Basic crucible questions

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LQFP-144
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Joined: Fri Mar 11, 2022 10:36 am

Basic crucible questions

Post by LQFP-144 »

Hello,
I've searched youtube, archive.org's books, and the whole internet for answers, but I've come up blank.

I've been reading up on how to create a backyard foundry, and I've come across a few terms and proceedures which I'm unfamiliar with. Perhaps someone here could explain. I'm 100% happy to read whatever material you'd care to direct me towards. I'm going to ask 3 at once (they should be pretty easy) instead of spaming the forum.

1: How do you "dress" a crucible?
2: What is "the ring test" that you can perform an a crucible?
3: What is a "ramable" furnace liner? (I'm familiar with what a castable furnace liner is.)

Thanks!
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Harold_V
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Re: Basic crucible questions

Post by Harold_V »

I'm not sure how to answer the "dress a crucible" question. One generally simply "seasons" a graphite/clay crucible by preheating slowly, to expel moisture. That is done prior to putting a crucible in service, and again if it is stored for periods of time in a damp atmosphere. One should avoid storing that type of crucible on the ground, even on concrete. That type of crucible is subject to cracking upon heating if that operation isn't performed. Silicon carbide crucibles do not suffer the same problem, so they are a better choice. Beyond that, my experience with dressing would apply to melting dishes, whereby the dish is preheated to redness with a torch, then a thin layer of borax, preferably borax glass or anhydrous is applied. Borax that contains water (although it is dry) will work, but it is troublesome to use because of its propensity to "puff up" and be blown about by the heat source. That process "greases" the dish so the melted charge flows easily when poured, leaving nothing behind.

The "ring test" is common for those who are familiar with grinding wheels. A wheel (or crucible) that is not cracked will have a distinct ring to it when it is tapped lightly with the handle of a screwdriver or other similar item. If there is a crack, the crucible will have a dull "thunk" sound to it instead of a ringing sound.

Rammable refractories come two ways (wet and dry), although for a crucible furnace only the wet type would be used. It consists of thick slabs (about two inches thick) of refractory, premixed with solvent (generally water based) to the proper consistency. This material is placed where desired, then hammered in place. It is plastic in nature and flows with hammering. The ramming process is relatively important in that it removes voids and condenses the refractory. A poor job of ramming generally results in short lining life, while the process properly done will provide a durable lining.

One comment that you didn't address.
I do NOT recommend one use a metallic melting vessel. Molten metals are strong solvents of other elements, so using one leads to a contaminated heat. Iron included in alloys changes their grain structure and often results in inferior metal. For that reason I suggest you avoid such a vessel. If, however, you insist on using one, it should be lined with a refractory to minimize contact of the charge with iron (steel).

I hope this helps.

Welcome to the board.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
FKreider
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Re: Basic crucible questions

Post by FKreider »

Harold_V wrote: Fri Mar 11, 2022 5:19 pm Borax that contains water (although it is dry) will work, but it is troublesome to use because of its propensity to "puff up" and be blown about by the heat source.
This is the exact issue I experienced when I "seasoned" my new graphite crucible.
-Frank K.
LQFP-144
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Re: Basic crucible questions

Post by LQFP-144 »

That answers all the questions nicely, thanks!
I have no intentions of using a steel crucible. I've been looking into alumina and SiC.
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Harold_V
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Re: Basic crucible questions

Post by Harold_V »

One thing to keep in mind is that fluxing of any nature influences the useful life of a crucible. Soda ash and fluorspar are exceedingly aggressive, but an essential constituent, depending on one's needs. Their use will liquefy the resulting slag, but are hugely detrimental to longevity, as they promote dissolution of the crucible. As borax is added, it forms a thick layer of slag which becomes difficult to remove. The addition of soda ash eliminates the problem, but at the added cost of also dissolving a portion of the crucible. For that reason, I suggest that flux be used ONLY when absolutely necessary, and sparingly.

I used to use #8 crucibles when I refined silver, which is recovered from solution with copper. The resulting silver is commonly known as "cement silver" and bears a resemblance to Portland cement. Once recovered and well washed (to remove copper nitrate), it was then melted with considerable flux, so the fine particles would agglomerate. The use of flux resulted in the crucibles being perforated after as few as six heats.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
LQFP-144
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Re: Basic crucible questions

Post by LQFP-144 »

Harold_V wrote: Sat Mar 12, 2022 5:22 pm One thing to keep in mind is that fluxing of any nature influences the useful life of a crucible. Soda ash and fluorspar are exceedingly aggressive, but an essential constituent, depending on one's needs. Their use will liquefy the resulting slag, but are hugely detrimental to longevity, as they promote dissolution of the crucible. As borax is added, it forms a thick layer of slag which becomes difficult to remove. The addition of soda ash eliminates the problem, but at the added cost of also dissolving a portion of the crucible. For that reason, I suggest that flux be used ONLY when absolutely necessary, and sparingly.

I used to use #8 crucibles when I refined silver, which is recovered from solution with copper. The resulting silver is commonly known as "cement silver" and bears a resemblance to Portland cement. Once recovered and well washed (to remove copper nitrate), it was then melted with considerable flux, so the fine particles would agglomerate. The use of flux resulted in the crucibles being perforated after as few as six heats.

H
Thanks! I'll be reading about fluxes (and probably asking about fluxes) as I get closer to a working setup.
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