The Shop > Metal Stuff
Foundry Furnace for the Tiny Shop
vtsteam:
Today I cast a base for the new lathe I'm building in the furnace, and after 10 heats in winter, no functional problems with the plaster lining so far, despite the extreme cooling after a melt. There is some cracking, due to expansion and contraction of the lining -- but they seem quite stable. I've noticed that they disappear when the furnace is hot, and reappear when it cools down. Basically expansion joints. And no worse than my old fireclay and sand furnace refractory, which lasted ten years. In fact, better so far -- no patching has been necessary with the plaster furnace.
There are a few small chunks broken off around the top of the lining, but these occurred when the furnace fell over (long after a melt) because the warmth gradually metled a layer of ice under the soil. The furnace gradually leaned over and toppled with the lid on and an empty crucible inside. I was eating dinner at the time. A few linilng dings were the result, but nothing needing repair.
After the melt today, the furnace was hot. Spying on a shelf the old brass POV valve removed from the propane tank I had made the furnace from, I thought maybe I should try to melt that. If the lining is going to fail, that should do it -- temps are a lot higher and a lot longer heats needed for brass.
And curious to see whether my small atmospheric propane torch was man enough to melt brass, and if it did, what kind of brass would I get out of an old valve? Lots of interesting questions to explore! If the lining gave out, it wouldn't owe me anything -- the whole furnace cost me $22, I think, and I'd had a good number of heats already.
So I popped the valve into a small clay lined crucible and turned on the propane at partial throttle and 10 psi.
vtsteam:
And the results after about 20 minutes:
Yes indeed, an atmospheric burner made from 1/2" EMT can melt brass in this furnace. :thumbup:
Nope, the lining showed no change after melting brass. :thumbup:
Yup, the brass looks halfway decent (Vermont expression, meaning "good") :thumbup:
Guess the furnace owes me even less than it did.....
Brass muffin:
The furnace lining, 11 heats incl. 1 brass melt:
awemawson:
Looks like you've hit on a useful formulation there Steve with your Plaster of Paris and sand - certainly seems to hold up well. Realistically it's only the first few mm that take the full heat I suppose. The temperature gradient from 'flame facing' outwards must be quite steep I expect.
vtsteam:
Well I'd been thinking about using plaster for quite some time Andrew, after some references to sprue pouring liners (for insulation), and a reference to using gypsum blocks in a bronze furnace, and plaster in statuary practice. The sand part was in a video posted on the forum, and I used a sand and clay liner before, so it seemed reasonable -- I just wasn't sure how well it would hold up and if calcining would be a problem. It's working so far. And the insulation value seems very good. 2" is fairly thin, but the outside of the furnace doesn't get very warm after an aluminum pour.
Another point of interest -- melting crucible of aluminum in the furnace took 20 minutes with propane and 15 minutes with charcoal briquettes. Propane is neater, and faster to light, but more expensive and slower to melt.
Charcoal is easier to do other things around -- I don't like straying from the furnace when using propane in case of a flame out.
While charcoal is starting (no blast) i can ram up a mold, and have a hot furnace ready to melt metal when the mold is set down. In fact, I've had aluminum in the crucible begining to melt a little if I took longer than usual making up a mould, even without a blast in my old furnace. That was quite a surprise the first time it happened! And a very fast melt afterwards. My usual procedure building the first lathe was, a little lighter fluid in a furnace 1/4 full of charcoal, light off, and when the embers had just started, add the crucible with initial charge of alumnum. Cover the furnace, and go make up the mould(s). When finished, things were already hot, and the blast made short work of the melt.
I can see using both fuels on different occasions for different purposes. It would be interesting to see how well charcoal melts brass. Might try that soon.
mattinker:
Hi Steve,
This is looking really good so far! definitely worth a try. Just a thought, any sign of glazing from the silica sand melting, the silica sand with the calcium from the plaster (if there's enough in the plaster present) should glaze over, if there isn't enough calcium, it should form water glass.
Time will tell, regards, Matthew
Edited to include glazing thoughts.
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