Making a test kiln

[Win G.]

I am still using an IFB test kiln that I made 50 years ago from a plan from Alfred University. the interior is 8x8 an it is made from 14 IFBs with 22 gauge Kanthal wire. It fires on household current. It draws 14 amps or 5840 watts. It fires to cone 6 in about 3 hours. I would like to make another one using fiberboard instead of bricks. How thick would the fiberboard need to be? Any other ideas are welcome. TIA. 

[neilestrick]

Your best bet is to talk to a refractories supplier. They'll be able to recommend the best product for your application. There are a lot of different types of insulating fiberboard out there. Regardless of the type, though, how do you plan to mount the elements to the inside walls of the kiln? IFB will probably be the cheapest, easiest and most durable way to build it, and it's clearly insulated well enough if you get to cone 6 in 3 hours. Why change the design?

14 amps x 120 volts = 1680 watts. 

[Kelly in AK]

I have a small gas kiln (8 cu. ft) made from fiber board. Two layers of 1” board. Has no problem getting to cone 6. It was cheap and easy to build but I won’t  build another. I agree with the points @neilestrick made and have a couple more. Fiber board has its strengths, being rigid and super insulating, but it has some drawbacks. One is that it will shrink. Once you fire your kiln (unless you pre-fire the boards) you’ll have to re-fit it because big gaps will form. The other is that it’s really friable. As bad as fiber blanket, in my opinion. Even though it’s rigid it’s not strong. Every time you lift the lid or open the door, you’ll be abrading it. The wear and tear is one thing, all the respirable silica it releases is another. 

[Bill Kielb]

3 hours ago, Win G. said:

. It draws 14 amps or 5840 watts. It fires to cone 6 in about 3 hours. I would like to make another one using fiberboard instead of bricks. How thick would the fiberboard need to be? Any other ideas are welcome. TIA. 

Definitely agree with all above but the simple answer is fiber or even rigid fiber generally perform better per inch than IFB. So to replicate or exceed what you have in fiber you need as much or more insulating value per inch as your IFB. Generally even rigid fiber thermally performs better and has less mass so less energy heating the bricks. 

Since fiber is not very structural, supporting the elements tends to be a functional issue and fabrication becomes a challenge. Just to mention, you may have a typo in the wattage posted.

The size must be fairly small to fire quickly. One method is to build it with fire brick and wrap it with rigid high temp fiber to reduce the losses further.. Done correctly, you should be able to increase its size but reduce losses enough for it to remain a low power consumer.

some material comparison below, comparing the conductivity per inch should give you an idea how thick to make it. Notice at the top temperatures, fiber begins to behave more like IFB.

Edited October 3, 2023 by Bill Kielb

[Win G.]

Thank you all. So many great insights will definitely help me make a decision.

[Win G.]

Yes, there was a typo in the wattage. Thanks for pointing that out.

 

[neilestrick]

There are a couple different types of silica board- high density and low density. I've used the low density stuff, and it is incredibly soft and not durable. I have not used the high density, but I imagine for it to be as durable as IFB it would not be much better of an insulator.