"Disposable" ground foundry

[Joey K]

I’m building a small ground foundry; okay “building” isn’t the right word. I’m only planning a one time use deal and later I will build something more permanent. The idea is to dig a hole in the ground using post hole diggers then run a pipe into it. The hole will just be filled with charcoal with my crucible in the middle. Is there something wrong with a build like this? Again, this will be a one time use “learning experience” for me.

 

I can only assume on the factors that will affect a build like this. Dirt density, the presence of moisture, nearby rocks, dirt getting in the charcoal or in the crucible. I may also need air supply being directed at both sides not just one. Thank you for any help.

[Jeroen Zuiderwijk]

I’m building a small ground foundry; okay “building” isn’t the right word. I’m only planning a one time use deal and later I will build something more permanent. The idea is to dig a hole in the ground using post hole diggers then run a pipe into it. The hole will just be filled with charcoal with my crucible in the middle. Is there something wrong with a build like this? Again, this will be a one time use “learning experience” for me.

 

I can only assume on the factors that will affect a build like this. Dirt density, the presence of moisture, nearby rocks, dirt getting in the charcoal or in the crucible. I may also need air supply being directed at both sides not just one. Thank you for any help.

If the ground is sand based, you'll need to line it with clay (mixed with organic fibers). Otherwise the pit will collaps as the fire dries out the sand. If the ground is clay based, then a hole is indeed all you need. Mind that the furnace design depends entirely on the way air has to move through the charcoal, the design of the crucible, how charcoal is added and how the crucible is extracted. Also keep in mind that the hottest spot in the fire is generally three fingers of charcoal away from the air inlet. Further away the charcoal is colder due to oxygen starvation, and closer to the air inlet you blow cold air against the crucible. For some more ideas on very simple furnaces, check these:

 

http://1501bc.com/metalworking/casting/casting.html

Edited December 7, 2009 by Jeroen Zuiderwijk

[Cylvre]

Joreon makes good points there, especially about the hole potentially collapsing in sandy soil. Depending on what you're trying to melt down simplicity is a good first route. My first aluminum casting experience was with an old stainless stockpot with a hole cut into the bottom edge buried in the ground using a shop vac for an air supply and a small cast iron fry pan for a crucible, worked well enough that we could barely make the pour because the cast iron heated to a bright orange and deformed as we lifted it out of the "forge." The best advice I can give, especially for a 'one off' melt, is not to overthink it. Be safe, but don't spend too much time on something you'll never use again. Most importantly have FUN (and take pictures, we like pictures.)

[Joey K]

Thank you all for the advice.

 

The dirt in my backyard is pretty hard, I don't think its clay but I also don't believe I need to worry about collapsing. Regardless, I will keep an eye on that. If the build is simply a hole in the ground, where should the air supply lead? Perhaps the very bottom?

Edited December 7, 2009 by Joey K

[Cylvre]

Thank you all for the advice.

 

The dirt in my backyard is pretty hard, I don't think its clay but I also don't believe I need to worry about collapsing. Regardless, I will keep an eye on that. If the build is simply a hole in the ground, where should the air supply lead? Perhaps the very bottom?

 

Directly on the bottom may cause problems with clogging from ash or charcoal. When I used this method the air supply was completely tangential (90 degrees) to the wall and pointing directly to the center and at the very bottom of the pot wall (pic A). If I were to do it again I would probably move the tuyure entrance up the wall slightly and then angle it downwards (pic . Also, to even the heat out even more, I would split the air supply and add another tuyure on the opposite side of the chamber (pic C).

[Joey K]

For my own clarification, I added a crucible and a line denoting the level of charcoal. The crucible probably sitting on a bed of coals and everything below the red line is filled with charcoal. Does this look alright? I know you suggested not over thinking something but for me over thinking is like breathing.

[Joey K]

That diagram helps a lot, thank you. For my own clarification, I added a crucible and a line denoting the level of charcoal. The crucible probably sitting on a bed of coals and everything below the red line is filled with charcoal. Does this look alright? I know you suggested not over thinking something but for me over thinking is like breathing.

 

 

Also maybe I can line the hole with a flower pot to prevent any collapsing.

 

 

EDIT: So sorry about the double post, I reported it already.

Edited December 8, 2009 by Joey K

[Cylvre]

Looks pretty good. You'll want the crucible to sit a little higher, just so you don't have any air blasting onto the crucible itself (and thus cooling it) but instead blowing down into the bed of coals just underneath.

[Jeroen Zuiderwijk]

Looks pretty good. You'll want the crucible to sit a little higher, just so you don't have any air blasting onto the crucible itself (and thus cooling it) but instead blowing down into the bed of coals just underneath.

Actually, the crucible needs to be completely buried, and you have to make sure the charcoal gets hot enough on top. The heat escapes much more rapidly through the top then it enters through the side. I've once tested this with a graphite crucible. I put the empty crucible in charcoal up to the rim, and blazed the charcoal around it to white hot. The inside of the crucible did not get more then faint red. With a lid on top, charcoal over it and making sure the white hot charcoal covers the lid, I very easily melt copper (750gram in 12 minutes f.e.). You can take as a guide that if the melt of a single charge takes more then 30 minutes, something's wrong either with the design of the furnace or the way you're operating it. If it takes 20-30 minutes, it's ok (particularly the first charge), but can take some finetuning.

 

N.b. about the placement of the tuyerre, if you use it like that, I would place it at the side of the crucible, otherwise you don't get the heat on top of the crucible. If you put charcoal below as suggested, it will burn away there within a few minutes, and the crucible will sit in the same position anyway. Just make sure there's about 8-10 cm of charcoal between the crucible and tuyerre, and keep filling up that area, as the charcoal burns away quickly (creating a hole that will guide the cold air straight to the crucible). Now also bare in mind that this system only works for small crucibles. If you have a bigger crucible, you have to get the air all around it, or you loose heat through the side that's not heated. So for melting say 0.5-3kg of bronze, it's much more efficient if you use a double floor, with the top floor having holes to lead the air all around the crucible (generally about 3-4cm away from the crucible bottom). This works very efficiently if done right. Over 3kg, this system doesn't work well anymore, as the heat doesn't go high enough into the fire to reach around the top of such large crucibles.

[Joey K]

Thank you all for the information! On a small note, after looking on the internet for some info about the infamous "flowerpot furnace" how does one drill a hole in a flowerpot?

Edited December 11, 2009 by Joey K

[Cylvre]

Thank you all for the information! On a small note, after looking on the internet for some info about the infamous "flowerpot furnace" how does one drill a hole in a flowerpot?

 

Carbide spade-style bit, made for ceramic or glass, or a diamond plug cutter.

[Joey K]

So I just need a bit and just go to town on it? No special preparation or procedure? Some websites say to let the pot soak in water for about an hour. But that website was talking about drilling a hole in the bottom, Ill need holes in the sides.

 

PS: This cursed weather is hindering my project, Mother Nature and I have never seen eye-to-eye.

[Cylvre]

It does help to lubricate the cut to pour water on the place where you are cutting with ceramics and glass but I've never cut on terra cotta personally so i don't know about soaking the whole thing. Just make sure to go nice and slow and let the bit do the cutting, too much pressure can cause chipping, binding, and catastrophic destruction.

 

Good luck!!

[Joey K]

Should be a fun exercise, like disarming a bomb. >_<

 

Thanks for the information, I appreciate it really.