Been thinking about it for years...

Greetings everybody. After thinking about it for years I might actually get around to building a simple furnace this time around. I have a small machine shop (manual lathe & mill), and getting materials for larger pieces have always been a challenge. Casting would increase my options a lot. While iron is my real goal I figure the best starting point would be a charcoal-fired furnace for casting aluminum and perhaps brass/bronze.

My main problem is the general lack of useful materials. Here in little Norway the "pre-mixed mafia" has taken over completely, so most of the really useful raw ingredients simply aren't available or just too expensive for now. Locally the best I can get is 1000°C+ pre-mixed mortar and leca. I also have some homemade green mullite clay I made by ball milling alumina and quartz a few years ago...

 

Hi Fulmen, That's the best idea, start simple, learn from your mistakes and make improvements from the knowledge gained. Isn't mullite what those older European crucibles were made from?, it sounds like it would do the job nicely....certainly much better than the YouTube plaster of Paris furnaces that fall apart after one use. I tried the fireclay/perlite/cement/silica sand recipe from the Internet at first but soon switched to a commercially available calcium aluminate furnace concrete as it's pretty hard to beat and can't easily be made unless you have access to a cement roasting furnace.

 

I have found CA cement online, but it's expensive even before shipping. So I'll try the fireproof mortar first. Mullite is basically an alumina silicate, so chemically it shouldn't be too different from say kaolin. Depending on the formulation the MP could be as high as 1800°C. I made it from some waste alumina absorbent and quartz rock ball milled together, so it's probably not as fine as proper clay. I did use it as a hot face in a butane fired coffee can forge, and it didn't even start to sinter at those temperatures. But that shouldn't be a major issue, even though it's wear resistance will be low. Perhaps it could be sprayed with a flux to drop the MP slightly?

 

The trouble with fluxes is they keep working every time you fire up the furnace again and tend to be corrosive: My furnace has melted iron oxide all over the inside which makes for a nice pottery glaze but erodes the insides every time it melts again: I think it's acidic. CA based concretes sell here for AUD$50 per 20 kilogram bag but it's a mechanically strong material with about 4 times the compressive strength of normal concrete, so if you're making a power plant furnace it's ideal for large arches and volumes. For the home user it's better to use it for a thin chamber wall and have mineral wool insulation behind it to get the best of both worlds. I think I paid about $150 for two bags delivered to my door and consider it good value for what it is. I bought it from a company that was advertising excess stock for pizza ovens, the same company would have sent it 1800 Kms to my city for free if I'd ordered a week earlier as a nearby sugar cane mill had bought a big order to re-line their furnace: you may find that a bakery or a crematorium or any place that has a steam generator of some kind has a few left over bags going for cheap.

For experimentation with charcoal, a common pottery clay should work fine: it will crack but the pieces will stay in place similar to how bricks in an arch are retained. It will let you experiment cheaply and gain information for any later designs.

 

Best deal I have found so far on CA cement was appr 3x of that, plus shipping. I might buy that for a future project, but for my first build I think I'll go for whatever I can get locally.
As for fluxing I agree it has it's risks. The idea was to use it sparingly on a material with a MP far above it's operating temp. As the base material starts to sinter it should in theory dilute the flux until the MP of the mix is higher than the firing temperature. But I think that's best left for future experiments.

 

Perlite is used in a lot of DIY refractory recipes but it melts and forms a foamy green glass at much above molten aluminium temperatures so Tobho's recommendation is a good one.

 

Welcome
I've never tried it so take this for what its worth- a large clay chimney flue liner might work for charcoal enclosed with sand for a heatsink. I've tried it in a thin steel cylinder on it's own and it worked, but having it surrounded by sand worked better.
I take you at your word about availability of materials but a brief search of foundry suppliers in Norway brings up a pretty sizable list. Of course I cant read any of it! lol.

Pete

 

The main problem is that most suppliers are whole-sale only. But I do have a few more leads to chase down. And either way I should be able to make a usable furnace using leca, mortar and rockwool, right? I also have a bit of vermiculite somewhere.

 

Well, first things first, I need to settle on a size. Is there a minimum recommended size to start with? Not all things scale well, and the risers and runners require a lot of metal as well.
I'd rather not build a monster on my first try, but a setup that's too small is fairly pointless...

 

here's my plan so far. The bucket is 15" tall and 14" at the top. The refractory is 3" thick, and the crucible is 4x8" (1,5l capacity).

 

I made my first (charcoal) furnace in a steel 5 gallon pail. 2" refractory, straight bore. The vee shape you illustrated will not be helpful and the plinth is unneeded. You need to have a bed of burning coals under your crucible and completely surrounding it all the way up to the top of the crucible. At least 2 or three inches all around. Air hole (tuyere pronounced tweer) no more than an inch off the bottom. A method that a couple of us have found very helpful is to make abase out of expanded steel supported by three legs that sits in the bottom of the furnace so that the grate is sitting right above where your air hole comes in. This allows the air to circulate and distribute much more evenly into the charcoal from below and helps to prevent hotspots in your coal bed. A cast iron cover used for floor drains supported by 3" bolts would work. It'll get burned up after awhile though. It takes a surprisingly small amount of air to run this furnace, but it takes a lot of charcoal. You want more air volume than pressure, so a hair dryer is enough. Too much air or pressure will act like a cutting torch and burn up anything ferrous, including a steel crucible.
Your charcoal furnace doesn't even need to be round. Gas and oil furnaces are generally round to promote combustion and heat circulation which are not as important with a charcoal design, but round seems more convenient and probably uses less charcoal. A neat stack of YTong bricks would do.

Pete

 

I agree with you regarding the shape, problem is it's the only suitable metal bucket I've been able to find. Seriously, there is nothing between a regular 10l bucket and a 55gallon drum to find anywhere. It's driving me insane.
And I do want to make it suitable for an oil burner, charcoal is just a simple way to get started.

 

Speak of the devil... Took an extra peek in the dustiest parts at work, and found an old 12x20" propane tank. So I made an executive decision that we shouldn't store pressurized gas at the premises

This looks a bit better, eh? It's drawn with 2" walls and a 6" crucible. I could go up to 3" walls, but that would limit it to 4" crucibles.


Edit: So the sketch indicates 20liters of refractory. if I remember correctly spheres has something like a 60% fill rate, so even with leca I still need at least 8l of mortar. Perhaps I could cut it further with vermiculite. That mortar isn't free either, and it should insulate better.
Don't know if I can get it liquid enough for chicken wire reinforcement either. But with that solid steel shell it might not be necessary.

 

As you are in the designing stage... Keep in mind, Flames should NEVER hit the crucible directly. We call that direct flame impingement. It will shorten the life of any crucible, steel, clay graphite or silicon carbide. The goal is to heat the environment around the crucible without heating it directly.

It looks like in your drawings the crucible is overhanging off the plinth. Only reason I point this out. I am not a charcoal guy. If I was going to try a charcoal rig, I wouldn't spend money on expensive commercial refractory cement. I'd just dig a hole in the ground and feed an air pipe to it. This is how it was done for thousands of years and is still done this way today. After you learn charcoal is a pain in the ass, that's when ya spend the bucks and build something more skookum. Charcoal is for BBQ!

Personally, I would think you'd be better off ordering a light weight box of Kaowool and coating it with cheap satanite. All in, I would guess about 300US for both shipped to Norway from the states. It sounds like you have a well outfitted shop so I'm sure you have an air compressor. Burn diesel, it's dirt cheap and use a hair dryer for a blower. This simple setup will take you very far in metal casting. The best part, you'll have enough material to build yourself a few furnaces!

 

I'm sure you're right, but I hate ordering stuff. It always takes longer than planned (especially these days), and I want to do this NOW not later. Me impatient? Nah, what makes you say that?
As for charcoal that's only to get started, this will be run on oil or diesel eventually. I'm currently looking at a propane-powered Babington design. Good point about the plinth though, makes sense.

I'm also considering building it from fire bricks with loose leca or vermiculite (or both) insulation. They're a bit too expensive at the hardware stores, but I might find some at the local "craigslist" for a sensible price.

 

Good quality firebrick would be way better than any homemade brew you could probably make. I get them here for 2bucks a piece. I'm not a fan of fire brick, but ya do what ya gotta do.

 

2 bucks a piece would be nice, I'm looking at 4-6 bucks at the hardware store. But I'm keeping my eyes up for a better deal, at a sensible price it would be a lot less work than casting something myself.