My cheap clay graphite crucible

Discussion in 'Foundry tools and flasks' started by Mark's castings, Oct 21, 2018.

  1. DavidF

    DavidF Administrator Staff Member Banner Member

    Guess I'll just have to give it a try and find out.
    When I smashed up a clay graphite crucible it looked to have large flakes in it. That's the only reason I was thinking flakes... maybe it was something else I was looking at... Sic??
  2. I was given an old unused AC18 and the black coating is flaking off exposing glittery silicon carbide of about 120 grit mixed in with the clay. I think it was coated with a carbon rich material, bitumen maybe?. My clay graphite looks like pencil lead where I've exposed it with an abrasive block.
  3. DavidF

    DavidF Administrator Staff Member Banner Member

    Any idea of a mix ratio I should try?
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Did you take a look at that video I posted? in Jimmy Cobb's thread? It's a lot of Dr Science but I think he mentions graphite and content. When I considered it I thought isostatic press was the way to go.

    .......Grabs popcorn and pulls up a chair.

  5. DavidF

    DavidF Administrator Staff Member Banner Member

    I watched the video on Morgan making their crucibles with the "rib press" . Looks fairly simple to build. Really just need to figure out a recipe for the crucible, then the coating that is sprayed on.
    Hessian crucibles look interesting also, but don't think they would hold up any better than clay graphite..
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

  7. I'm trying to find information on the process, some articles mention silicon carbide, clay and pitch other articles mention using graphite, silicon carbide and clay. According to this article: We would be looking for method 3 or possibly method 1.

    "There are three types of silicon carbide used for engineering purposes:

    1. Reaction Bonded Silicon Carbide – Also referred to as self bonded SiC, mixtures of SiC and silicon metal or carbon powder are formed into green shapes. They are then fired under controlled atmosphere, carburising (with silicon powder) or infiltrated with molten silicon metal. Under these hot conditions the silicon and carbon react to form SiC that bonds the original silicon carbide grains together.
    2. Sintered Silicon Carbide – Fine SiC powder can be sintered with the addition of sintering aids like Boron Carbide (B4C). This process results in high strength provided exaggerated grain growth and any other strength limiting process can be avoided.
    3. Bonded Silicon Carbide – SiC can be bonded using secondary phases e.g. resin, glass, silicon nitride, clay or metal. Often the bonding phase requires a suitable treatment (e.g. firing) such that it will bond the SiC particles together. This type of SiC is most commonly used in refractory applications."
  8. Jimmy Cogg

    Jimmy Cogg Silver

    On Kellys posted video, "Making a Graphite Crucible", the formula used there is at 2.45 onwards.
  9. The new silicon carbide crucible arrived today: it's an AT30 size from Vesuvius and made in Mexico. The price for Australian players was $242 including tax or USD $176.66, freight and insurance was $36 extra. For some reason the AC20 size I wanted was not that popular here and lead times were long. The crucible sales people tell me it's considered acceptable to use a masonry disc in an angle grinder to cut a ring section off the top of the crucible to get the height you actually want. I'll use the smaller one in the middle for brass, the AT30 for aluminium and the old crucible for iron experiments.

    Jimmy Cogg likes this.
  10. PatJ

    PatJ Silver Banner Member

    Cut them off. Yikes !
    Is that legal?
    But seriously, why not just use them as-is?
  11. I was as surprised as you, but yes apparently it's no big deal to modify them with a masonry cut off wheel. It's a tight squeeze in my furnace, I can remove the plinth or cast a ring at the top to increase height.
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I like the idea of a removable section to extend furnace height. Will your lid lifter still work? How about tongs and shank for the big one? Looks like they have pretty generous sized pouring spout compared to Morgans.

  13. The crucible will fit in but there's no clearance for the gas flow out the lid. It'll be a permanent change maybe a thin ring over an inch thick, and I'd have to modify the lifter geometry to suit, it seems the most sensible modification as the plinth area refractory stays thick. New tools will have to be fabbed but I expect once you have a few to choose from, you can stop making them. I haven't tried the existing shank to see how it fits.
    Last edited: Nov 20, 2018
  14. Zapins

    Zapins Silver

    I ran into the same problem with my #16 bilge. I need another 2-3 inches of height to use it. I'll weld on an extension and add some more refractory to make the furnace taller. Shouldn't be too hard.

    Good progress on the crucibles. Have you fired and used on yet?
  15. No not yet, I'm still trying to debug the fuel and air system: I can report that the cheap crucible I've been using has decent mechanical strength when I was trying to melt iron. While it was hot I swung it into the dirt to try and loosen the hard iron mass, this failed but upon cooling the mass shrunk to have about 1/8" inch gap between it and the crucible and I was able to loosen the mass with a hammer and tip out the cool lump of iron. I did a trial fit of the AT30 inside the furnace chamber and it does have enough side clearance to work but the lid clearance is a few millimetres. I'm wondering if my half bag of refractory will be enough to raise the chamber two inches or so: I think it should be more than enough

    My crucible gripper tool comes very close to working, it just doesn't grip below the widest point so longer arms would be needed to reach around the curve for secure lifting.
  16. Just had this video show up as a suggestion: A fairly detailed 12 minute video on how Morgan make their clay graphite and silicon carbide crucibles. It shows what kind of binders get used (coal tar) and steps such as converting the coal tar to carbon in a furnace over three days. Then the crucibles get vacuum chamber impregnated, glazed, baked again in a kiln and so on.

  17. Melterskelter

    Melterskelter Silver Banner Member

    Another fascinating video. An interesting point for me was the worker blows into the mold to release the clay/graphite casting at about 3 min 28 secs into the vid. You'll miss it unless paying close attention. Interesting to me because even a very sophisticated high tech operation still needs the occasional very basic human touch.

    Now I don't feel quite so bad plunking out bucks for my Morton Super Salamander crucibles.

    Thanks for posting.

    Mark's castings likes this.

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