# Nuked Crucible



## bitgold (Jan 5, 2022)

So this happened as I was rerunning the slag from my previous experiments with lead as a collector this time. The xrf showed the slag still had pt pd and rh that I has run with sterling as a collector metal previously in the same furnace. I did use the appropriate tongs that came with the furnace that fit the upper rim of the crucible not the channel locks LOL Thoughts please?


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## bitgold (Jan 5, 2022)

bitgold said:


> So this happened as I was rerunning the slag from my previous experiments with lead as a collector this time. The xrf showed the slag still had pt pd and rh that I has run with sterling as a collector metal previously in the same furnace. Thoughts please?


I am running finely crushed catalytic converter material with cryolite 50-50 mix.


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## bitgold (Jan 5, 2022)

bitgold said:


> I am running finely crushed catalytic converter material with cryolite 50-50 mix.


2200' F maxed out temp on the table top furnace.


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## Williamjf77 (Jan 5, 2022)

Get a new crucible? I’d go for one turned on a lathe instead of molded if that’s possible. Those types are so popular I’m sure there is a variety.


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## Yggdrasil (Jan 5, 2022)

bitgold said:


> 2200' F maxed out temp on the table top furnace.


The reason to use Cryolite is its ability to break down refractory materials. Any crucible will be destroyed over time with any aggressive flux, even more so with Cryolite.
That is why one use thick crucibles like Salamander crucibles.
Your furnace are designed to melt mostly pure metals, i guess, according to the crucible design.
You could get a better lining inside the outer graphite crucible, but it will decrease the maximum charge significantly.
Or find/build another furnace.
This is my ideas around your problem anyway.
There are far more experienced people in here, so I hope they chime in.
Regards Per-Ove


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## bitgold (Jan 6, 2022)

Yggdrasil said:


> The reason to use Cryolite is its ability to break down refractory materials. Any crucible will be destroyed over time with any aggressive flux, even more so with Cryolite.
> That is why one use thick crucibles like Salamander crucibles.
> Your furnace are designed to melt mostly pure metals, i guess, according to the crucible design.
> You could get a better lining inside the outer graphite crucible, but it will decrease the maximum charge significantly.
> ...


I had only used it 4-5 times it was in good shape before I fired it with the lead with the lid closed.


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## Yggdrasil (Jan 6, 2022)

I'm no master of fluxes, but if you used this crucible 4 times with this flux, you may have been lucky it did not fail earlier.
One should have designated crucibles for each kind of materials.
Especially, do not smelt and melt in the same crucible.


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## Yggdrasil (Jan 6, 2022)

Get yourself a Salamander or other similar Crucible and build/buy a gas fired furnace for it and you would be good to go.


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## Elemental (Jan 6, 2022)

Are you leaving your crucible sitting out while it is red-hot? I have a similar furnace and the oxygen in the air was reacting with it and eating the crucible. I noticed most of loss is around the top lip, which normally holds it in the insulation bricking. This is even more prone to happen when you take the graphite to the max temperature of the furnace. 

My recommendation, only pull your crucible out while it's hot to pour then put it right back in the furnace and close the lid. This won't prevent the damage, just slow it down some so you can get more pours out of your crucibles. These types of crucibles are designed to be expendable parts, you'll have to replace them eventually.

As a side not, I'm not a huge fan of this type of furnace and in hindsight I wish I would have gotten a front opening muffle lab furnace and gone with a silicon based dish. Don't get me wrong, it's cheap and it works, but the idea of my crucible dropping into the bottom of my furnace due to the crucible lip disintegrating and dumping out all my molten values while also destroying my furnace just doesn't make sense. (I solved this problem by placing a steel block in the bottom of my furnace to rest the crucible on. Live and learn.

Elemental


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## orvi (Jan 7, 2022)

Graphite is just carbon. If you heat it past 500-600°C, it will start to burn forming carbon dioxide and monoxide. That is the fact you must live with. As it was suggested by Elemental, do not leave hot crucible "uncontained" out of the furnance. When the hot crucible is in the closed space, all oxygen inside that space is used for burning the carbon, but it STOPS BURNING when no more oxygen is present. More you expose the crucible to fresh air, more of it will burn away. This is also the case when some folks are smelting the stuff in graphite crucibles in the gas fired furnances. Curious why the crucibles are pouring their melt from the bottom, which develop holes 
Graphite is excellent material for crucibles, for nearly every temperature. But for gas powered furnances, it does not last very long due to obvious fact of burning in the air 

I do not particulary know if any flux will attack graphite alone, but i am sure that ferrous metals could eat some portion of carbon/dissolve it - like iron does when steel is produced. But compared to the portion eaten by oxygen, it is still minor damage I think.

Cost of the crucible is one of the things you must consider when doing regular smelting/melting. Graphite burns, clay/silica crucibles/melting dishes are on the other hand attacked and slowly/quickly dissolved by fluxes. It differs from flux to flux. It is the running cost, which every smelter who do this is well aware of. You can only figure the most compatible flux/crucible type combinations to prolong their service.
Cryolite or soda ash can destroy basic crucibles even during the first melt 
Silica is in my eyes least resistant, clay is better. For aggressive fluxes consider magnesia/calcia materials, which have lowest solubilities in the fluxes (mostly). Even ordinary portland cement, properly blended with some other additives could make good and cheap crucible material (if you are willing to make some low budget ones at home), but it often crack badly. Silicon carbide-clay-graphite composits crucibles are also very good choice in terms of long term performance.


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