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Burno crucibles


I have been using my Burno crucible for melting sterling silver, and
notice that the borax which used to completely coat the inside of the
crucible is all pooled up on the bottom of the crucible.

Is it necessary to recoat the crucible, so that the section that
holds the metal is completely coated with borax?. If so, should I
remove the old glassy borax at the bottom of the crucible? If so, how
should I remove it? The old borax is sort of brownish red, caused I
believe by the cuperous oxides in the sterling.

I read somewhere that one could coat the cruicible by filling it 1/3
full of borax, and putting it in the kiln to heat, rotating it around
so that the molten borax completely covers the sides of the crucible.
If this is the proper proceedure, what temperature should I heat my

Even with heavy leather gloves and tongs, this seems like a
difficult proceedure. Is there a better way than using the kiln?

I will appreciate advice on the proper care of my crucibles so as to
insure longevity.

Thanks Alma


Hi Alma,

I believe the purpose of coating the crucible with borax is to
separate the molten metal from the silica in the crucible. The borax
does not make the crucible last longer nor does it prevent cracks
from appearing, but it does flow into cracks filling, but not really
bonding them. What does prevent cracks is slow even heating and
cooling of the crucible. As you are using a kiln to melt the metal,
cracking should not be a problem unless you put cold metal into a hot
crucible, or place the crucible on a cold surface after pouring.

What you describe is consistent with using a kiln for melting metal.
The pool of borax at the bottom of your crucible is not a big problem

  • the borax will melt and float on top of the molten metal and should
    coat the crucible in the process. To coat the crucible more
    completely using the kiln, the method you describe sounds like the
    correct way.

Cheers, Alastair