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Electric kiln design - ceramic fibreboard, kanthal wire

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Hi all,

I'm planning on building a hobby kiln for my partner, and am in the process of choosing refractory materials and kanthal wire for the heating element. Was hoping I could get some advice from anybody who knows about these things.

I want the kiln to be light weight and very efficient. I'd like to use ceramic fibre board as an inner layer of insulation (rated to 1400 C / 2550 F), and calcium silicate board as a secondary layer (rated to 1000 C / 1830 F). The ceramic fibre board is very expensive compared to the calcium silicate board, but i can't only use the calcium silicate board as it's only rated to 1000 C / 1830 F. So my idea is to cement the two boards together, with the calcium silicate board on the outside, so that it won't exceed 1000 C / 1830 F. I'm wondering how thick the ceramic fibre board needs to be such that the calcium silicate doesn't reach that temperature. We'll be firing the kiln to about 1280 C / 2340 F.

My other question is about kanthal wire for the heating elements. I'm wondering what gauge of wire I need. The kiln will be about 40 litres / 1.3 cubic feet. I want it to get up to 1280 C / 2340 F. Based on similar designs that I've seen online, it looks like I'd need it run at about 3600 watts. I'm wondering what gauge kanthal wire I should use for a well-insulated kiln that meets these specs.

If anyone has any idea about either of these questions I'd really appreciate your thoughts!

P.S. I'm aware of the health risks of working with fibreboard, and will be using the proper PPE and precautions.

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There was discussion on these forums previously around the grooves for the elements of such a kiln.

Do a search maybe in "equipment use and  repair " forum.

Good luck.

 post this query there too.

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If you're going to use anything other than IFB brick, you'll need to figure out how to attach the elements to the walls.

As for the elements, there are several things to consider- resistance, coil diameter, coil spacing, wire thickness, and watt density. Figuring those out will depend on the size and shape of your kiln, how many times the elements will wrap, etc, so we can't say which wire to use, nor do I know that anyone on this forum as that much knowledge in rolling elements. It would be fastest and easiest to simply buy elements from somewhere like Euclids.com, as they are experts on that sort of thing and can set you up with whatever you need.

As for your insulating board, the manufacturer of the board should be able to give you hot face/cold face numbers based on thickness and interior temperature.

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Thanks Babs and Neil.

For attaching the elements, I was thinking of just cementing strips of fibre board to another sheet of fibre board. I would cut the strips at an angle such that the resulting grooves slant downwards. You probably get the idea but I'll attach a diagram in case it's not obvious. I haven't seen anyone else use this approach, so I'm not sure whether it would hold up, or whether the strips would fall off from the weight. But I thought it could be a good way of avoiding the airborne fibre dust you'd get from carving out grooves.

Neil, I saw your comment in another thread about how it would be hard to replace elements from a fibre kiln. I take it that's because fibre is more fragile/brittle compared to IFB?

I'm in Australia so I don't think Euclids.com would be an option, but I can check if a similar company exists here.

Thanks for the tip on hot face / cold face temperatures. I've got the data sheet for the fibreboard and it doesn't have that information on there, so yes I'll probably get in touch with the manufacturer and ask them.

Has anyone heard of calcium silicate board being used as back up insulation for a hobby kiln? I've seen people use fibre board and fibre blanket as back up insulation, but not calcium silicate. It seems like a good option since it's stronger than fibre blanket, and cheaper than fibre board.

Thanks again!

 

Screenshot 2020-02-14 22.59.52.png

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for what its worth I thought I would toss in some unsolicited opinion and of course just ignore if this seems off target. I could see building and using a kiln for myself. Bought a book on small wood fired kiln designs so I can do just that when pottery is all I do every day and I have the right property. I love projects and I am somewhat an equipment junkie. I wouldn't want to use a kiln built by a family member or friend though because with that comes now a tie to someone I like/love. If I was happy with the end result forever and it always met or exceeded my needs then obviously no problem. Now instead of the kiln being a tool that I can use as long as it serves my purpose and I can toss, sell, upgrade or even give away, whatever, when the need or want moves me, it is something someone else contributed and worked very hard at building this elaborate tool and so it now has some emotional tie that has to drive my decisions about this tool, probably for almost ever (kilns last a long time). Just a thought. I know kilns are very expensive in your country and of course you guys may not have any of the issues I would have in the same situation. Kilns and wheels are the main two pieces of equipment and if those don't measure up for any reason it's got to be fixed. Shelves, counters, drying racks even slab rollers and recirculating sinks can be a little off the mark and still be fine but the kiln and the wheel have to be dead on and as you go along the needs often change and they have to be changed up to something else for your tools to keep up. 

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I've not used calcium silicate board, so I can't say whether or not it will work. I have never seen an insulating board product that I would trust to hold elements like that, though. Will the board hold pins? You would most likely need to hold pins to get the elements to stay in place. I would also not trust mortar to hold the board pieces together like that.

IMO, this is one of those situations where there's a good reason everyone builds things a certain way. In this case, with IFB. It's the best combination of strength, ease of building, durability, and cost. I think using insulating board products as a backup layer is great, but not as a  face layer. There are definitely ways of hanging elements on ceramic rods, but it's an expensive way to go because the rods aren't just something you can make in your studio. There are fiber kilns with the elements imbedded in the fiber, but those elements aren't replaceable- you have to replace the whole fiber module. They're used in glass kilns where elements rarely burn out. Some glass kilns have elements inside quartz glass tubes. You can buy those, but they won't be cheap, and I don't know if they hold up at stoneware temperatures.

If you're willing to spend the time and money testing something new, I say go for it. It's never bad to experiment and discover different ways of doing things. But if this is a one shot kind of deal, where it has to work, then go with a system that's proven to work.

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Thanks for the comments.

There are ceramic kiln manufacturers out there that make good quality fibre kilns, including Woodrow in Australia and Blaauw in the US. Paragon also are starting to use fibre. So I don't think it's impossible to build a good kiln with fibre, but since it's my first kiln, I'm probably better off going with IFB (with calcium silicate backup), for all the reasons mentioned, not least because of cost. I think it would end up costing about twice as much in materials if I go with fibre. I may still use fibre board for the door though, and potentially the ceiling.

Neil, I'm not sure whether pins would be needed to hold the elements in place. Are you familiar with Woodrow's fibre kiln design? They carve grooves out of the fibre board, as you would carve grooves out of IFB (https://keaneceramics.com.au/products/woodrow-hobby-fire-large-kiln). I haven't heard anything about their elements not being replaceable. Why would that be the case? Because the fibre board would crack as you're pulling out the elements?

 

 

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4 hours ago, ojb said:

Thanks for the comments.

There are ceramic kiln manufacturers out there that make good quality fibre kilns, including Woodrow in Australia and Blaauw in the US. Paragon also are starting to use fibre. So I don't think it's impossible to build a good kiln with fibre

You can definitely build a good kiln with fiber, it's just a whole lot more difficult with electric kilns. If you were planning on using gas it wouldn't be an issue at all. It looks like Blaauw uses ceramic rods to support their elements, but also uses IFB to support the ends of the rods. Paragon uses fiber in a lot of their glass kilns, but they embed the elements in the fiber- the element is actually underneath a thin layer of fiber- so you can't remove the element to replace it. In a glass kiln it's not a big issue since the elements last forever. In a ceramic kiln it wouldn't be practical.

4 hours ago, ojb said:

Neil, I'm not sure whether pins would be needed to hold the elements in place. Are you familiar with Woodrow's fibre kiln design? They carve grooves out of the fibre board, as you would carve grooves out of IFB (https://keaneceramics.com.au/products/woodrow-hobby-fire-large-kiln). 

That's a neat design, and probably insulates very well.  It must be a very rigid fiber material to be durable enough to hold up to that.

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I would question the glue that holds the two boards together at 2200 hundred degrees as well. Some sort of fiber cement not a brick mortar .

If you make it please post some photosm and let us know how its holding up?

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On 2/13/2020 at 3:04 AM, ojb said:

My other question is about kanthal wire for the heating elements. I'm wondering what gauge of wire I need. The kiln will be about 40 litres / 1.3 cubic feet. I want it to get up to 1280 C / 2340 F. Based on similar designs that I've seen online, it looks like I'd need it run at about 3600 watts. I'm wondering what gauge kanthal wire I should use for a well-insulated kiln that meets these specs.

We used to wind our own elements, did so for quite a few years. Had a front loading 10 cubic foot kiln that we used a thick gauge wire for. We bought the wire on spools from Euclids. (note if you do make your own and they don't put it on a spool it's a royal pain to untangle it) I would look up the specs for a similar sized commercial kiln and which element(s) it uses and make the kiln size to fit the elements. 

Advantages of winding your own is you can make elements for odd sized kilns or make your own adjustments to the elements. Euclids could do this too though. Disadvantages are it's really not much cheaper to make your own versus buying ready made ones, at least for us it was this way. Gotta get the math right in regards to size of the mandril, coils, wattage etc. Any little kink or scratch on the element will probably shorten it's life, it's not easy winding yourself. We used both the drill method and later on a metal lathe, lathe works way easier than the drill method.

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 17 hours ago, Mark C. said:

If you make it please post some photosm and let us know how its holding up?

I think I'll just go with the plain old IFB bricks after all, at least for the floor and walls.

On 2/15/2020 at 3:12 PM, neilestrick said:

That's a neat design, and probably insulates very well.  It must be a very rigid fiber material to be durable enough to hold up to that.

Yes I really like the design, but I'm not sure I could replicate it and even if I could, it would be out of my budget due to the cost of the fibre board.

17 hours ago, Min said:

I would look up the specs for a similar sized commercial kiln and which element(s) it uses and make the kiln size to fit the elements. 

I've looked up a few similar kilns but so far haven't seen any that specify what gauge wire they use. I'll keep looking.

We don't have Euclids in Australia so I'll have to find another retailer for the wire. I'll get a quote from Kanthal (the company), I suspect they won't be cheap though.

I think I'll be using the drill method, since I don't have a lathe.

Thanks for the tips!

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Quick Google search brought up Northcote Pottery Supplies in Australia which have a 40 litre / 1.5 cubic feet electric kiln. Instead of getting  custom made element(s) you could just use their elements in your kiln with similar specs. If they are coiled too wide for your IFB channels then go with another Australian kiln that uses bricks rather than fibre.

If you do land up winding your own with a drill then make up a little wood piece to run the wire through and hold onto this and the wire to get a smooth feed, its easier for two people to do this smoothly, every time you stop the drill you will get a noticeable "bump" in the winding so you want to avoid this. Also, wear heavy leather gloves as the wire needs to feed slowly and it gets hot to hold while feeding it onto the mandril through the wood piece. 

Edited by Min
added a thought

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