Bouncing kiln floor

[charlie holden]

This came up not long ago when somebody complained about oddly bent glass. Now it has happened to me.

I have always had two layers of IFB in my kiln floor. I recently took one layer out in order to get my kiln to cool down faster between firings. I've got a sand bed in it and want to reload it every day. Things went fine for about three days of heavy firing. Then after the fourth firing I noticed there were cavities in the sand under my glass. I thought it was weird but didn't think too much about it. But when I got the glass out and cleaned up I noticed it was warped. Apparently my kiln floor, 1/4 steel plate under the brick, bowed up when it was hot. Sand had filtered down between bricks when they were bowed up. Everything settled back down once it was cool enough.

So I tore all the brick out, put in a layer of one inch mineral board insulation and put one layer of brick back on top. We shall see if that is enough to keep the steel cool. May not be able to reload five days a week though.

As I've written before, next kiln I build is going to have steel grating under the floor rather than plate, as per Warren Langley's design.

ch

[rosanna gusler]

hi charlie, what kind of steel grating? like expanded metal or some sort of welded strips on edge tm kind of thing ? would you have some sort of small i beam or angle iron support under the grating? i am in the verry beginning stages of designing my big kiln. rosanna

[Bert Weiss]

This came up not long ago when somebody complained about oddly bent glass. Now it has happened to me.

I have always had two layers of IFB in my kiln floor. I recently took one layer out in order to get my kiln to cool down faster between firings. I've got a sand bed in it and want to reload it every day. Things went fine for about three days of heavy firing. Then after the fourth firing I noticed there were cavities in the sand under my glass. I thought it was weird but didn't think too much about it. But when I got the glass out and cleaned up I noticed it was warped. Apparently my kiln floor, 1/4 steel plate under the brick, bowed up when it was hot. Sand had filtered down between bricks when they were bowed up. Everything settled back down once it was cool enough.

So I tore all the brick out, put in a layer of one inch mineral board insulation and put one layer of brick back on top. We shall see if that is enough to keep the steel cool. May not be able to reload five days a week though.

As I've written before, next kiln I build is going to have steel grating under the floor rather than plate, as per Warren Langley's design.

ch

Charlie

My kiln has the layers of brick and then piles of blanket and boards on top of that. With a 3/8" layer of sand on the top layer of blanket, I can fire 10mm glass up and down in 24 hours. Without the sand, I was able to push by cracking the kiln just below 500 and get in out and reloaded in 12 hours.

I have been thinking about pulling out all the fiber and a layer of bricks, and replacing it with a 1" or maybe 2" layer of vermiculite board. Another idea would be to do a 1" layer with 1" studs of vermiculite board underneath it so there would be air vents beneath the sand bed. I'm not too sure about that as it could set up uneven gradients. I use thin sheet metal underneath my bricks. I have tried to discover the relative insulating factors for IFB and vermiculite boards but I haven't had much luck. The board is cheaper than brick though.

Keep us informed about what else you discover.

I have recently noticed that most of my glass that is sand cast has some grits of sand sticking on the top surface. I'm not exactly sure just when they get on there. It could be when I lower the bell, although I was pretty careful last time and it seemed to be worse on both ends. Perhaps I knocked some up when I was cleaning off the suction cup rubber.

I am intriqued by the idea of getting some coarse alumina powder and messing around with a mixture of olivine and alumina and CMC. Make the bed, dry it, and then put the glass in and fire. Bentonite might do the same as CMC.

[Tony Serviente]

In my two big kilns I have floor made of two layers of low densiity brick with mullite shelves, and even with regular runs of over 1500 F, I have had no warping of the floor. One of my older kilns has only one layer of the brick, and the floor experiences "tides" on heatup and cool down.

[charlie holden]

hi charlie, what kind of steel grating? like expanded metal or some sort of welded strips on edge tm kind of thing ? would you have some sort of small i beam or angle iron support under the grating? i am in the verry beginning stages of designing my big kiln. rosanna

I would use one of these, probably the first one -- welded bar grating. It is made for pedestrian traffic, which should be plenty rigid enough for a kiln. If you click in a couple of pages you can find load tables.

http://www.mcnichols.com/products/bargrating/

I would probably have square tubing welded across in a couple of places but I haven't calculated out the specifics. It will be some time before I can afford another kiln.

I think a big advantage to this stuff would be that the grating can cool down much more easily than steel plate while also having more resistance to bending vertically. If you can compress the bricks, mortar them together or drill them and run bars through them for support, you may be able to get by with very thin steel, which wouldn't have the strength to move the brick if it wanted to warp. But grating seems the easiest.

I would also think seriously about aluminum boxes instead of steel. A lot of the Australian kilns are being made out of aluminum. I found a bit of rust under my bricks even though the plate had been painted with heat resistant stove paint before I put them in the first time.

ch

[charlie holden]

I have recently noticed that most of my glass that is sand cast has some grits of sand sticking on the top surface. I'm not exactly sure just when they get on there. It could be when I lower the bell, although I was pretty careful last time and it seemed to be worse on both ends. Perhaps I knocked some up when I was cleaning off the suction cup rubber.

I am intriqued by the idea of getting some coarse alumina powder and messing around with a mixture of olivine and alumina and CMC. Make the bed, dry it, and then put the glass in and fire. Bentonite might do the same as CMC.

Oddly enough that was one of the things about the bent glass -- it had grains of sand on the top surface, which almost never happens to me. I don't know if it was coincidence or not. I don't know if the steel bends up slowly or pops up all at once, flinging sand into the air. It happened in the middle of the night.

I recently added some silica play sand to my olivine and plaster mix so i could get more texture. The play sand wasn't very well cleaned. I would use commercial grade sand for plaster next time.

I'm drawing a blank on CMC. What is it? I want to try a bentonite mix to see if it will hold steep slopes after it is dampened and dried.

Too much to do.

ch

[Jacques Bordeleau]

Hi Charlie. I had similar issue years ago when I first built my 27x57"I.D. box. Puzzled my feeble brain, I tell ya. The floor was one sheet 1/8" steel plate, with 1-2 layers of firebrick, dependin'. Well.... of course there was a temp differential...the edges not heating nearly as much as the center area, and so the center area would rise ... a lot. Since the steel sheet floats on a grid of 3/4" steel sq. tubing, I had it cut into smaller (like 8x12") sections and reinstalled all in place. Works just fine now.. small gaps allow each piece of the 1/8" sheet floor to expand indiviually with never a rise out of plane.

Sometimes I 'flat-fire' glass on plain un-washed steel sheet at low temp...and even then the ends of the large sheet will still rise due to temp differential ... so I lay weights on the ends and it stays flat....only going 1,100 f or such. That gets me a piece of glass that is truly flat after whatever I've previously done to it.

Hope this helps, Jacques Bordeleau

[Jacques Bordeleau]

Charlie.... mine bends upward slowly as the kiln hit fusing temps. Doesn't pop-up. I've watched it happen many times. The floor arcs upward and flattens back down as the kiln cools....but the glass arc is locked in place much sooner and does not flatten back down as the steel does. Sound familiar? As I recall, it happens pretty late in a firing cycle, as it takes a while for the heat to penetrate the floor brick enough to cause that kind of upheaval. Probably you will get the same effect using a grate or expanded metal for the same initial cause... if it is still in one sheet, as it will still stay cooler near the edges and hotter in the center. It's no mystery once you realize why it's happening. Cut it up.

Regards, Jacques Bordeleau

[Bert Weiss]

My kiln floor has 1" x 1" x 1/8" angle irons running every 9" so that each brick is supported by the steel. On top of the angle irons are 2 pieces of thin galvanized sheet metal, then the bricks. I have never seen any warping, ever.

I suspect that the thick steel beneath your bricks is problematic.

I see no good reason to use square steel tubing anywhere on a kiln design. It adds considerable expense and only minimal strength addition. Angle irons do the job for a lot less dough.

My gantry is made with round tubing that is flattened at the ends to make welding possible without having to grind out the radius. This is also much cheaper and easier than using square tubing.

On Carol Swann's kiln we used no tubing and it worked out to be very stable.

[Brock]

I suspect that the thick steel beneath your bricks is problematic. . . Bert

I agree. My kiln floor is expanded metal mesh, then 1800F board, then fibre. Only 4, half bricks near the corners to act as bases for the bricks that support the shelves. No warping ever. These kilns are 20" x 20", but I see no reason why it wouldn't work on a larger scale. I think some kilns are over-insulated, hence the ridiculously long cool down times. Brock

[Jacques Bordeleau]

Gee Bert.... dunno.... square tubing never broke the bank way out West here...heheheh. The point of my post wasn't the tubing. I'd use round, square or angle stock.... whatever. The point of my post was if you have a thick steel floor, cut it into pieces and the problem is solved. My sub-floor is 1/8" steel... Your galv sheetmetal is so thin I suspect it can't pose the same problem ... not enough strength to lift the bricks, so it seems like a good material for next time...cheaper too. But I think I'd use plain, non-galvanized, just to be safe. You may have so much insulation on your kiln floor that it would never get hot enough to off-gas, but galvanized CAN give off some noxious fumes. I reckon you knew that, but others may not.

Regards, Jacques Bordeleau

[Bert Weiss]

I suspect that the thick steel beneath your bricks is problematic. . . Bert

I agree. My kiln floor is expanded metal mesh, then 1800F board, then fibre. Only 4, half bricks near the corners to act as bases for the bricks that support the shelves. No warping ever. These kilns are 20" x 20", but I see no reason why it wouldn't work on a larger scale. I think some kilns are over-insulated, hence the ridiculously long cool down times. Brock

My experience tells me that mass is way more a factor than insulation. Without a sand bed I can cool twice as fast with the same insulation. Charlie's problem is possibly due to not enough insulation, causing the steel to warp. Smaller pieces of steel might solve it, though.

I understand the idea of underinsulated and overpowered, but there are certainly limits to those parameters like the box being too hot or the floor warping.

[charlie holden]

I suspect that the thick steel beneath your bricks is problematic. . . Bert

I agree. My kiln floor is expanded metal mesh, then 1800F board, then fibre. Only 4, half bricks near the corners to act as bases for the bricks that support the shelves. No warping ever. These kilns are 20" x 20", but I see no reason why it wouldn't work on a larger scale. I think some kilns are over-insulated, hence the ridiculously long cool down times. Brock

My experience tells me that mass is way more a factor than insulation. Without a sand bed I can cool twice as fast with the same insulation. Charlie's problem is possibly due to not enough insulation, causing the steel to warp. Smaller pieces of steel might solve it, though.

I understand the idea of underinsulated and overpowered, but there are certainly limits to those parameters like the box being too hot or the floor warping.

For all I know the floor could have been warping before I took the second layer of brick out. I was mostly firing on fiber shelves so the glass wouldn't have been effected. When I was firing on sand I wasn't firing as hard and fast as I was last week.

The sand slowed down my cooling a lot. Taking the layer of brick out was a pretty good balancing act and got me back almost to my previous rate of cooling without sand. I probably took out more mass in brick than I added back in sand. So now I've put back an inch of low mass insulation and we shall see. I do think this kiln is over insulated for purposes of trying to cycle through relatively thin glass every day. It's great for casting thick stuff. I could cut more vent holes in it but that would be work.

I built it interpolating from Halem's and Gibberson's books. (The only guy near me at the time that had big kilns wouldn't let me look at them and wouldn't offer me any advice. He won't let anybody in his studio.) They use 1/4" plate under their kilns but those kilns aren't as long and wide. A 20" x 20" piece of plate might not warp at the same temperature as a 48" x 80" piece.

As far as cutting the plate into smaller pieces, I only have one bar of C channel across the center for support. It is welded to the plate. So I would have to add in lots of frame members to hold smaller plates. If I had to do that I would just put in the bar grating. I think it doesn't warp because it has very little area exposed to the warm bricks and lots of area to radiate heat into the air.

ch

[Bert Weiss]

Gibberson's book is way off base as far as I am concerned. Dudley is not a fuser and I have no clue why he decided to include fusing kiln designs in his book. It's been a while since I looked at it, but as I recall his large design is way underpowered.

This was in part an impetus for doing my book, which is in process.

[Phil Hoppes]

Charlie,

Was the steel plate welded to your frame? If so that is your problem. I have
1/8" steel plate under a layer of K23 IFB and it is fine. The plate simply sits on a fame structure of box steel. There is a 3/8" gap all around the steel edge to allow for expansion with heating and cooling. You can't weld the steel floor of a kiln be it expanded metal or solid steel. It will buckel under the heat. That is of course unless you want to build your kiln of inconel or hastoloy

I agree with Bert, mass has a MUCH larger effect on the heating and cooling of a kiln. I would suspect that an expanded metal vs solid steel bottom would show little difference in actual thermal resistance. For starters, steel is a very high thermal conductor so the temperature of one side of the plate is going to be very close to the other side. The largest effect would be the difference in how IFB performs as a black body radiator vs steel plate. Don't know the answer to that one off the top of my head.

Phil

[Brock]

Charlie,

Was the steel plate welded to your frame? If so that is your problem. I have
1/8" steel plate under a layer of K23 IFB and it is fine. The plate simply sits on a fame structure of box steel. There is a 3/8" gap all around the steel edge to allow for expansion with heating and cooling. You can't weld the steel floor of a kiln be it expanded metal or solid steel. It will buckel under the heat. That is of course unless you want to build your kiln of inconel or hastoloy

I agree with Bert, mass has a MUCH larger effect on the heating and cooling of a kiln. I would suspect that an expanded metal vs solid steel bottom would show little difference in actual thermal resistance. For starters, steel is a very high thermal conductor so the temperature of one side of the plate is going to be very close to the other side. The largest effect would be the difference in how IFB performs as a black body radiator vs steel plate. Don't know the answer to that one off the top of my head.

Phil

The main problem in this situation is allowing THAT MUCH heat to reach your framework. That is what the insulation is for, but like the 3 bears porridge, it's gotta be just right. Not too hot, not too cold. My kilns are a angle iron superstructure, with expanded metal mesh WELDED to the frame. No problem!

[Phil Hoppes]

Ah....but mesh will expand and contract without a problem. Lots of holes for flex. Solid steel wants to expand but if the sides are welded there is no place to go but up as they say.

I stand corrected. Mesh floors should work fine. Solid floors should NOT be welded.

Phil

[Phil Hoppes]

Charlie,

I did a little more analysis on your problem. Steel, or any material for that matter, will change shape equally in all directions if it is unconstrained. The simple formula is:

Delta (inches) = COE (Material in in/F) * DeltaTemp (F) * Length (inches)

In these units the COE of standard alloy steel is between 6.3 and 8.6. For a 20" square plate the amount of expansion is the same REGARDLESS of if it is 1/8" or 1/4" or 1/2" ...etc. assuming all the different plates are heated equally. I have some cursary data on my kiln which has one layer of IFB on the bottom. When it is heated to 1600F and held for one hour the temp on the bottom steel plate raises 260F from room temp. For this amount of temperature rise you would then have the following expansion:

Delta = (8.6 X 10 E-6) * 260 * 20 = 0.0447 or almost 45 thousands. That is almost 1/16", which if both sides were welded fixed would be more than enough to pop your bottom up.

Take the other example of a piece 48" x 80". Across the longest dimension of 80" you will have 4x the previous example or almost 3/16". That is quite a bit of movement. As I corrected myself (thank's Brock) expanded metal will have a lot of give since it is an open material. You could weld expanded metal, which actually the sides and top of my kiln are made of, and not have a problem with it poping.

Phil

[charlie holden]

I think there can be some warping even if the plate isn't welded. I've seen plenty of marvers that are a half inch thick or more of floating plate that have warped over time from hot glass being rolled over them. This came up in a small way when I was building the kiln and the guy doing the welding said I should weld it rather than float it. What did I know?

I read an article on the collapse of the World Trade Center and it said that a big part of the problem was that the remaining steel posts and beams warped towards the heat of the fire.

Did a pretty high fire today so I'll see if the extra insulation made any difference. Might have to go several days in a row to really tell.

ch

[Bert Weiss]

I think there can be some warping even if the plate isn't welded. I've seen plenty of marvers that are a half inch thick or more of floating plate that have warped over time from hot glass being rolled over them. This came up in a small way when I was building the kiln and the guy doing the welding said I should weld it rather than float it. What did I know?

I read an article on the collapse of the World Trade Center and it said that a big part of the problem was that the remaining steel posts and beams warped towards the heat of the fire.

Did a pretty high fire today so I'll see if the extra insulation made any difference. Might have to go several days in a row to really tell.

ch

Long ago in the early 80's I did a session at Haystack in stained glass with Paul Marioni. Paul showed us how to roll sheet glass in the Haystack hotshop. We clamped down some 1/4" flat bar on a 1" thick steel marver, ladled molten glass on the marver and rolled it out with a piece of iron pipe that was kicking around. It was very cool.(I mean hot) I got a large cows tongue made of clear glass. We did a dozen or so sheets and the marver ended up super warped. I was always pretty freaked out at how the marver warped considering it was so thick. I relaxed a bit about the situation when my glass blower budy explained to me that he would rather work on a warped marver as it gives you more opportunities for shaping the glass.

I appreciate your sharing the warping problem. Kiln design is certainly an interesting subject. I feel extremely fortunate that I lucked in to a pretty good design for my big bell. Especially because much of the input I got from people was from people with zero kiln building experience.

What I really want to discover is just how thick a slab of glass I can anneal on the floor of my kiln. Eventually I will get around to doing comprehensive testing on this. I can borrow the necessary equipment for the tests. I just need a compelling reason to do the tests, beyond curiosity.