Propane tank/fueled furnace build

Discussion in 'Furnaces and their construction' started by Shawno, Dec 27, 2020.

  1. Petee716

    Petee716 Gold Banner Member

    Cast the plinth tall enough so that it brings the bottom of the crucible a little above the tuyere so the burner flame won't impinge directly on it. Make the diameter at least a little larger than the base of your largest crucible. Make at least a couple. I have several additional ones of various thicknesses so I can bring smaller crucibles up closer to the lid (I try to get about 2" from the top). Use a cottage cheese container or the like. Fire them with the same care as your furnace.

    Pete
     
    Jason likes this.
  2. Use a plinth as thick as you can stand. Get the top of the crucible within three or four inches of your lid opening.

    And it needs dryout just like the furnace.

    While it's drying you can run without a crucible, or use half of a hard insulating brick like used in fireplaces.
     
  3. Like he said.
     
  4. Shawno

    Shawno Copper


    Oooohhhh..

    Glad I asked! Not what I was picturing but it makes sense. Thanks!
     
  5. Shawno

    Shawno Copper

    Taking measurements, I think the crucible may be too large for my finished furnace. I only have about 2 1/2 inches, top of crucible to the inside of the lid with the crucible sitting on the bottom. So I poured a plinth 1 1/2" thick for now but will look into a smaller crucible. Hard to tell from the photo but about 1" all around and just over 2 to top
     

    Attached Files:

  6. Petee716

    Petee716 Gold Banner Member

    Yeah, that burner impingement is a real thing. I think one of our members actually extended the height of his furnace after the fact. Mark maybe?
     
    Jason likes this.
  7. Shawno

    Shawno Copper

    That is actually doable, fairly easily I think.....mmmmmm

    Thanks!!
     
  8. Yes I added half a keg on top for a variety of reasons, Shawno might be able to get away with a taller, thicker lid that is domed and hollow on the inside to allow the crucible lip to be level with the furnace rim.
     
  9. Jason

    Jason Gold

    ALWAYS lay a couple of pieces down on the plinth before you place your shiny new crucible on it. If you don't you'll have the new 1 piece plinth and crucible combo!
    The cardboard burns up and leaves some black ash behind. This prevents sticking together.
     
  10. Tobho Mott

    Tobho Mott Gold Banner Member

    I have noticed white ash from using cardboard discs to prevent crucibles sticking to plinths, never seen black ash. I don't think the colour of it matters though. I use 2 layers of the corrugated stuff. Maybe that's more than I need, but none of my crucibles are glued to any plinths.

    Jeff
     
  11. Jason

    Jason Gold

    Good point Jeff. I see black ash outside the furnace and white ash inside. Gotta be the heat. I'm just glad it works. I forgot once and thought I was screwed. Got lucky and twisted the crucible off the plinth.
     
  12. Tobho Mott

    Tobho Mott Gold Banner Member

    Makes sense. I don't usually bother with cardboard when I'm just setting them down for a second outside the furnace to grab with the pouring shank, but now that you mention it I think I remember seeing you do so in one of your videos. I don't put new cardboard under the crucible in the furnace after the pour either. First time I melted bronze I found my silicon carbide crucible lightly stuck to the plinth when I came back for the next heat. Luckily that came off easily too. Only happened the one time though.

    Jeff
     
  13. dennis

    dennis Silver

    Any particular kind? Mizzou, perhaps? Kast-o-lite30?

    (I can get those two easily in my area.)
     
  14. Shawno

    Shawno Copper

    So since my crucible is too large for my furnace, and I can use a piece of fire brick for a temporary plinth, would I be fine to use a piece of steel pipe as a crucible for now? I know I have read that the graphite crucibles are better but is a steel crucible for early burns OK?

    And, ultimately, I started down this path so that I could get larger aluminum chunks to machine. If I was to melt some aluminum, is pouring it into a tin can as a mold acceptable or will I need to go through the process of a sand mold? I really just need 5"ish chunks of aluminum but maybe shaped more into pulleys when I get "fancy"....;-)

    And if an aluminum can is sufficient for a mold, I suspect I should preheat it or burn it before dumping in the aluminum to get rid of the coating?

    I know, newbie questions....:confused:
     
  15. Petee716

    Petee716 Gold Banner Member

    You can use a steel crucible. The thicker the better. I've used 1lb camping propane cans. They're still too thin to last for more than a few melts but they'll work. I shoot a hole through the top with a .22, fill with water, dump out, and cut the top off with an angle grinder. If you've got too much air (lean) you'll burn right through it like a cutting torch and the molten aluminum will also attack the steel from the inside. Heating your steel crucible red hot and letting it cool will give a coat of oxide on the inside which will offer some protection from the Al, but you'll get a hole in it, probably sooner than later. I think Master Yoda has good luck with a stainless milkshake cup.
    I pour aluminum into dog food cans all the time. Yes you want to burn the coating out. That goes for the propane cylinders too. There's some kind of coating inside those cylinders that pops off when it gets hot, so watch the eyes and bare skin!
    You can probably get larger empties from a local diner. Don't try it with bronze though. It'll burn right through, or if it doesn't it'll weld itself to the can.

    Pete
     
  16. Iron from steel is considered a contaminant to be minimized in aluminium as it affects strength and machinability properties. I haven't confirmed this personally but if you think of thermite powder it's a mix of iron oxide and aluminium powder that chemically reacts to convert the aluminium into aluminium oxide and the iron oxide into metallic iron. So on a smaller scale a heat oxidized steel crucible is going to supply iron oxide in contact with molten aluminium and convert the aluminium into oxidized dross and supply metallic iron to alloy with the remaining molten aluminium and contaminate it. Also like Petee716 pointed out the steel will gradually dissolve into the aluminium too making the crucible get thinner.
     
    dennis likes this.
  17. I use one layer of cereal box cardboard. The thin layer of ash prevents sticking.
     
    dennis likes this.
  18. dennis

    dennis Silver

    Good for specifics. I'd thought to use thicker, in my ignorance - up to 1/8 inch scraps of tempered hardboard, or some sheet stock I use for signage. (Since I no longer eat cereal, due to illness)
     
  19. Jason

    Jason Gold

    I really try not to eat garbage, but I have been eating cinnamon toast crunch like it's going out of style lately.:D Just cant get enough of that sugar!:eek:
     
  20. dennis

    dennis Silver

    While I don't consider cereal to be garbage, it seems I've finally woken up to the fact that I'm substantially gluten-intolerant. Eliminating gluten has helped a lot with my fatigue and mental clarity.

    Sugar helps a lot when you've low blood sugar.
     

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