Been thinking about it for years...

Thanks for the link, Jeff. Your build is pretty much what I'm aiming for, guess I'll just walk in your footsteps.
FishbonzWV: Iron will have to wait a while, I'm happy if I can melt alu and bronze in this furnace.

 

I used my eight year old furnace every day this week, four moulds a day because that uses all my sand.
If the weather holds and the grandkids don't show up, I'll be back at it next week.

 

Not being contentious, I just don't know. Is your furnace unaltered from your first build? Most of us have to try a second or third time to get something that suits us. My brick furnace was my second build, my first being loose bricks.

 

I beg your pardon..... Kaowool, Mizzou and an oil burner right out the gate. Monkey see, Monkey do, this isn't rocket science. Find someone's rig you like and do what they did.
Screw reinventing the wheel. I wanna cast metal, not build furnaces.

 

Lucky for me I like doing both... All 3 of mine still work great FWIW.

Jeff

 

Yep, all original.
The only thing I've done is add a blower to the burner.
I did build an A10 sized furnace the same way but I like my propane tank A6 better. It's my go to furnace.

 

I'm glad to stand corrected guys.

Jason has the best advice, look around and when you see what you like, copy it. Later if you think you can make an improvement go for it. Most people, once they get to melting, don't want to spend time modifying a furnace if it's working relatively well. I know mine works good enough I don't want to spend time trying to improve it, especially with free fuel.

 

Sigh. The math keeps ending up with slightly more than 1 bag of refractory. I'd like to reduce that to say 3/4 bag to be on the safe side. No point in buying an extra bag for a few pounds of clay, don't know how the shelf life is.
So I need to decide on some dimensions. For aluminium I can use steel crucibles, but I expect I need proper crucibles for brass and beyond. So I should probably size everything around those. How much clearance should I go for? With a 8" ID I would probably have room for an A8 crucible, right? What about height above the crucible?

 

A case for the second bag: You'll need plinths and a lid. I have several plinths of varying heights and I also have a 2"x6"diameter refractory disk that I set my hot crucible on outside the furnace before lifting to pour. Casting the small amounts of refractory will also give you a feel for how the refractory behaves before you mix the big batch and get surprised by it. The plinth in the furnace has to be tall enough to keep the bottom of the crucible just above the tuyere so it's taking the brunt of the punishment from the burner, not the crucible. I try to keep the space between the crucible and lid around 2".
I'll assume for now that you're looking at Jeffs lid design. It's been successful. It's important not to skimp there. Mine is poured the full thickness around the outside for about 3" then drops to 1-1/2" thick to accommodate a layer of kaowool above, and then covered by sheet steel. It's suspended only by the annular depression and handle holes of the keg lid. I have a 4" exhaust hole.

It's developed a single crack visible on the bottom surface that goes the full diameter. I'll just have to watch it for further degradation and take action as needed.
Charcoal and oil need 2" at a minimum from the furnace wall, 3 is better, but you can get away with less using propane, but you need space for your lifting tool to fit without scraping the walls too much.

Pete

 

I like that lifter, very elegant. I was always aiming for some sort of "side swing" solution, I can only imagine the amount of heat coming off a hinged lid.
Having an extra plinth and something to put the crucibles on is a good point. But even with those I should be able to keep this build to one bag by reducing the height a bit. It's not the end of the world or anything, but it's so wasteful to let an almost full bag go bad...

Here's the tank, I just got done filling it with water so I can cut it.

 

I have a hinged lid and I stand right in front of it. It's simpler to build so that's why I went with it. It may direct the heat at you, but the blower is still going then so it goes pretty much straight up.

The width of a furnace is more important than excess height. Mine is too tall by 4-5 inches. You only need 2inches MAX above the top edge of the crucible. Keep in mind, the bottom edge of the tuyere should sit about 1/2" above the floor. (in case of crucible failures, you don't want metal flowing out your burner) The top edge of the tuyere should be BELOW the top of the plinth. Keep the flame from directly hitting the plinth. Petee suggests making several different sized plinths while you are pouring mud. This is so you can adjust the height of the crucible for this very reason. No direct flame impingement. It drastically shortens crucible life. Don't worry about refractory going bad. If you keep it dry, the stuff lasts for years. That date is for guys that have to deal with govt oversight and insurance companies. You're in your backyard and won't get visited by the osha pricks.

You buy a crucible yet? I HIGHLY suggest you get something like an A6 from fleabay and have that on hand NOW. This will greatly assist you when building. They really are pretty cheap and last for years if you follow some simple tips. If I had done this, my furnace would have been a hell of a lot shorter. Live and learn Smart man learns from others!

 

I haven't bought any crucible yet, but the salamander dimensions are easy to find. Turns out I can shorten the tank by at least 6", that's good to know before I start casting the refractory.

The hinged lid has a major advantage, it doesn't move the center of gravity much. A swing lid might require wider legs. But I kinda like it anyways. Here's my plan:

 

My furnace weighs a ton. Not a chance in hell it's gonna tip over. When building, I was careful to limit it's travel on the hinge or it could get tippy I suppose.. Anyway, that's a bag and a half of Mizzou. The rest of the 2nd bag went for my forge and a few plinths and a ribbon burner. I get the stuff pretty cheap. Current price is 72bucks a bag and no shipping charge. I pick it up down in Austin.

 

Heavy is good if you can handle it. My back isn't what it newer was, so I wanna keep it as light as possible. That's part of the reason for keeping this build to one bag. The ground where it will be used is pretty rough, so your dolly wouldn't work. And an "all-terrain" build would increase cost, size and complexity, I'd rather keep it simple and "haulable".

 

That's why I recommended building with wool and a coating.
My furnace weighs about 13 kilo's, I pick it up and put it away.
Forget the charcoal, forget the oil burner. Build a propane burner, it's all you need.
What Jeff didn't mention is his oil burner furnace is covered in cobwebs since he built his wool/coated furnace.
Kelly ripped out his castable insert and replaced it with wool and a light weight hot face.
What are your plans for the lid insulation? See where I'm going?
There's no forms to build, place the wool, paint the coating on, you're done.
Make life easy. I guaranty you'll like it.
Edit: Just keep it a lift off lid, keeps the weight down and simplifies the build.

 

So far I haven't found kaowool or a decent coating, that's why I was thinking of making a lightweight castable leca filler and then coat that with refractory. But as others have pointed out it won't come with any guarantee.

 

Here's my

The nay say-ers have never used one.
Castable furnaces are old school. Step into the 21st century.

Here's my larger A10 furnace in action

 

I figured I'd raise a few hackles with that one.

 

My big furnace has I think it was 3 bags in it, one in the floor, one in the hotface, one in the lid (Give or take... Actually less in the lid and floor and more in the hot face IIRC). I did have to crack open a 4th bag but hardly had to use any of it. The cart it's on is really heavy as a result but with lawnmower wheels on it I'm able to move it across the yard when needed. Steering it and getting it to bump up over the threshhold into the casting shed is a real PitA, I have to lift the front of the cart and turn or pull it up, then go push from the other end again. I definitely built it with having room to grow into fully in mind.

So it is way oversized for anything I have done yet in the 5 uears since I built it. It has a 12" bore that is also probably too tall. My "big" crucible is a #12 and it looks kind of tiny in there and sits too low for heating it up quickly IMO. My small, lightweight furnace also has room for that crucible if I'm running propane so that is what I find myself using when I only need to melt one pot full (nearly always the case for me).

Jeff