollow quatrz tube idea then blow air thruogh that

I put news paper on the glass when safe 2 do so

This stops thermal shock n U can realy open up the door

Brian