Firing a clay graphite crucible without drying or seasoning.

Discussion in 'Foundry tools and flasks' started by Mark's castings, Mar 15, 2021.

  1. So as mentioned in an earlier post, I had a poorly packaged A25 crucible arrive with a crack in it. As I'm trying to zero in on tuning the furnace for reliable iron melts, the cracked crucible went in the furnace as a "Dummy load" for fifty minutes of run time. Roughly 15 minutes into the session I heard some loud pops and assumed it was the crack lengthening from the heat. After the session was over and there were some blisters on the outside only of the crucible in the coating. They come from the factory with a gloss black coating that looks brushed on and in previous uses the coating had peeled off like stripped paint when hot. The fired coating is rough and glittery, almost like a silicon carbide rich coating and is the layer popped off by escaping gasses, probably steam. When the blister is disturbed you can see the shiny factory glaze underneath it. You can see a painted on coat in this video here at the 11:36 minute mark and I suspect this is how the glossy black coating is applied with it's brush marks and so the factory wouldn't fire it for a third time just for a protective coat. So if the crucible is dried before use and fired empty, the painted on layer may just be a carbide rich protective coat to extend the crucible least that's my theory and would explain the longer life of quality crucibles.

    There's a PDF about drying wet crucibles at the bottom:

    Unfired A25 Salamander Super with nasty crack and the glossy brushed on coating:
    clay graphite crack unfired 1.JPG

    Same crucible after 50 minutes in an oil fired furnace reaching molten iron temps, note the multiple blisters and the rough glittery surface finish now. clay graphite blister 1.JPG

    The same blister "popped" in a very thin layer exposing the fired glaze of the crucible from the second factory firing.
    clay graphite blister popped 1.JPG

    Attached Files:

  2. So I have word that it's "normal" to see this if you put your new crucible straight into use, with no drying or seasoning first.
  3. dennis

    dennis Silver Banner Member

    Dry in oven at 50 C, then ramp up slow? Do we all need hopped up kilns?
  4. I'm going to put mine in the kitchen oven and slowly bring up to 200 deg C and store in a plastic pail with silica gel.
    dennis likes this.
  5. dennis

    dennis Silver Banner Member

    I've wondered about ceramic "reptile heaters" and a Super-Wool "cocoon" for, uh, keeping crucibles "good."

    The heaters look vaguely like stylized mushrooms, and come in various wattages from about 35 up to 150 (?) watts. Upend the pot over the heater, and then put the cocoon over the pot. Use a dimmer, and gradually warm things up.
    Last edited: Mar 16, 2021
  6. That would certainly work but use electrical power over long periods, I would expect a sealed container and dessicant would do the trick or an oven bake the night before a casting session.
    They mention in that drying note that the Salamander Super paint can lift after firing and that it's cosmetic....but it does appear to be a silicon carbide paint so it would give some protection if intact.
  7. rocco

    rocco Silver

    I'm thinking Mark's probably on to something, fill an old sock with something like this, take your still warm crucible, toss in the sock and seal it in a container.

    Click on the pic.

    Attached Files:

    Last edited: Mar 16, 2021
  8. Even the irregular silica gel crystals used for kitty litter could be baked for a few hours to recharge and used, or exchanged from time to time and the used damp crystals made into sodium silicate.
  9. Today I ran the furnace again, loaded up the cracked clay graphite Super Salamander and five minutes into the run there was a loud bang!. I shut the furnace down and opened the lid to find one side of the crucible missing and lots of glittery 4 centimetre flakes of crucible scattered all around the furnace. Hmmmm, Oh Wait!!!, maybe storing the crucible under the back awning of the shed while there's been multiple tropical downpours and daily 90% humidity then giving it a flogging in the furnace is possibly a BAD idea? o_O:eek::oops:. So glad I experienced this firsthand with a sacrificial crucible.

    wet clay graphite.JPG
    Jason, Tobho Mott, dtsh and 1 other person like this.
  10. DavidF

    DavidF Administrator Staff Member Banner Member

    Well shit!
  11. dennis

    dennis Silver Banner Member

    Given that these things are made in India, shouldn't that noise be B'hang?
    dtsh and Mark's castings like this.
  12. Just to add further information, the damage occurred on the opposite side to the original cracked area but where the flames wrap round the crucible. Also there is a series of cracks forming a crosshatch pattern all over the crucible surface. This is only the second time the crucible has ever been fired too.
    wet crucible cracks.jpg
  13. dtsh

    dtsh Silver Banner Member

    How did the original crack happen? I'm curious if this one was perhaps defective and this is just more effect of the defect.
  14. It wasn't packed at all by the seller, they said a new guy in the freight department wasn't putting them in protective boxes before shipping them in the original box with a single layer of corrugated cardboard.

    In hindsight the crucible recently got wet a few days before this last firing.
    dtsh likes this.
  15. Jason

    Jason Gold

    Rock and Roll! Look at the blow out on that sucker! I guess there is something to that real slow heat up to red hot and slow cool over night. I did notice when I got mine, they had that brushed on look too. Very interesting!

    Tell that company it was defective and see if you can score a freebie off them. Meanwhile, we all know why the ass blew out of it and won't say a word.;)
  16. It's already been claimed: it was damaged in shipping and I was using it to test the furnace tuning. I will be baking any new ones in the kitchen oven and storing with dessicant after this episode .
    dtsh likes this.
  17. Peedee

    Peedee Silver

    That was a smart move to use the crucible as a test piece.
    Mark's castings likes this.
  18. Peedee

    Peedee Silver

    By my last comment I should have added we all get to see what happens when crucibles fail and the circumstances, never stop learning as my dad always said
  19. It was a simple spur of the moment decision to run the furnace Friday afternoon while there was a break in the weather. There have been some absolute downpours in the last week so that crucible had been wet and seemed dry on the surface. What was impressive was the complete destruction of the clay graphite into small bits a bit bigger than 50 cent pieces. I told Peter about it and he related how when he was in a rented shed, the neighbours in the same building hosed out the dust on the concrete floor and water got under the wall, wet his crucible and then dried out. He went to lift the hot crucible out of the furnace and the base was broken off in a perfect line an inch or so up the side. I had the new crucible stored upstairs in the relative dry and put it in with no preheating so the painted on glaze is all blistered uniformly with 1/2" size lumps like chickenpox. I let it cool in the furnace overnight and I'll look at storing it with dessicant from now on.

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