Aluminum frame for kiln?

[Terry Gallentine]

I am building a bell kiln and I am considering using aluminum tubing for the frame construction with aluminum sheets for the sheathing. I will be using some softbrick for element holders but mostly the kiln will be ceramic fiber with mineral wool backing. The aluminum would be good because I want to keep the weight down. This kiln will have a firing chamber 54"x54" with a 12" ceiling height. I have built a smaller kiln using aluminum and didn't seem to have any problems but I thought that I would see what you folks thought about it.

[Bert Weiss]

Aluminum has a melting point of 1221ºF. That means that it softens and weakens at some temp beneath that. One site I found says it loses 50% of it's strength at 1000ºF. Keep it cool, man.

Simple question: What is gained by making the bell lighter? You will still need mechanical help to raise and lower it. How much does it matter if you have a 300 pound hoist or an 800 pound hoist? I worry more about structural stability than weight.

[Bert Weiss]

I realized the #1 reason I wouldn't use aluminum. My design for a framework is all about having steel at the points where I need to weld on the studs that hold the insulation in place. You can weld stainless to mild steel, but I'm pretty sure you can't weld stainless to aluminum.

[Terry Gallentine]

Thanks Bert,
I will be lining the kiln with 2" of fiber frax and 4" of mineral wool so the cold face temp should pretty minimal and far below anything that will have an effect on the aluminum. What I have done in the past with blanket anchors and element holders is to use 304 stainless welding rod to hook internal features and then travel to the exterior of the kiln where it is held taught with a compression nut. It worked pretty well on a kiln that I built twenty years ago. It also allowed me to remove element holder modules easily. That kiln was constructed with steel and had cement board sheathing and weighed five or six hundred pounds (it had a firing chamber that was 5' by 9' by 18" tall). I just want to make it lighter so that it can be moved more easily and will not put as much strain on the electric winch.
What I was wondering about was whether anybody had any idea of what the vapors that result from firing would do over time to the aluminum. I assume that it will oxidize somewhat but how much?

[Bert Weiss]

Thanks Bert,
I will be lining the kiln with 2" of fiber frax and 4" of mineral wool so the cold face temp should pretty minimal and far below anything that will have an effect on the aluminum. What I have done in the past with blanket anchors and element holders is to use 304 stainless welding rod to hook internal features and then travel to the exterior of the kiln where it is held taught with a compression nut. It worked pretty well on a kiln that I built twenty years ago. It also allowed me to remove element holder modules easily. That kiln was constructed with steel and had cement board sheathing and weighed five or six hundred pounds (it had a firing chamber that was 5' by 9' by 18" tall). I just want to make it lighter so that it can be moved more easily and will not put as much strain on the electric winch.
What I was wondering about was whether anybody had any idea of what the vapors that result from firing would do over time to the aluminum. I assume that it will oxidize somewhat but how much?

There are 2 places that tend to get hot, the bottom of the bell, and the place where the elements exit the kiln. I have no clue what would happen at these points. I cover the outer surface of my bells with galvanized steel. I suspect this would be problematic contacting aluminum.

Mineral wool is a good insulator, better than the frax. However, I suggest you look at calcium silicate board as a backup insulation. It is more stable than the mineral wool.

I would still be evaluating the cost of a more powerful hoist verses using aluminum to lighten the load. You can also rig the hoist with 2 cables, instead of one, which both adds stability and cuts the load in half. Northern Tool sells a variety of hoists.

[Terry Gallentine]

Thanks again Bert,
I appreciate the information about the backer board but I have quite a few of the mineral wool panels in reserve waiting to be used. I am pretty familiar with using dissimilar metals and problems that they present and I think that I can isolate the aluminum well enough. In the past I have used ceramic tubes to shield the elements both thermally and electoconductively from the frame and sheathing materials.
Again, what I am most interested in is the question about the possible effects of the kiln vapors on the exterior aluminum.
I have worked with very heavy equipment in the past and have almost been killed by the same. Lighter is safer at least for me.

[Bert Weiss]

Thanks again Bert,
I appreciate the information about the backer board but I have quite a few of the mineral wool panels in reserve waiting to be used. I am pretty familiar with using dissimilar metals and problems that they present and I think that I can isolate the aluminum well enough. In the past I have used ceramic tubes to shield the elements both thermally and electoconductively from the frame and sheathing materials.
Again, what I am most interested in is the question about the possible effects of the kiln vapors on the exterior aluminum.
I have worked with very heavy equipment in the past and have almost been killed by the same. Lighter is safer at least for me.

I always include chains in my design that hold the weight of the bell when lifted. The cables are used only for raising and lowering.

Keep us informed with your design adventures. There are always numerous good approaches.