Kiln Building Materials and Design

Dean Hubbard · Post by **Dean Hubbard** » Thu Oct 30, 2003 5:09 pm

I'm a new kid on the block, just joined this forum this morning. I've been working with glass (cold) for the past 25 years and have recently warmed up. I currently use a Nova 21 ceramics type kiln and it's time to go bigger. I've been looking over past postings about kiln building. It seems I now have more questions than answers.
My objective is to build a 48" x 96" bell type kiln that I can slump, fuse and also cast with. I would like to use up to date efficient materials and still save a lot of money building vs. buying. Materials that have caught my eye so far are, isofrax blanket insulation and quartz tube kiln elements. I have a 200 amp single-phase service coming into my shop. (I live and work out of a decomissioned fire station) I have read that you need 3-phase to use quartz tube elements and that to cast in a kiln of this size, I need to heat from the bottom as well. I am not sure what to use on my table (soft firebrick or fiberboard or ?)
If any one has any advice regarding design, material selection please let me know. Thanks!

Dean Hubbard

Tracy · Post by **Tracy** » Thu Oct 30, 2003 7:21 pm

Dean,

I don't know a thing about how to build kilns, however, I'm now reading an old book called Kilns Design, Construction, and Operation by Daniel Rhodes. Maybe some of your questions can be answered by this book. Good Luck!

~Tracy

jerry flanary · Post by **jerry flanary** » Thu Oct 30, 2003 8:57 pm

Isofrax is good stuff but not cheap. Cheap is available from Tom at Hi-Temp Refractories, or Clarks in Virginia. Clarks is listed in the back of Henery Halem's Glass Notes. If you don't have it, just go get it you don't have to re-invent the wheel. And while your at it, Dudley Gibbersons Glassblowers Companion is another good source. If you want really cheap: bulk fiber instead of blanket: just pour it in between the bricks and the exterior metal wall and pack it in tight. Build a smaller oven first; Make smaller mistakes.
Quartz tube elements sound great. I was unaware of the three phase requirement. But that isn't saying much. Still it seems counter-intuitive. Current is current, Resistance is Resistance- it's not like it's a motor. Dudley has a system of supporting the elements on a ceramic rod or tube that you might consider.
Currently I am building a big oven with an element in the floor. The floor is softbrick. If you trash the floor, it is possible to get a softbrick to lift straight out using a shopvac which you can't do with a hardbrick. I flip them over and put em back. So I had a straight level floor of IFB and then I cut some so that they were 1"x 4 1/2"x 9" and I laid out a maze that my element snakes through. Then the kiln shelves sit on top of the maze so that the element is not visible.
Draw it before you build it. It's a lot easier to erase on paper!
Building your own equipment is good because when it breaks (and it will) at least you have a good idea of how to fix it. If you consider the cost of your labor though it is not "cheaper" unless you have a bunch o free time. If you have the money go for a pro model.
Hope this helps some.

charlie holden · Post by **charlie holden** » Fri Oct 31, 2003 2:48 pm

There may be a bit of confusion about quartz tubes. Most glass kiln builders that say they are using quartz tubes are putting normal, coiled wire kiln elements inside a quartz tube that is open at both ends. The tube is a way to protect, insulate and hang the element.

If you do an internet search on quartz tubes you will get a lot of sites that sell quartz tubes that are sealed and, (I think), filled with an inert gas and an element. These quartz tubes get much hotter much faster than the elements we typically use. They get white hot. The one place I have seen them used is in a kiln used for firing stains and paints onto thin pieces of small glass. That kiln gets to 1000 F in five minutes.

My experience has taught me that it is unlikely that you will need floor elements in a kiln that big. In fact, if I was building a kiln that big I would build it with an open iron grating under one layer of soft brick, so that heat would move out of the floor of the kiln fast enough to cycle it every 24 hours. I would use it for sand bed slumping.

You only need floor elements when you are casting thick glass, more than two inches or so, and when you are using ceramic or silicon carbide shelves. The heat from a floor element won't get through a fiber shelf well enough to make a difference, and it is hard to find a shelf that big made of anything but fiber. Weight begins to become an issue when the glass gets thick, so designs tend to be broken down into tiles. How heavy would a 4'x8'x2" piece of glass be?

The better option is to build a big kiln with ceiling elements only for large slumping and a smaller, deeper, better insulated one for casting, that has elements on all six surfaces.

Which brings up the issue, if you want to slump a 4'x8' piece of glasss, the outside of your kiln needs to be about a foot longer and wider than that. You don't need to have three phase power, but it is more efficient. Fiber blanket, about 4 inches thick, is good for the bell and IFB good for the floor.

Dean Hubbard · Post by **Dean Hubbard** » Fri Oct 31, 2003 4:47 pm

Thanks Charlie,

Whew!
I stayed up way too late last night going through the archives searching for more information. (I think I hit the sack @ 4:00AM, kinda tough getting myself and the kids up and off to school this morning)

One thing I didn't bring up here is, I looked at a commercial bell type kiln that mentioned air induction for effecient cooling as opposed to crash cooling. (I'm not sure if it was air induced into the open end quartz tube only or into the firing chamber itself) That sounds pretty touchy to me, gee if you could do that without hurting the kiln, why couldn't you add quenchers and rollers to do tempering as well when needed?

I haven't been too impressed lately with the tempering results my commercial glass supplier provides, so anything I do that needs to be safety glass, I laminate myself with a product called Glasslam.

charlie holden · Post by **charlie holden** » Fri Oct 31, 2003 7:41 pm

glazinguy wrote:
One thing I didn't bring up here is, I looked at a commercial bell type kiln that mentioned air induction for effecient cooling as opposed to crash cooling. (I'm not sure if it was air induced into the open end quartz tube only or into the firing chamber itself) That sounds pretty touchy to me, gee if you could do that without hurting the kiln, why couldn't you add quenchers and rollers to do tempering as well when needed?

I haven't been too impressed lately with the tempering results my commercial glass supplier provides, so anything I do that needs to be safety glass, I laminate myself with a product called Glasslam.

I have power vents in the base of my big, (admittedly over insulated), kilns. They draw a small amount of air into the top of the bell and out through the floor, which helps circulate air, but isn't enough to really speed cooling. I think that you'ld have to be very careful about thermal shock if you wanted to pull enough air volume through to crash cool. Tempering is beyond considering on my own.

I don't crash cool any more unless I want to chill the surface of a cast piece that has lots of bubbles in it. Then I bring it back up to casting temp anyway, to smooth the surface out. I vent my kilns when they are coming down at about 350 F, and then only in a way that the glass isn't exposed to a direct blast of cold air. I don't vent thick cast stuff until it is at room temp.

Bert Weiss · Post by **Bert Weiss** » Sat Nov 01, 2003 4:48 am

The reason we're not building tempering ovens is a simple matter of needing a 750 HP blower, and the high cost of alumina rollers.

I am lucky to have a very cooperative tempering company 2.5 hours from my studio. They will temper while I wait as long as I am transporting the glass both ways and I hit their schedule just right. It takes 12 minutes to run a sheet of 10mm up and down to room temp.

Glass lam is not the best laminating resin to use for other than flat glass. Zircon and Bohle make resins that have low shrinkage. Glass lam and several other resins have a high shrinkage percentage.

I've seen some kilns with quartz cooling tubes. The tubes extend all the way through the roof. At the end of the tubes there is a fan on one side to blow away hot air and help the kiln cool faster. The fan comes on when the kiln is cooling slower than the program.

Dean Hubbard · Post by **Dean Hubbard** » Sat Nov 01, 2003 3:01 pm

Wow, 750HP for a cold blast of air! The only thing around here that has that kind of horsepower is my neighbor's dragster. I think the other neighbors would frown on that one, not for twelve minutes anyway. Hee-Hee!

I haven't had any shrinkage problems with Glasslam, I'm using their newest version of resin and I've poured the resin interlayer as thick as 1/2" in some unusual pours. I live in a very mild climate, I wonder if that attributes to not seeing any shrinkage problems.

Bert, do you think incorporating the quartz tubes with cooling fans would be a worthy investment?

Dean

Bert Weiss · Post by **Bert Weiss** » Sat Nov 01, 2003 4:36 pm

glazinguy wrote:Wow, 750HP for a cold blast of air! The only thing around here that has that kind of horsepower is my neighbor's dragster. I think the other neighbors would frown on that one, not for twelve minutes anyway. Hee-Hee!

I haven't had any shrinkage problems with Glasslam, I'm using their newest version of resin and I've poured the resin interlayer as thick as 1/2" in some unusual pours. I live in a very mild climate, I wonder if that attributes to not seeing any shrinkage problems.

Bert, do you think incorporating the quartz tubes with cooling fans would be a worthy investment?

Dean

750 is a big team. As I understand it, the thicker the glass, the less air is required to temper it, so it could be possible to create a home oven that could do thick pieces, but I have never heard of somebody pulling it off. The 750 is required for window glass.

I do think that the quartz tube cooling program is a good idea. There are several kilns in the Portland OR area that are built this way. They are not all that far from you. Bullseye has one, I think, as well as some private studios. I am thinking about adding empty tubes beneath my straight wire elements. I tried a stainless steel tube that went all the way through and had holes every few inches. It caused more problems than it solved.

My straight wire elements give a very even heat and cost a lot less than coiled elements. They are not compatible with quartz, at least not with the element inside. You need Kanthal wire to put inside quartz. I use nichrome 80/20. They oxidize differently.

It is good to hear that your glass lam is working for you. They may have improved the formula. I have played around with lamination and hold on to it in my bag or tricks. It is a difficult technique and messy when the seals fail. I have some theoretical fixes for that, but I haven't taken on any laminating projects since doing my prototypes.

jerry flanary · Post by **jerry flanary** » Sun Nov 02, 2003 2:38 pm

Bert,
The stainless tubing might have worked better w/o the holes. Run air through the interior and just pull the heat through the metal because of the cool air on the interior. This way though, the cool air wouldn't interact directly with the kiln environment. Just an idea. A little time w/ a welder might revitalize that experiment; just fill those holes back up...

charlie · Post by **charlie** » Tue Nov 04, 2003 12:14 pm

charlie holden wrote:Weight begins to become an issue when the glass gets thick, so designs tend to be broken down into tiles. How heavy would a 4'x8'x2" piece of glass be?

~410 lbs

Dean Hubbard · Post by **Dean Hubbard** » Tue Nov 04, 2003 1:14 pm

Charlie,

I come up with 839.68 lbs. (assuming what I read was true: 1/4" float glass weighs 3.28 lbs. per square foot) I also read that float glass weighs 150 lbs per cubic foot, which would mean 1/4" would weigh 3.125 lbs. per square foot. Figured that way I come up with 800 lbs. Either way, it would be too heavy for me to set by myself!

Speaking of float glass, I've been going through the archives (what a treasure!) looking up things about bubbles, float glass compatable frits, firing schedules and such. I now see why I gotta get this new kiln built.
I've been doing fusing and casting tests with float glass in a side fired cermaics type kiln.
I see why I am having so much success getting bubbles.

So far the best I can do, is to get flat air pockets that aren't bubbles yet, they kinda look like ameabas. (topped out at 1550F) I haven't tried using chads yet. I've got a fuse test in right now and I'm going to soak @ 1220F for 1 hour, then up to 1440F for 1 hour, then crash to 1050F anneal soak.

In regards to flaot glass, I do have a questions that I couldn't find and answer to.

When casting multiple layers of 1/4" float glass into an open faced plaster mold, can you soak or raise the casting temperature so all the bubbles and air pockets from the bottom of the mold rise to the surface , pop and leave a smooth upper surface?
Or should I try making float frit? (I've read that float glass doesn't flow very good, so I haven't tried the flower pot drip technique.)
If I make a float frit, is there a flux that I can add to get better flow?

Lionel · Post by **Lionel** » Tue Nov 04, 2003 3:46 pm

I just got my tubes and elements from Duralite. My interior is 43 x 96 and the 8 elements draw 88 AMP - Single Phase.

I worked backwads because of limited studio power. My max available amps with 20% overage came out to 88. So I asked Durilite to calc the max envelope, wall and ceiling thickness. The number to watch is watts per cubic foot. The higher the number the faster the ramp potential. At 1025 I should be able to get to 1550 in just over an hour.

With the elements,tubes, mulite insulators, wire harness, power block and all connectors I came in at 734.45 including freight.

My kiln todate is at $3,241.00 - not including the more than 150 hours of research and construction time.

I look at it this way - it's a lot less than th 13K to buy a complete unit.

PS - Don't uses SPS it took them more than 30 days to ship the fiber and brick and they were short 2 cases. No calls No credits No help at all.

charlie · Post by **charlie** » Tue Nov 04, 2003 4:02 pm

20% overage or underage? i hope the latter.

charlie holden · Post by **charlie holden** » Wed Nov 05, 2003 1:04 pm

glazinguy wrote: In regards to flaot glass, I do have a questions that I couldn't find and answer to.

When casting multiple layers of 1/4" float glass into an open faced plaster mold, can you soak or raise the casting temperature so all the bubbles and air pockets from the bottom of the mold rise to the surface , pop and leave a smooth upper surface?
Or should I try making float frit? (I've read that float glass doesn't flow very good, so I haven't tried the flower pot drip technique.)
If I make a float frit, is there a flux that I can add to get better flow?

Sure, you can always get the temp high enough and soak long enough that the bubbles will rise and pop. The problem with float is that it takes a very high temp and a very long time. The better option is to build a mold that is porous enough that air below the glass works its way out through the mold. This is why sand casting is so interesting. You can make a plaster mold porous by adding in some chopped paper, cellulose or vermiculite. And there are some plasters like Hydroperm that have a soaping agent in them that form small bubbles when they are mixed.

It's not just that float doesn't flow well, it also devitrifies easily. You can add borax to frit or cullet that will act as a flux. I've never done it. I think Lundstom talks about this some in "Fusing Book Three" in the section on crucible casting. It would take lots of testing, careful measuring and record keeping to get consistant results. You would also change the COE of the glass so it might not be compatible with your colors anymore. In the end you would still end up with green tinted base glass and I don't think you would have saved much money over casting cullet from Bullseye or Spectrum.

ch

Bert Weiss · Post by **Bert Weiss** » Thu Nov 06, 2003 2:20 am

My take on bubbles is multifold. First never fire above 1435ÂºF. At that temp the glass is not soft enough for the bubbles to rise much. Next I soak at 1220 or so for 1 hour to help squeeze out bubbles. Lastly I use the heaviest glass I can for the application. 1/4" is not heavy enough to push out much air. 3/8" is my glass of choice for stacking.

Bert

Dean Hubbard · Post by **Dean Hubbard** » Thu Nov 06, 2003 2:30 pm

Thanks for your take on bubbles Bert, which by the way, I'm going to stop playing with for a while anyway. I need to get back on track with the kiln project.
I just got off the phone with Duralite, I've got some info to discuss and hopefully get some good input.

Using an inside finished dimension on the bell of 48" x 96" x 18" and based on 1100 watts for casting ability (1800F range), I amped myself out of the park, so to speak, 40 amps over my service.
Lowering the finished inside height to 12", which gave me 32 cubic feet, we came in at 160 amps. (1100watts x 32 cubic feet / 220 volts = 160 amps)
Duralite also recomends using the Watlow controller, using a solid state (not mercury) relay. They told me that way I can throttle down the power when slumping and fusing. I like that idea, although most kiln builders I've talked with have advised me to use a mercury relay.
These were just preliminary calculations. Duralite can fine tune everything and might get the amps down a bit more, as soon as I give them final specs. on material construction and kiln loads (maximum weight of glass to be placed in the kiln)
So, here are my questions:
1. Will 12" be enough inside height, or does that seem a bit too low? I plan on working mainly with float glass and perhaps casting billets.
2. The Watlow controller is a single zone controller. If the kiln is designed right and element placement correct, giving me good even heat, is a multi zone controller really needed?
3. mercury vs. solid state?
4. 1100 watts per cubic feet with the ability to throttle down when not casting, Sound good?
5. Is a raduis bell corner on the long sides a good idea to cut the cubic feet down, or just an added design nightmare as far as even heating goes?
6. Movable bell or movable table? I originally planned on building two tables and moving the bell from one to the other via track. I'm now leaning towards a single table and movable bell on a shorter track.

Input would be greatly appreciated. Well, I'm off to the archives!

Edit Note: Well, the archives say 800-900 is a good number for watts per cubic foot. Maybe Duralite advised of 1100 watts because of casting range or the design of the quartz tube elements. I know not. I do know that I get to pay my electric bill though.

charlie holden · Post by **charlie holden** » Thu Nov 06, 2003 7:30 pm

First, I trust the guys at Duralite. They know a lot more about this than I do. I think capacity to 1800 F is more than you will ever need unless you want to fire ceramics. I do flower pot pours of Bullseye at 1600. Float, if you need to get it that soft, will take 50- 100 degrees more. And remember, you won't be able to slump a piece of 48"x 96" glass in this kiln since you will need a couple of inches all the way around between your glass and insulation.

Otherwise:

1. 12" is plenty. You can always add a ring of IFB around the outside of your table to raise the table 3".

2. I don't think multi zoned controllers are a must for this kind of kiln. I would hang the elements so that you have an element right in the outside corners of the bell, and space the others so that they are further apart from each other in the center of the bell and closer near the edges. These types of kilns tend to be hotter in the center. I can't give you precise measurements. You might be able to find a commercial kiln and measure their element placements.

3. The tricky thing about solid state relays is that they tend to fail on when they fail. So you should put in a seperate thermocouple, controller and mechanical relay as an overheat safety circuit. Solid state relays do fire more precisely and extend the life of your elements. Mercury can be very expensive to clean up if they leak.

4. I think 1100 is plenty.

5. I'm not sure which corner you're talking about radiusing.

6. If you're only going to have one table, all you need to do is get the bell up high enough, and safely hung enough, that you can work under it. If you're going to move things, I prefer a mobile bell, as moving the table tends to shake things up and shift them around.

Dean Hubbard · Post by **Dean Hubbard** » Fri Nov 07, 2003 3:06 pm

Charlie,
I was impressed with Duralite. I spoke with an engineer named John.
He told me that there is a backup system incase of failure, they won't sell it without one.
I'm trying to finalize the material list, so I can proceed with John at Duralite, but I'm not making any progress with a company out here on the west coast that unifrax directed me to.
I only have two decesions to make at this point.
1. Insulation configuration. I like the products I see at the unifrax site, hd and ld boards, fiber blankets, but I wish I could get a response from this company out here, to find out sizes available. I'm hoping to use a one piece board on top of the IFB.
2. Interior size and configuration of bell. I'm leaning towards going 48" x 90" x 12". It would have be nice to slump a stock sheet of float 48" x 96" however, I didn't consider the need for extra spacing around the perimeter. I think this way with a 48" x 90", I'll be able to easily fit a 42" x 84" piece of float and be able to also get a 46" wide piece if needed. Also this size got me down to a 150 amp. requirement which is well below the 20% underage of my 200 amp service.

elin · Post by **elin** » Fri Nov 07, 2003 10:10 pm

I work at EK Miller Co. in Portland. We build FuseStar kilns, which have the cooling system Bert described-- and there are quite a few of our kilns in use in the area, and also in the bay area. I think it works great-- cool air is never actually being introduced into the kiln, but heat is being removed quickly. The elements are wound on the outside of the quartz. There are also venting chimneys on the roof of the bell, if you need to crash cool the glass surface.

I'm not an expert, but I have some experience, and I have a few answers for you--
1) You do not need three-phase power to have quartz elements.
2) 12" might be too low for some things, but you can build up a brick wall to increase the height-- Just don't skimp on power if you want that flexibility.
3)A multi-zone controller can be a very nice luxury, but it should not be necessary-- you'll be able to tell if the heat is uneven.
4) Mercury relays tend to be more reliable, but solid state ones work well too.
5)I strongly reccommend a movable bell. Once you have your pieces carefully laid out, the last thing you want is for your table to hit a bump while you roll it into place.-- You always have the option of building a second table later.

I have also used Glasslam, and have always had success with it-- even on contoured surfaces. I hate gluing! But it seems to work.

Anyway, good luck with the kiln