BUILDING YOUR OWN KILN

By James Howell

 

When we decided to feature James on our cover, we thought a story about how he builds his small kilns would be interesting to the readers. He is so well known for his glass points, for which he makes from his own glass slabs. He has learned how to select glass shards, recombine them and get some of the wildest looking slabs you have ever seen. Here are his instructions.

The kiln I build is 4½ inches tall and 10¼ inches wide inside. It works very well for cooking slabs and testing small batches of rock to verify its temperature range, rather than take a gamble on blowing up a whole load in a big kiln. This little kiln only takes 18 bricks to make, and when you finish cutting it out, there isn’t much more than a double hand-full of scraps left. This kiln is 6-sided, so you need to cut the side bricks at a 30 degree angle on a miter saw. Cut exactly on the corners so you leave the brick the full length. Be sure to get the soft firebricks, they are so soft that you can cut these with a regular saw blade. The bricks are 2½ inches thick by 4½ inches wide by 9 inches long. Just be sure to wear a good respirator and safety glasses when cutting the brick.

I begin by cutting the corners on side bricks. Then I cut the grooves in the flat side for the coils to go in, on the table saw set at ½ inch deep. I set the rip fence at 1 inch between the blade and the fence. I run each brick through the saw on edge, then turn the brick around to the other side, run it through the saw and cut it. I do this to all six bricks, then move the fence over the width of the blade and run them all through at this setting. Repeat this step until you have the grooves 7/16 of an inch wide.

Next I lay the bricks out on a piece of cardboard and draw around the outside of them with a pencil to make a pattern for the bottom. I draw in the lines for each brick on the pattern. I then work against the pattern, mark the 1st brick, and then cut it on the miter saw. Then I do this for the rest of the bricks on the pattern, putting a little piece of masking tape on the top of each one and numbering it so I can assemble it in the same place. I make one for the top and one for the bottom.

I use a high heat mortar to glue the bricks together. The name of it is “Sairset” and I get it from Harbison and Walker Refractories in St. Louis, MO. Their phone number is 314-521-3314. The Sairset comes in a 15-pound plastic bucket and costs about $25, shipping and all. I use about half of the bucket to build this whole kiln. If you want a bigger kiln, all you will have to do is make 2 more rings without the grooves in them, for the body. I think that if you put the ring with the coil in it in the middle, it will heat this size of kiln just fine. Three rings in one would give 10 ¼ inches wide by 11½ inches tall, inside measurements.

To assemble it takes a piece of plastic sheeting, because I lay it on a flat worktable to keep the mortar from sticking to it. I use a putty knife to put a very thin coat of this mortar on the edges of the joints and push them together as tight as I can. I assemble the body first. As soon as I get it put together, I wipe any excess mortar off because it looks and sets-up like regular cement mortar. It sticks so well, attempting to clean it off after it has set up will only pull off pieces of the firebrick. Be sure to get any excess wiped out of the grooves for the coil.

Then I assemble the top and bottom the same way. I let it dry overnight, turn them over and check to see if any of the joints on the other side need any mortar in them. I let them sit 24 hours before I put the coils in. I have to cut a groove across one of the firebricks between the 2 grooves for the coils, so the coil can cross over to the other groove. This I do in the middle of one of the bricks, I just cut it in there with my pocketknife. I cut this groove in at a 45-degree angle, drilling one hole in the end of the top groove next to where the coil crosses between the 2 grooves, doing the same at the bottom. I use a ¼ inch drill bit for this hole.

Next I mount the box for the control. I use a metal conduit box that is 4 inches square and 2¼ inches deep, centering this box over the holes for the coil, sticking a pencil through the holes and marking the box where I need to drill the holes. Then I drill the holes in the box out to ½ inch. This is to make sure the coil wires do not touch the box.

Next, using some of the holes in the corner of the box, I screw the box to the kiln. I leave one of the screws loose because I will put the green ground wire on it. Next, I drill a pilot hole in the brick for the screws, like you would in wood, so the screws go in the brick without stripping out.

I make the coil out of .045 stainless Mig. welding wire. The coil I use is 15.1 ohms. I cut a slot in the end of a ¼ inch rod and leave about 6 inches of wire hang out. I put a variable speed drill on the other end of the rod to turn it. It takes two people to wind these coils, one person to run the drill and one to feed the wire on the rod. When you wind the coil, keep the wire tight and the coils tight against each other. I measure from the inside end of the slot up the rod to 37¾ inches and mark the rod; this should be 15.1 ohms using .045 wire. If you want a different ohm, make it longer for more ohms or shorter for less.

I leave about 6 inches of wire on this end too, laying the coil around the kiln and seeing how much I need to stretch it out to fit in the kiln. You need to stretch it out longer than the distance around the kiln so when you put it in, you have it push the coils together. This creates enough pressure that they won’t stick out into the kiln when they get hot. You can also push little nails into the bricks up against the coils to hold them in, like the factory does.

I mount the control on the side of the box, being sure to center it both directions. I run the coil wire directly to the switch on my kilns. I use a blue spade connector, taking the insulation off from it and squeezing the end the wire goes in just a little bit for the coil wire. I bend about ¼ inch of the end of the coil wire back against itself tight, then slide the wire in the connector and crimp real tight.

I put a light on mine, which comes on when the control kicks on. This tells me that the control is working. Wire the control up like you see in the photo. I use a #10 gauge 3-wire cord. Also to protect the firebrick, I make a basket for the inside of the kiln out of expanded metal. I put a solid metal bottom in mine to protect the bottom too, because the firebrick is so soft that the rock will chew it all up. You will have to check the temperature on different numbers on the switch, because every coil and switch can heat at a little different temperature. I hope this helps you.

If you have any questions, my e-mail address is jhowell@coxinternet.com. Be sure to put “kiln” in for subject so I will know it is not junk mail.

Fig. 1. The kiln with control box and basket.   Fig. 2.  The inside of the kiln with the coil in place.

Fig. 3. The inside of the control box.