Why is graphite crucible becoming porous?

[anon136]

I have used my electric melting furnace 3 times now to melt silver and I noticed after this most recent pour that the crucible is becoming very porous. It worries me because surely it must be having an effect on the structural integrity of the crucible. I fear that soon it will come apart mid pour or that the bottom may fall out i the furnace and break my furnace. When i first got my crucible it looked so nice, like a flawless solid object, now it looks like a kitchen sponge, any idea whats going on?

Some stuff that may be relevant: I have been using borax instead of boratic acid. I am told it does the same thing and is cheaper. Also during my first pour i noticed that the borax will not coat the areas near the bottom of the crucible because its just so hot down there that it turns the borax into a pure liquid that just floats on top of my molten silver and since the areas above that line where it is just too hot still have a thorough coating i only used borax in the first pour and not the 2 after that.

Thanks in advance for any help you guys can give, and if im not on the right forum for this question, perhaps you might be so kind as to point me in the direction of the right forum.

 

[Lou]

It's clay bonded graphite and the graphite is oxidizing away.

 

[anon136]

It's clay bonded graphite and the graphite is oxidizing away.

is it normal? is it a problem? what should i do about it? in general how much life can i expect to get out of a single crucible?

 

[Lou]

If you melt under a cover gas, many melts.

Be happy if you get 30-50 melts out of one.

 

[justinhcase]

I returned my electric furnace with a stern letter about a product being suitable for it's stated purpose.
But while I had it I found that a crucible paint was very important.
As it reduces the speed of carbon burn of it will double it's life span and make it slightly safer.
http://www.ebay.co.uk/itm/GRAPHITE-PROTECTOR-CONCENTRATE-25-ML-4-LONG-LIFE-OF-GRAPHITE-CRUCIBLES-MOULDS-/310956675985?

 

[anon136]

one more more question. is it possible to buy a porcelain or quartz or some other type of crucible that i wont have to replace all the time? im sure it would cost a lot more, but 1 up front payment could be a better idea than something i have to replace all the time.

 

[Lou]

Then you would want silicon carbide.

And replacing crucibles all the time is part of the business! Part of the reason we have an "X number of uses/crucible" policy is part of loss mitigation and accident prevention.

 

[anon136]

Then you would want silicon carbide.

And replacing crucibles all the time is part of the business! Part of the reason we have an "X number of uses/crucible" policy is part of loss mitigation and accident prevention.

how many pours does a silicon carbide crucible do?

 

[Lou]

That's like asking how many licks to the center of a tootsie roll pop.

Short answer: it depends.

Less short answer is that it's contingent on what you melt, fluxes, how hot, how long.

Let's just put it this way: silicon carbide is much better than graphite.

If you don't want to go that route, go with the crucible paint. It does help. Boron nitride mold release is nice too.

 

[Anonymous]

That's like asking how many licks to the center of a tootsie roll pop.

Reading that analogy whilst taking a slurp of hot coffee wasn't the cleverest thing I've done in my life.

I now have two screens to clean.....

:roll: