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Thread: Bill wants to make a crucible

  1. #1

    Bill wants to make a crucible

    Update! Pictures are up, please go to the end.




    I have made my large mortar pattern and want to cast it. This calls for a larger crucible!

    I bought some plaster of paris today. I plan to use that for the outside mold. The inside will be made of wood, sanded and polyurethaned. The bottom board will have a aluminum plate over it to prevent sticking. All surfaces in contact with the refractory will be vaselined.

    I will clamp the whole mess to the bottom board, and pour in the refractory. When the crucible is dry, I will put a plug under the crucible and re-clamp to push the crucible out.

    The refractory I will use is rescobond AA 22 S. It is supposes to be a fairly sticky refractory used for gun- spraying, but hte vaseline should stop that.

    Questions:

    Will this work?

    Will the refractory cure in a mold like this?

    Should grog be added?

    I was thinking of adding graphite to increase conductivity. Will the graphite burn out?

    Will pounding the mold up and down get all the air pockets out?

    This will be a large crucible. I am planning 10" high by 8" wide at the lip. As such, I am planning for 3/4" thick walls and 1" bottom. How does this sound? Lastly, I plan to follow the refractory instructions to the letter for mixing. I have heard people say of mixing drier, but I don't think I will.

    Any tips for casting the plaster?

    Finally, how would I line up the core piece?

    Thanks
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    Last edited by Bill Toomey; 07-20-2012 at 02:39 PM.
    Use those coffee makers with the spout out the bottom to preheat you oil!

    My foundry: http://www.alloyavenue.com/vb/album.php?albumid=1

    My waste oil burner:http://www.youtube.com/watch?v=NwpKCsiWKls
    Metal casting video:http://www.youtube.com/watch?v=8ui24..._order&list=UL

    My website : maniacmechanics.blogspot.com

  2. #2
    The simplest way to cast the plaster outer mold would be to find a cup shaped object the shape of the outside of your new crucible, put that in a box, and pour in the plaster. All coated with Vaseline of course. A kitchen supply store should have any number of cup shaped objects.

    I find that the best way to get the air bubbles out is to probe it thoroughly with a piece of steel rod, lots of stirring and punching.

    Use graphite for sure. I made a crucible with pure Mizzou. It would not melt bronze. The Mizzou wouldn't pass through enough heat; all the heat went up the outside of the crucible and on out the vent hole. I took that crucible out of the furnace and put the same charge of bronze in a clay graphite crucible and it melted in just a few minutes.

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

  3. #3
    Wow! How thick were the walls on your crucible? Have you tried the graphite trick yourself?

    I also wonder how rescobond compares with mizzou. I'd imagine they are very similar.
    Use those coffee makers with the spout out the bottom to preheat you oil!

    My foundry: http://www.alloyavenue.com/vb/album.php?albumid=1

    My waste oil burner:http://www.youtube.com/watch?v=NwpKCsiWKls
    Metal casting video:http://www.youtube.com/watch?v=8ui24..._order&list=UL

    My website : maniacmechanics.blogspot.com

  4. #4
    Senior Member Fredo's Avatar
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    The MSDS on this Salamander clay-graphite crucible lists it as having 10-25% silicon carbide. I would guess it's added to help with heat transfer, so you could throw some of that into the mix. http://media1.riogrande.com/Content/...ibles-MSDS.pdf

  5. #5
    The silicon carbide forms by heating the graphite up long enough. I don't think just adding it would do much, but I could be wrong.
    Use those coffee makers with the spout out the bottom to preheat you oil!

    My foundry: http://www.alloyavenue.com/vb/album.php?albumid=1

    My waste oil burner:http://www.youtube.com/watch?v=NwpKCsiWKls
    Metal casting video:http://www.youtube.com/watch?v=8ui24..._order&list=UL

    My website : maniacmechanics.blogspot.com

  6. #6
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    Another candidate as thermal conductive media would be silicone carbide, its cheap and "pretty" hardcore stuff as well.
    nvm me then.
    Last edited by daniel; 06-26-2012 at 04:24 PM. Reason: oh fredo.

  7. #7
    So you could just add it in? If so that would be better as it is in sand blasting media, easier to find in the pound sized bucket.
    Use those coffee makers with the spout out the bottom to preheat you oil!

    My foundry: http://www.alloyavenue.com/vb/album.php?albumid=1

    My waste oil burner:http://www.youtube.com/watch?v=NwpKCsiWKls
    Metal casting video:http://www.youtube.com/watch?v=8ui24..._order&list=UL

    My website : maniacmechanics.blogspot.com

  8. #8
    Not only did the Mizzou crucible not melt the bronze, but it also cracked. I buy Starrbide crucibles exclusively now. When you factor in the long life expectancy they offer, the cost per melt is negligible.

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

  9. #9
    Yea, I know. I'm not making any money off what I do yet though, and I have no job besides mowing some lawns, so I have to penny pinch. I have to spend my time to save my money right now.
    Use those coffee makers with the spout out the bottom to preheat you oil!

    My foundry: http://www.alloyavenue.com/vb/album.php?albumid=1

    My waste oil burner:http://www.youtube.com/watch?v=NwpKCsiWKls
    Metal casting video:http://www.youtube.com/watch?v=8ui24..._order&list=UL

    My website : maniacmechanics.blogspot.com

  10. #10
    I hear ya on the money there, Bill...

    Have you checked out 4CF's crucible thread? Some good ideas there, including centering and grog... he has learned a lot here about refractories and ceramics and you could certainly learn from that thread. I'm not saying that you should follow him exactly, but there is some good stuff in there.

    I got faith in you though, Bill... you will come up with something great. Just be sure to share it here (first)

    As far as my oppinion, I'm working on some now that are 5" x 7" and using plaster for the mold. I'll post some pics, but I'm working my butt off at the moment. Hopefully pics by the weekend.

    I suggest thinner than 3/4 walls and VERY disciplined thermal shock care. As far as grog, see if your castable has much in it already. Some times they suggest you mix the whole bag, if so, it probably has grog in it. If it doesn't say to mix the whole bag, and instead says a workable proportion, you can add grog, I believe.

    I tried the rope gasket and had some luck, but I think it is pretty temperature limited. Need to make thicker crucibles to get the benifit, which is what I'm working on now.

    However, I do think 3/4" is too thick. Maybe try it and, depending on how you make the molds, try it in different thicknesses. I'd suggest 1/2" to 5/8"m but I'm no pro on these things either.

    I believe it's generally an up-hill battle with refractory crucibles. It's good to know how to make one when you need it, but it won't out perform a clay-graphite or SiC.

    Kiln's aren't hard to make though, and then you could fire some Clay-Graphite, I believe.

    And just to make this post even longer... adding graphite should help a great deal and reading 4CF's thread will give you some better ideas on that too...
    Last edited by GypsyTinker; 06-27-2012 at 03:31 AM.
    Knowing how to make something is far more valuable than knowing where to buy it...

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