How to pretreat refining crucibles

I get a lot of questions about pretreating crucibles for refining. Do they need to be prefired with flux to "coat" the inside? Do they need to be preheated?

The simple answer is: Yes, do preheat crucibles. This will drive off any moisture that may have been pulled out of the atmosphere during manufacturing, storage, and cooling from the previous firing. And for high value fires (like refining), it is good insurance.

Coating the inside of the crucible on the other hand probabaly does not help. If you want to coat the inside of the crucible with borax, go ahead as it probabaly will not hurt anything. But, the borax is a low melting temperature component so the coating will not form much of a barrier after you start heating things up. And, it will "cost" you one of the fires.

Some people do some refining in assay crucibles. The fire clay used in assay crucibles is not rated for nearly as high of temperatures as graphite or silicon carbide, and this is not really recommended. But, if you have had success doing this, make sure and preheat the crubible before use (as mentioned above).

Preheating can be done by placing the crubible on top of the furnace, or on a hot plate, or by putting them in the furnace and raising the temperature slowly.

Minimizing crucible erosion

We used to use boric oxide during melting of gold scrap in a graphite crucible. The nature of the induction furnace doe not allow usage of any other type of crucible. However, boric oxide forms a very viscous flux at 2000F and could trap gold particles and prevent easy separation. So, we started to use a 50-50 mix of boric oxide and borax to thin the flux. But this flux mixture attacks graphite (though we have an argon gas cover) resulting in very short crucible life. Is there another flux mix that is fairly fluid at 2000F that will minimize crucible attack? The flux chemistry need not be active as the scraps are all clean.

Shan Aithal

Re: Minimizing crucible erosion

Slag formation and flux composition is fairly complex. It would be difficult to predict what flux will work best for your material.

However, if you are having trouble with erosion of your crucible this would indicate a deficiency of one of your flux components. During slag formation this deficient component will be pulled out of the crucible. I would suspect (but am not certain) that your slag might be silica deficient.

Although it does not deal specifically with refining fluxing, the Fire Assay book by Shephard & Dietrich has an excellent section on proper fluxing:
https://www.lmine.com/assaying-processing-c-223_307/fire-assaying-shepard-dietrich-p-5929.html
You can also review the fluxing section in the "How to Smelt Gold And Silver" book:
https://www.lmine.com/assaying-proc...-your-gold-silver-by-hank-chapman-p-5937.html
Premixed refining fluxes are also available (but you may need to adjust them for your particular material):
https://www.lmine.com/fire-assay-fl...efining-flux-20-lb-chapman-recipe-p-8083.html
https://www.lmine.com/fire-assay-fl...d-refining-flux-bucket-net-24-lbs-p-5213.html

Yes I would suggest sodium silicate this protects the crucible somewhat more than without it. There are many recipes for flux. It's kinda ore specific. There is a website that offers metallurgical insight and help to this I can't remember the name I'll post again when I find it.