Removing flux from graphite ingots molds?

[Hartbar]

I poured some silver into a graphite ingot mold, from a fused silica heating dish. I glazed the dish prior with borax.
Some of the flux poured into the graphite mold with the silver.
How do I remove it? The graphite moulds are quite delicate. I remove flux from gold and silver bars by boiling in dilute sulfuric acid, not sure how the graphite mold will respond if I try that?

 

[Ayham Hafez]

I poured some silver into a graphite ingot mold, from a fused silica heating dish. I glazed the dish prior with borax.
Some of the flux poured into the graphite mold with the silver.
How do I remove it? The graphite moulds are quite delicate. I remove flux from gold and silver bars by boiling in dilute sulfuric acid, not sure how the graphite mold will respond if I try that?

Graphite mold will react with sulphuric acid, try to heat the mold a little bit then put it in freezer, after half hour try to scratch the flux it should be removed since freezer will shrink the flux

 

[Marcel]

@Ayham Hafez : Interesting way putting the crucible in a freezer. Will try that out next time. Though I usually prefer to dissolve the flux and then:

Do not forget to collect these little Gold beads that were trapped in the flux!

Sometimes there is a lot of them ;-)

 

[4metals]

Do not forget to collect these little Gold beads that were trapped in the flux!

Sometimes there is a lot of them ;-)

This is exactly why scrap gold is best melted in a flux containing a thinner like fluorspar. The thinner flux drops the beads easier and keeps them where they belong rather than in your refiners slag bucket. (Where he remelts them into a house bar!)

Refined metals are usually melted with no flux. For small quantities simply glaze the melting dish.

A properly glazed melting dish has no free flowing borax, it simply has a glaze that should not pour out with the melt. If you feel you have over glazed a dish, it can be heated until the borax can flow and the flowing borax can be poured out with an assist from the torch flame.

While we are talking Borax, there are different types of borax all based on how much water is in the borax. If you heat up your borax, does it foam like popcorn? If it is you are using the wrong borax for melting. If you notice, all formulations I post for Borax state anhydrous borax, that is borax with no water. In reality borax is borax, but anhydrous is borax that has been heated to 800-1000ºC and crushed and sieved to powder for melting. Typically borax can be had as borax decahydrate, borax pentahydrate, or anhydrous borax. The off the shelf in the USA 20 Mule Team Borax is actually 10 waters borax or decahydrate.