I finished a small scrap copper melt today and after everything had cooled down, I took a look at the crucible. I am wondering if this crucible is still usable or if I should retire it. It is a PMC Procast #8 that up until today, had been only used for about half a dozen aluminum melts in my old furnace with a venturi propane burner rather than the forced air diesel burner I used today. There are two cracks that concern me, as well as the fact that the inside appeared to be melting off during the melt. It appeared to create a film across the surface of the molten copper. The crucible also seemed to stick to the plinth even though I placed a piece of cardboard between the 2 before I lit it. I planned to replace it with a Super Salamander in the future. Any advice would be appreciated as I haven't encountered this before.
That's typical of budget crucibles that don't seem to have much graphite or silicon carbide in the composition. Name brand crucibles like Morgan or Vesuvius actually seem to have better economy due to the longer life if you work it out as dollars per melt. I'd be reluctant to use such a cracked crucible as the risk versus rewards is not there from unexpectedly dumping a load of molten metal.
There's a lot of crucible shaped objects that seem to be made by the Fwang Dang pottery and flowerpot Co. out there. They can be great for tuning your furnace but once they start to melt or metal oxides begin to dissolve holes in them it's time to buy the real deal and have one less point of failure.
I had a similar learning curve with crucible quality. I melted aluminum exclusively for several years starting with steel crucibles in a charcoal furnace which sometimes ended with a burn hole and flooded furnace. At least they didn’t catastrophically fail like a ceramic crucible might, and aluminum easily separated from the furnace floor when cool, so I just dealt with it for awhile. Then I moved to “budget” clay-graphite crucibles from Legend Inc and the burnthrough issue went away. I completely destroyed a #10 one time using anthracite coal as fuel. That’s a whole other topic dealing with a super hot fuel and too much oxygen, but I digress. I could tell it was compromised by the damage done to the exterior. I soon went full time with a diesel burner and all went well with aluminum in those crucibles until I started melting bronze. I went along ok for awhile until one day I kept adding metal to a #10 and it just wasn’t getting any fuller. My crucible had developed cracks that looked very much like yours and was leaking out. I had a couple of lbs of bronze in the bottom of the furnace. Unfortunately it doesn’t just peel off of the refractory and I ended up chipping out and replacing my furnace floor. The experience was unsavory to say the least. I broke the crucible apart later and discovered that the graphite had been almost completely depleted progressively toward the rim. The clay-graphite just couldn’t withstand the beating. I got off of my wallet and switched to the Salamander Super line and have had no trouble since then. I’m not surprised to see the glaze melting off your crucible. You’ll see that on better crucibles as well. They are consumables after all. Most of us here-the novices among us anyways- only make suggestions. This is an exception though. That crucible is junk and should be thrown away. Pete
There is a good discussion on here about new crucibles. In short don't flux. Also the third photo looks like it is burnt, with the crucible in the burner flame. As far as the crack's...A nice New Flower Pot for the wife.
