Annealing a clay/graphite crucible

grinch - Saturday, September 1, 2007 - 9:14am

I have a new A6 clay/graphite crucible to replace the welded steel pipe one I've been using in my Gingery style blast furnace for the past couple of years. I also recently constructed a propane burner for the furnace to make tuning the temperature a little easier than with charcoal, as well as to leave more space to work in the furnace. Can someone share with me their procedure for initial anealing of such a crucible? I'd hate to crack it on the first try!

P.S. I'll be forging some purpose-made tongs for the crucible to handle it before I stick it in the furnace.

Thanks!

-Roy Grant

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Roy, I may just have been

Rich Waugh - Saturday, September 1, 2007 - 7:11pm

Roy,

I may just have been lucky, but all I've ever done with my graphite crucubles is put them in the burnout kiln and bring them up to about 600F and hold them there for anb hour or so and then bring the kiln up to around 1200-1500F and shut it down. I let it cool down to room temp with the door closed, which takes about ten hours or so. That's it.

I was told many years ago in school that the crucible should be dusted with flux while it's at the 600F temp and let it glaze the inside of the crucible when it comes up to heat. I think I did that with my first crucible, and then forgot with the next few. It didn't seem to cause any issues, but I'm only reporting my observations, not making any recommendations.

Rich

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Thanks for the input, Rich.

grinch - Monday, October 1, 2007 - 10:40pm

Thanks for the input, Rich. I don't have a kiln in which I can control temperatures, so I went ahead and slowly (?) brought it up to about 600 degrees (F) in a closed barbeque, let it sit for an hour, then transferred it to the furnace and brought it fairly quickly up to yellow heat, then let it cool in the furnace. I've done several melts with it since then with no signs of trouble.

As I melt different metals in this crucible, I'd be afraid of flux for one metal re-melting and contaminating something incompatible the next time around.

-Roy

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Crucibles

bpfink - Tuesday, October 16, 2007 - 6:57am

I find the costs of crucibles to be minimal to the costs of everything else, including the gases and just general other parts of casting.

So I simply use a different crucible for each different metal. As far as preparing each one for use, just pre heat it slowly and hold it for awhile before the big heat. I do it in the very furnace I'm using for the final use but do a series of on's and off's over a period of several soaking hours. These are rather large crucibles, with the main ones being #100 or ones that will melt 300# of bronze per melt. For smaller crucibles the time is much shorter. bpfink

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glad you mentioned the

Bill Roberts (not verified) - Tuesday, October 16, 2007 - 7:14am

glad you mentioned the "different crucible for different metals", Bruce. I kinda cringed when I read the "One crucible for all metals" approach.

I'm sure that there a many that take that approach.......but not the way I was taught.
Bill

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I'll second that on the "one

Rich Waugh - Tuesday, October 16, 2007 - 8:05am

I'll second that on the "one metal per crucible" point. Not only do you risk contamination by using multiple metals in a crucible, you risk possible injury. All sorts of unpleasant things can happen when you mix metals accidentally. Why risk it to save the price of a relatively inexpensive crucible?

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Me = Expert.

Jahm Mitt - Wednesday, June 17, 2009 - 12:59am

Yeah I used to run a big foundry a log time ago, mainly doing brass's, bronzes and aluminium, and it's vitally important that one keeps a separate crucible for EACH alloy type.

Off the top of my head I can't remember the exact details of all the possible reactions;

With metal alloys and phases and all that with the zinc, tin, copper, silicon, aluminium, etc., etc., etc., it will only take a small part of a percentage of contaimination left in the crucible from a previous melt to turn a few hundred dollars of brass or bronze into a tub of gunk that cannot be poured and cast.

I think it's much like a how a very small amount of moisture from a damp atmosphere, permeates a bag of cement and causes it to go off.

So it's absolutely important that you keep all your crucibles for that ONE particular alloy that your running through it - including it's own storage area and tagging.

It's probably OK to have a spare general purpose crucible for non descript aluminium alloy cut off's and spares - to make "not terribly important" aluminium castings with.

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