Kiln wall/floor thickness questions

[AaronRotchadl]

I'm planning to build a ~30 cubic ft. natural gas, downdraft, soda kiln that will be fired to ^3, possibly ^6. I'd like to keep the cost as low as possible, and I'm thinking about doing an inner course of super 2.5" thick duty bricks(I have ~300) and an outer course of IFB. I've been thinking of stacking both courses so that they're 4.5" tall, making for a wall thickness of 5"(half hard/half IFB)

Given that some electric kilns have only 2.5" thick IFB walls, I feel like my plan is more than sufficient, but I wanted to reach out to see before I start building it. 

I'd also be interested to hear if anyone has stepped down the quality of brick as the chimney gets higher? I imagine that it's not necessary to have super duty bricks at the top of a 15' chimney.

Any info greatly appreciated. Thank you!

[Mark C.]

I fire to cone 10 in all my kilns-that said the floors are 7.5 inches thick with a soft brick in the middle of sandwich . If the floor has burners on that level that area can really get hot.

firing to cone 3 for me is an unknown-cone 6 is also a temp I just go thru.

Most of my kilns have 9 inch walls-some with soft brick 4.5 inside and hard outside.

A 5 inch wall is marginal-especially since the hard brick is on the inside.

It can work but the walls will be hotter.

Just keep in mind 30 cubic feet is a lot of HOT area not like a smaller electric so the thermal mass is also larger-hence more insulation

I think you will find thinner walls not a good idea as you are not the 1st to try this-its a lot of work to find out the hard way

The burner areas as well as the flue exits can really get hot (bag walls as well)

Let us know how this turns out after a few fires

In terms of lesser bricks in the chimney the answer is yes. I have just plain fireplace bricks (bought at any lumber yard-they are unregular in shape as well so tghey need mortar more) in my stack starting about 7 feet up (3 feet beyond damper ) they tend to crack more so mortar them together and or use steel supports.

[neilestrick]

If I was going to go to the trouble of building a 30 cubic foot gas kiln, I would build it the standard way, with 9" walls. That way if you ever want to fire hotter, you can without too much more effort. A kiln that size will radiate a ton of heat, so better insulation will be a good thing. Also, thinner walls will mean more energy costs to fire it, so you won't be saving much in the long run. There's also the issue of bricks not being super stable when laid on their sides. It works in electric kilns because they are cut at angles and compressed by a metal jacket. It's a totally different beast. Front loading electric kilns, which are more similar to a gas kiln, are built with the bricks laying in the standard 4.5" wide position, not on their sides. Having built several soda kilns (one just last month), I would not trust 5" walls to remain at all stable in a large, straight walled kiln.

[Mark C.]

I think you will need more than the 300 brick-there must be some near salt lake as the mills south of town had furnaces-still do I think. You can have them trucked to you as well-new or used.

also some for sale on Widbey Island (you can drive there -no ferry) on potters web-hard soft and arch bricks. The midwest must also have bricks-Idaho also has had them. You are in the middle of these areas so getting them will take some travel.