I'm planning to build what they call a masonry heater or Finnish heater. A problem is the cast iron doors required cost $800 and up. I'm an OK welder and I have plenty of scrap plate around. I'm trying to find out if it would be a good idea to fabricate my own doors out of plate.
My concern right now is the finish: First question, is the flat black appearance of cast iron a black iron oxide or something else?
There are apparently more than a few phophatizing/bluing/parkerizing methods available but the few I've seen cant take 750F heat. My only other criteria is that I want a flat brown or black or dark gray color. What finsh or treatment (if any) is available to the DIYer?
Sure. That's the way I made the door to my central wood furnace.
Depends. Cast iron comes out of the mold a dull grey color. It can be oxidized, painted or use one of several other finishes. The burnt oil finish that develops on cast iron cookware when it is seasoned is quite tough and can be attractive. That finish will develop on non-cast metal too, as anyone who's ever been a restaurant griddle operator can tell you.
If the finish matters a lot then I suggest polishing, perhaps bead blasting if you don't want a shiny finish and then coating it with one of the automotive ceramic header coatings that are now available. These coatings are completely heat-proof within the temperature range that steel can be used. The best coatings are applied by specialists where the coating can be baked. There are several DIY kits, however, that have gotten good reviews.
Something else you can do, something I did on my furnace door. I got tired of the door always being scorching hot. I therefore installed a radiant heat shield on the inside. It consisted of nothing more than a sheet of stainless steel mounted about a half inch off the inside surface of the door. Mounts were hunks of all-thread welded to the door. The metal blocked the radiant heat from the fire and set up a semi-dead air space between the shield and the door metal. Even with a fairly large fire, the door stayed cool enough that it could be momentarily touched.
John
-- John De Armond See my website for my current email address
"Stove polish" or "stove black" works well on cast iron, but rubs off of steel. Thurmalox paint is remarkably durable, at least the stuff I was buying from Dampney 25 years ago was.
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addition to using it on stoves, I painted the bottom of the aluminum pot I use for backpacking as an experiment to see whether a black bottom would improve heat transfer. Though I didn't expect it to last very long, it's still on there after about 15 years of regular use. I'm sure there are other comparable brands, but Thurmalox seems to be widely available.
A bigger issue is getting a tight seal on your door. I went thru several iterations before coming up with a simple design that seals well. I'd be glad to describe it if you're interested.
I made doors for our maple syrup evaporator out of 1/2" mild steel plate. They worked fine. Over the years they got rusty, which is sort of brown. Things get real hot in a sap evaporator and for long periods at a time. Then it sits there, in an unheated building, unused for 10
1/2 months. So you might not have as much rust. But I'd think that the stuff they call "Hi Temp Stove Paint" would work just fine. I put it on my forge hood 15 years ago and it's still in great shape. We also use it on the inside of pierced tin candle carriers and it doesn't get burned off.
Here's a few pics of our emergency stove, the only 12" stove I built. I used it as my only heat for a couple years in a small place. The others I made with the same details ranged in size up to 3 that accepted 48" logs.
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The door panel is made from the piece cut out of the door opening. The flange around the opening is 1/4 x 1 flat bar, fillet welded on the outside corners and seal welded to the stove inside the opening. The door frame is 1-1/4 x 1-1/4 x 1/4 angle. A ceramic rope gasket fits in the gap between the door panel and the frame. You can use cement to stick the gasket to the door, but if you get the groove right the rope is a snug fit and the cement isn't necessary. It's not necessary to seal weld the angle to the door.
Hinge pins are turned from bar stock and placed away from the flange to avoid problems with the door interfering with the flange as the door swings. Weld the hinges up with door in position and the gasket in place.
The damper turns on a bolt welded thru the door. The damper has a piece of round stock tapped to fit the bolt welded to the back. The door and damper are tight enough that it's possible to all but put out the fire.
It's convenient to have a lathe to make the various parts, but with a little ingenuity, not required. Any questions?
I used the lathe to turn a tapered mandrel with a shallow 4 pitch 'thread' and wound SS welding rod around it to make wood stove door handles that don't burn the hand.
Thanks for the thorough reply. The explanation and the pictures make pretty clear. I will have to give a little more thought about mounting something imilar to this to masonry. I will not have much clearance for the hinges. One thing, what to you mean by "seal weld"? Full penetration?
Placing the center of rotation of the hinges well away from the flange makes getting the clearance between the door and the flange less fussy, but you can certainly move the hinges in closer as long as you make sure the door won't hit the flange as it swings.
I was using "seal weld" to mean "only enough to prevent leaks." The object is to keep air out without running a heavy bead that'll warp the flange.
For a door assembly to be imbedded in masonry, I'd think about making the door jamb out of angle or small channel, like a picture frame, rather than plate. That would provide some resistance to twisting and a way to key the door into the masonry.
Let us know what you do - my uncle is building a camp and is planning a Russian fireplace, so I'm interested in what works for you.
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