My cheap clay graphite crucible

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

  1. After getting a quote for a silicon carbide crucible (AC20) of about $350 AUD of which more than half was for freight, I had a look around online and bought a roughly A20 crucible online for $170 AUD delivered from China (the crucible was $30-40 USD). It came from Qingdao Tenry Carbon Co. and was 30% silicon carbide they assured me. When it arrived it had a generous coating of graphite that burned off after the first run. I used a carbide abrasive brick to make a tiny spout on the rim and it exposed more graphite in the crucible.

    Anyway this is what it looks like after ten runs in an oil fired furnace: the surface it beginning to craze, tiny bits are breaking loose and the whole surface is crazed like dried up mud in a puddle. On the side is some oxidized copper that dribbled down the side on my last run and has eaten into the surface of the crucible like a corrosive substance.

    I'm thinking of retiring it and breaking it open to see if there actually was any silicon carbide used in the mix in the first place. So I'd be interested in hearing everyone's opinions on this product and whether it is at all usable.

    crucible 4.jpg
    crucible 3.jpg
    crucible 2.jpg
    crucible 5.jpg
    crucible 1.jpg
     
    Last edited: Oct 21, 2018
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Looks like clay graphite to me... o_O
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    After ten heats that would be very discouraging. Doesn't look any better than I'd expect out of just fire clay. At those prices and results it might actually be worth making your own with fireclay, graphite, and/or SiC blasting grit.......if half your cost is freight. Are there no Morgan dealers in AUS that could do better than $350AUD?

    Best,
    Kelly
     
  4. I wonder if something was lost in translation, it never really looked like a regular silicon carbide crucible: I think Morgans, etc. have tar or something similar coated on them for the initial firing.

    I'll keep shopping around, I may try another freight company for a quote since I discovered a cheaper company. The thought of joining the pottery club and whipping up some crucibles has crossed my mind, I'd need a good recipe.
     
  5. DavidF

    DavidF Administrator Staff Member Banner Member

    Here is a budget clay graphite from legend mining supply after 5 or so iron pours.

    20181021_225311.jpg 20181021_225315.jpg
     
  6. Did that crucible have any flux used on it?. It looks like a foamy, porous mess.
     
  7. DavidF

    DavidF Administrator Staff Member Banner Member

    I think it had some sort of glaze on it. They only last for 10-15 iron melts in a best case scenario. May sound odd, but im ok with that for the price.
     
    Mark's castings likes this.
  8. I used a silicon carbide abrasive block to remove some material on top of the spout area: this crucible didn't have a pouring spout so I cut a tiny 90 degree V earlier. There's about 1/8" to 3/16" of graphite free outer layer with a graphite core, probably where air can't get to it to burn it. It seems to have a graphite depleted surface with a cracked/crazed appearance and a solid core. Makes me wonder just how many uses it's supposed to have.

    crucible 6.jpg


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  9. Rasper

    Rasper Silver

    I tried Chinese crucibles years ago and learned my lesson. Never again.

    Richard
     
  10. HT1

    HT1 Gold Banner Member

    I had really good luck with the Legends Budget crucibles... but they dont have the case deal anymore, or my size... If i remember right
    V/r HT1

    I order my Brass crucibles from Mifco, and the Zamak and Aluminum crucibles from Ebay

    V/r HT1
     
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  11. Rasper

    Rasper Silver

    I only buy Starrbide silicon carbide crucibles sold by Morgan. I melt only bronze. They seem to be indestructible at those heats. Worth whatever they cost.

    Richard
     
  12. Been getting quotes today: a Morgan AX25 in silicon carbide is $247 AUD or $174 USD and a Vesuvius AT30 in silicon carbide is $240 AUD or $169 USD. I used the crucible again for a test run of the furnace with a higher volume blower: I rapped the crucible lightly a few times and it gave a sound like a clay flowerpot, no dead sounds indicative of a big crack. The furnace ran nice and hot, the interior and crucible was glowing orange in daylight and there was a blue flame from burning zinc. The crucible worked ok this time but then it's a gamble with steadily worsening odds until it fails.
     
    Last edited: Oct 23, 2018
  13. HT1

    HT1 Gold Banner Member

    In case you all dont know the Regulator on my furnace failed wide open , so I got 30 PSI of Propane rather then the 27 I like, so I did most of a brass heat quite rich. this caused probably the equivilent of 20 Heats of damage to My Starrbide Crucible FYI

    V/r HT1
     
  14. I took that crucible over to Pete's workshop and foundry along with my brass casting: He thought the crucible looked OK to use....for a clay graphite crucible one. He pushes his silicon carbide crucibles to the limit and gets 50 melts maximum which is really pushing it (flux erosion). My crucible has copped a lot of fuel rich runs and thermal differences of running hot on top and cool at the bottom while sorting out the furnace design. The manufacturer is quite a large company if the website is anything to go by, you'd think they'd have mastered making a crucible: http://www.tennry.com/en/
    http://www.tennry.com/en/Prodetail.aspx?id=349
     
  15. Rasper

    Rasper Silver

    I looked at their product sheet. It says the material is carbon. No mention of clay or another binder.
    My experience with Chinese crucibles was with three crucibles I got from Budget Casting Supply years ago. They claimed to be silicon carbide. I ordered two: a number 8 and a number 40. The number 8 cracked immediately. BCS was glad to replace it. That cracked too. Later they discovered, and admitted on their web site, that they were really clay graphite crucibles with a silicon carbide coating. (Which burns off in short order.) Still, they shouldn't crack on the first or second use.

    On the other hand the number 40, which really measures as a 30, has been a great crucible. It was obviously from a different manufacturer (labels and design differences). I am more than happy with it.

    So all Chinese stuff is not junk---but it is a crap shoot. If you figure half of it is junk and half is not, then if it costs half as much as a well respected brand, you come out even---maybe.

    Richard
     
  16. DavidF

    DavidF Administrator Staff Member Banner Member

    Anyone have a source for graphite in flake form?
    I've got the fire clay ;)
     
  17. That second link refers to clay crucibles and they are a graphite processing company. Your experience closely mirrors my experience, either there is some sort of translation error regarding the phrase "silicon carbide" or they are telling you what you want to hear to make the sale. Silicon carbide can be made by mixing clay and graphite together and heating it to high temperatures, so it could be a misunderstanding.
     
    Last edited: Oct 23, 2018
  18. It gets sold as "Plumbago", apparently graphite is mined and has abrasive contaminants that need to be processed out, so you don't want the more expensive "Lubricant" quality. I'd be more inclined to mix in silicon carbide to aim for a better, more durable crucible .
     
    DavidF likes this.
  19. DavidF

    DavidF Administrator Staff Member Banner Member

    I can find graphite at tractor supply at a reasonable price, problem is that it's a fine powder. Pretty sure you need a flake graphite for crucible making ??
     
  20. It gets sold as "Plumbago", apparently graphite is mined and has abrasive contaminants that need to be processed out, so you don't want the more expensive "Lubricant" quality. I'd be more inclined to mix in silicon carbide to aim for a better, more durable crucible .
    I don't know about that, flake graphite is mechanically weak so maybe a really uniform mix of clay and graphite powder would have the best strength?.
     

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