Building An Electric Kiln For Pottery

Question

I intend to build an electric kiln for pottery, clay ( an other materials ) sculptures. The internal dimensions would be 40cm(height) x 40cm(width) x 40cm(deep). Therefore, its internal surface is 9600 cm2. I need a maximum temperature of 1300 Celsius.

1) I would like you to confirm or correct this information. I was informed that in a typical kiln the calculation of the total Electric Power necessary is given by this formula P ( Watts ) = 0.6 X Total Internal Area ( cm2). If that is true, then I would need 5760 Watts. However, I found in the internet values that are different from 0.6. So, if you are an expert please give me your informed opinion on that.

2) In order the temperature inside de kiln could be as much homogeneous as possible, its internal walls should be filled with as much heating element wires as possible. But there is a trade-off to be made. The more heating wires the bigger will be the resistance and the weaker will be the production of heat.Furthermore it may unnecessarily increase the cost of heating elements. Additionally, there will be the need to carve too many grooves on the refractory bricks to fit the elements inside which will weaken and possibly fracture them. By looking at the pictures of many different kilns it seems that the optimum distance between two parallel coiled wires is about 7cm ( axis to axis ). But if the grooves should be carved directly on the bricks and in the middle of their height then the distance between parallel grooves would be equal to their height or a multiple number of it. Would you agree with my reasoning or do you have a different one ?

3) It seems to me that 3 resistors ( heating elements ) electrically connected in parallel is a good circuit because they would use only three relays and probably could be controlled by an electronic controller. What is you informed opinion on that ?

4) Another important aspect of the kiln design is the coiled wire ( heating element ) diameter. The bigger its diameter the bigger will be its durability. However bigger diameter reduces the resistance of the heating element and produces lesser heat. I intuitively think that a diameter of 1.30 mm would be OK. ( Of course, later on, I should also consider the coil diameter and the distance between its turns ). Do you have a better informed proposition ?

( later on I may need further advice on the same project )

Answer 1

I have been repairing kilns since the 1970s, and would like to point out that it is very important, for homogeneous heating, that you use grooved firebrick all around, with coiled Nichrome heating elements in the grooves.

Typically, the kilns I work on have a multi-position switch to control the heating. For a small kiln you would want 4 elements used in pairs. Relays are great but with the right switch you won't need them. You can settle for two switch positions - one that turns on every other element, and the other where it connects all the elements.

The top and bottom should be a single piece of firebrick if possible, or just cobble several together, wrapped with a steel strap. If you want the top to open you will need a metal hinge (so the top part is grounded) and a handle. It is OK to not cover the top of the firebricks, but remember that they are like limestone and if you start putting things on top of it they will wear down and ultimately break.

If you want a door in the front then you will need it to have firebrick on the inside of it, aand you can even drill a peephole - with a swiveling metal cover for when you're not looking. The door will not have elements in it so will cause uneven heating.

The entire assembly should be tightly wrapped in shaped sheet metal so no heat escapes between the bricks. When in use, the sheet metal will get hot, so you will want it up on legs. Maybe on a cement floor, also.

This is not a "my first project" kind of thing to build. And if you can't afford the electricity bill, then don't bother. I have worked on many that have required the electric company to come out and put in a 100 Amp circuit just for the kiln. And these are medium-size personal kilns, not the large walk-ins (which mostly work on natural gas).

Your biggest worry will be grounding. You need to make absolutely certain that the entire outside is well grounded. These things can be lethal.

Your next worry will be the amount of power you will need. And that determines the size of your supply wiring and your power supply. It will be a lot more than you imagine. But as long as you buy professional heating elements the manufacturer will tell you all you need to know.

You will learn a lot more by reading catalogs of these things than by trying to do a bunch of math. Forget the math and just read the specs. They have done all the math for you.

There is a lot of literature online about how they are built, and I suggest you do it as professionally as you can. Don't just throw some firebricks into a microwave oven and use it for heating. Not only will it cause uneven heating but the hot spots can do strange things like melt metal that flows down between bricks and becomes a permanent piece of the oven. From then on it also acts like an antenna, attracting microwaves.

Building kilns is a lot of fun. And the elements have predictable resistance and are made for whatever voltage you are using. Buy professional elements, in other words.

Once you have all that, the only real trick in using kilns is timing. Remember that power multiplied by time equals total power delivered. With only two settings for heat level you will have to experiment with your timing.

And remember that even when you shut it off, the pieces inside will be H-O-T for a while. You'll want a good set of tongs with a decent reach. Not the kind you use in the kitchen, either.

And finally, once you have used your elements the first time, never touch them or they will break.

When they do, that's when you would have called me. I use an ohmmeter and go through the switch positions to determine which element is out, unless it is obvious. But when I was active my goal was to restore kilns back to their factory specs, which means that sometimes even unbroken elements must be removed when one breaks, so the resistance is the same for that pair. Their resistance changes with age.

Have fun!