Strange crucible issue after first copper melt.

Discussion in 'Foundry tools and flasks' started by Haus, Sep 25, 2021.

  1. Haus

    Haus Copper

    So I've been tinkering and playing around with this hobby for a while now. Prior to today it was all aluminum. Today I was going to do my first attempt at a copper melt. I had read that it's a good idea to have different crucibles you use for different metals. SO I ordered this one off Amazon, seemingly pretty highly reviewed and whatnot :

    I went through the usual prep, an hour in the oven at 450, allow to cool naturally, then an empty run in the furnace up to melting temps, let it cool naturally in the furnace. Afterwards it had a nice "pebbly" surface on it which apparently it normal.

    So this morning I decide to do my first copper run. I've been collecting copper where I could find/scrap it. Most of this was from a cache of dead ceiling fans I got ahold of, plus a nice sized old copper pipe which was laying around a junk shed at my mom's place. Came up to just under 2lbs total. I load up all the fan copper into the crucible and start up the furnace. It runs for a while and I see the wires starting to compress down a little, so it's melting, yay! I put the copper pipe in (and poking up out of the hole in the top of the furnace) and let it keep going. 5 minutes later the pipe is getting shorter so I watch it melt down into the crucible.

    Once it's all melted it's got a little junk on top. I sprinkle on around half a tablespoon of borax, as I heard it's good to help get the impurities out. Stir it with a graphite rod and let it cook for another 5 minutes then skim the slag off the top. Things are looking great, and dare I feel almost competent at this... Take out the crucible to pour, all good, red hot crucible with lovely copper in it and pour my first copper:


    That's post a little wirebrushing, and obviously my sandcasting needs some work...

    I put the crucible back into the furnace to let it cool with the furnace while I run errands. Fast forward 5 hours. I'm back and ready to put away the now cooled off furnace. I take the crucible out since I figure I'll do an aluminum run tomorrow. When I take the now cooled #3 out of the furnace a sizable chunk falls out of the bottom?!?!?!


    Now am I right to assume this Crucible's days are done after one melt? Don't really feel safe putting it back in and loading it up with molten metal now. But have any of you guys had this happen on a first run with one? Did I do something wrong? Did I just get a not so great crucible? Was that a crap manufacturer I should avoid? Very odd.
  2. That crucible is too damaged to use again, I suspect it had moisture in the base from sitting on a damp surface and part of it stuck to the furnace floor on cooling. I had been warned against storing crucibles in contact with damp concrete for this reason, it only takes a small amount of moisture to flash off to steam on initial heating and do this damage. I'll see if I can find photos on this forum of my steam damaged crucible, in my case it was an already damaged crucible which I was using to test my furnace and it had been stored in a wet environment before heating.


    Last edited: Sep 26, 2021
  3. Haus

    Haus Copper

    In the case of this crucible, it was stored for around a week after it's initial dry run on a metal shelf in my garage. And it's been a normal rather dry Texas autumn here. But I suppose moisture could have gotten in somehow. Well, at least it was only a $18 crucible and nothing blew up. heh
  4. If that's the case, then opportunities for moisture pickup are limited, did it make any noises when you fired it for the first time?. I store mine upstairs now on a wood floor where things are drier as I live in the tropics and I'm considering sealed plastic containers to keep them dry. I notice from your photos that the fragments are glazed on the surface so the cracked faces have been hot after the intial cracking. So it looks like it was cracked when you poured the copper and returned it to the crucible where the glaze stuck it to the floor. I use small circles of thin 4mm sheet cement under my crucible and cardboard can be used as it burns to ash under the crucible preventing adhesion.

    While I think of it, copper can absorb oxygen and be fairly thick to pour, if you can get some phosphorous copper shot pellets, a tiny amount will deoxidize the copper and make it very fluid to pour.
    Last edited: Sep 25, 2021
  5. Haus

    Haus Copper

    I have been using the "piece of cardboard" method and it's made it where my crucibles don't stick to the firebrick on the bottom.

    There were no noises I was aware of at any point during the firing with this one. It could have been the post pour cooldown when it cracked for good.

    As for the fluidity of the copper. This was a bunch of motor windings and some copper plumbing pipe. The most I used was a little borax, but you're right it poured pretty thick which combined with the sand cooling it the second it hit it I figure that was the problem there. I'm probably going to re-melt the skull and pour it into something other than sand to test the sand theory. But thanks for the top about phosphorous copper, I'll look into getting a little of that. Also, since the back of the skull has a pourous look to it like it gassed out some gases while cooling. (tiny "holes")

    Here's the back of that skull (i.e. the top of the mold as I didn't use a 2 part sand casting, just one sided) :
  6. FishbonzWV

    FishbonzWV Silver Banner Member

    Cheap chinese crucibles.
  7. Something is outgassing to generate that foam, possibly gas coming out of solution as the metal cools. I recently discovered that unexpanded perlite is used in foundries to coagulate dross on the metal surface, possibly plain old garden perlite will melt and clump the dross into an easy to skim lump. Pure copper is difficult to cast at the best of times as you've found and adding almost any alloying element would go a long way to solving your problems: silicon, tin, zinc, beryllium in small quantities will help. Aluminium is great for fluidity but causes fine porosity if too much is used.
    Last edited: Sep 26, 2021
  8. amber foundry

    amber foundry Copper

    Copper absorbs oxygen during melting and this is the source of the porosity. Try putting charcoal on the copper during melting and if possible a lid on the crucible.
    Haus likes this.
  9. Petee716

    Petee716 Gold Banner Member

    I’d guess the crucible was defective to begin with or possibly cracked sometime between the factory and the furnace. You appear to have handled it properly.
  10. theroundbug

    theroundbug Copper

    I'm guessing defective. I have the same budget crucibles, treat them poorly and still have no issues like that.
  11. oldironfarmer

    oldironfarmer Silver Banner Member

    Try a Super Salamander crucible.

    I set mine on an old firebrick on the shop floor to cool. Never a problem. Did you put fresh cardboard under the crucible to cool? It looks like it stuck for some reason.
  12. metallab

    metallab Silver

    True. I also use charcoal and put the charcoal first in the empty crucible and then the copper. This forces the charcoal to float on the copper and make more contact with the melting copper when starting to float. Then on top of the liquid, it captures the oxygen from the air. And *all* copper has to be molten: unmolten (or re-frozen because of a too cold crucible bum) copper goes unnoticed as it sinks in the liquid, like most substances and metals / alloys and unlike water/ice.
    Moreover a reducing atmosphere (i.e. not too much air) inside the furnace is important.
    When meeting these conditions, even unalloyed copper is as mobile as water and looks like a lustrous mirror.
    When returning the crucible into the furnace, put it deliberately skewed to prevent sticking to the walls or the plinth. Possible 'bump' the crucible when it is still dull red. It is like putting a wet cup in the freezer which will stick.
    Last edited: Oct 3, 2021

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