Question regarding graphite crucibles

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jscarmozza
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Question regarding graphite crucibles

Post by jscarmozza » Wed Sep 12, 2018 5:48 pm

I needed to make a few very small castings to complete a project and didn't want to fire up the big crucible furnace so I purchased two very small graphite crucibles and melted the metal I needed in my small heat treating furnace. Everything went well so I did another small pour today, I made three melts and noticed that each time I handled the crucible, the tongs were looser and looser until I wasn't able to grip the crucible at all. My question is, do graphite crucibles burn up and decrease in size with each use? Thanks, John

Harold_V
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Re: Question regarding graphite crucibles

Post by Harold_V » Thu Sep 13, 2018 3:21 am

Yes, they do, and it happens quite rapidly.

I used to own a Handy Melt furnace, which was used for casting small silver ingots. The crucibles were (are) made from solid graphite, and rapidly burned away at the top when used. So fast that you could see the flame resulting from the ignition of the carbon. This link will display one, and if you scroll down you'll see a picture of a crucible to give you an idea what I'm talking about. http://www.ottofrei.com/Handy-Melt-Digi ... rnace-110V

You might enjoy better luck with a silicon carbide crucible, or even one made from alumina. Also, some graphite/clay crucibles are coated to limit the amount of combustion. Unfortunately, they're only coated on the outside, so the inside is still subject to degradation. With the solid graphite type, they are converted to CO2, so there is no residue that might lead you to believe they're burning away.

If you prefer melting small amounts, you might consider exploring melting dishes. They're used extensively in the manufacturing of jewelry, and are available in a few sizes. They are now made from a white substance that may be alumina, and resist degradation from heating. The older type, usually made from tan clay, readily cracked from rapid heating and moisture, but the type shown are very robust and yield a large number of heats.

Here's a picture one, so you can get an idea what I'm talking about:
Melting dish.jpg
H
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jscarmozza
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Re: Question regarding graphite crucibles

Post by jscarmozza » Thu Sep 13, 2018 6:53 am

Thanks Harold, when everything cooled down yesterday I inspected and measured the crucible, the wall thickness was reduced to half the original dimension, I was wondering why the walls were so thick on the new crucibles. Well, the two that I bought cost me $12.00 each and served their purpose, but I won't be buying graphite crucibles again. John

John Hasler
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Re: Question regarding graphite crucibles

Post by John Hasler » Thu Sep 13, 2018 7:57 am

Flooding the furnace with argon, nitrogen, or CO2 (only practical with an electric furnace) would solve the problem. Better to just use silicon carbide unless you really need carbon, though.

RONALD
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Re: Question regarding graphite crucibles

Post by RONALD » Thu Sep 13, 2018 8:11 am

I run a small electric furnace for lost wax, and as you can see in the photo, graphite crucibles do wear down.

Here is a site I buy my crucibles: https://www.riogrande.com/Product/Rio-G ... ace/704037

Of course for bigger stuff I use #30 or #60 Silicon Carbide Crucibles.
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jscarmozza
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Re: Question regarding graphite crucibles

Post by jscarmozza » Thu Sep 13, 2018 8:35 am

Exposed to open flame these things burn up like a piece of coal, do any of you know a source for small ( a half pound or less capacity ) silicon carbide or clay graphite crucibles? Every now and then I want a small casting and don't want to fire up the big crucible furnace for just one item, a small crucible in the heat treating oven seems to work for the odd casting to keep the project moving ahead. Thanks.
John

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steamin10
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Re: Question regarding graphite crucibles

Post by steamin10 » Thu Sep 13, 2018 12:20 pm

If you want to use such carbon crucibles for small doses of melt, one theory is to dip them in ceramic slip such as used for casting figures and drying them out to coat the carbon and prevent burning. Slip for ceramics can be reconstituted many times if it dries with a bit of water a drop of dish soap and a used blender. Once smooth it can coat just about any vessel it is painted on for a barrier. If chipped, the Bisque like coating can be repainted or dipped and dried to use again. It works vey well on pourous and rough vessels , not so much on a piece of pipe, where the expansion tends to drive it off in flakes. Once slip is baked it becomes china like and cannot be reused.

I have used this method to extend the life of worn down vessels but it has no strength, its only value is in a surface coat to prevent further erosion of he base.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
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Harold_V
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Re: Question regarding graphite crucibles

Post by Harold_V » Thu Sep 13, 2018 3:11 pm

I applied a wash to those graphite crucibles, but keeping oxygen away from the graphite was hard to accomplish. The graphite still burned away, although at a somewhat reduced rate (not enough to make a difference, considering the added cost and effort). That left the wash unsupported, and it was quick to flake away as a result. Can't help but think it was partly to do with what you said about a piece of pipe. All of the degradation occurs at the top, in spite of having a loose fitting cover.

At any rate, unless one is able to control the atmosphere, graphite crucibles can prove to be a rather expensive route to pursue.

H
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steamin10
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Re: Question regarding graphite crucibles

Post by steamin10 » Thu Sep 13, 2018 9:16 pm

I have not the occasion to use graphite, only relaying what I was told. I used clay and carbide so it is a different animal. I started casting aluminum out of a cast iron dutch oven, and will never go back, due to iron contamination. I cant stress that enough for the newbe that is going to get started. Little bolts and rivets can really mess with the melt as can paint and all the pop cans. Better to get good heavy scrap and pay attention to process and yield good results.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

jscarmozza
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Re: Question regarding graphite crucibles

Post by jscarmozza » Fri Sep 14, 2018 6:55 pm

As I looked into the matter it became clear that graphite is okay for induction ovens and a total misapplication for a gas furnace. My mistake. I found a source for for small clay/graphite crucibles and I'm awaiting delivery. Thanks to all. John

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