QuestionI'm about to undertake my first casting project after a series of successful simpler projects, and I'm trying to collect as much information as I can so I understand the process fully before I actually put fire to metal. The one thing about which I can't seem to find any information is the removal (if even necessary!) of flux from metal after the cast is complete. I guess the few things I'm just not understanding are these: If the flux forms a thin glass-like coating on the metal (which is what I seem to be reading, if I've understood correctly), then that would have to be removed, surely...? Somewhere I read the metal 'can be released from the hardened flux under light heat', or something to that affect, but I'm not understanding: Isn't the flux there to, amongst other things, prevent oxidation under heat? If one uses heat to remove the flux, wouldn't the metal just oxidize the second time heat was applied, during the removal of the flux? And what will that look like, anyway; the flux 'being released from the metal'? If I have a bit of gold or silver that's coated in leftover flux from the melting, and I apply heat again to the newly cooled metal to remove the flux, where does the flux go?? Surely it can't 'burn-off' or it wouldn't have worked under extreme heat in the first place, and it can't melt off, or the same would be true. Does one polish the flux off? And if so, does that remove any of the metal under it? And if not, what sort of polish removes flux? I read something about putting the metal into sulfuric acid to remove the flux? If that is, in fact, the best method, then where on earth am I to find h2so4??
As you can see, I'm really confused, and pardon my ignorance! Any clarification you can offer would be GREATLY appreciated!!
AnswerGreetings,
I'm wondering what the simpler projects were and whether they were casting or other metal process.
Anyhow, I'll see if I can walk you through it.
Whenever you cast or anneal metal it's commonplace to soak it in a pickle solution. The solution 10~15% sulphuric acid at 60C will remove fire scale form the surface and also remove melted flux.
There are other ways to remove the flux, but this is by far the simplest and most effective way to achieve this.
The flux has quite a low melting point and stop the hot metal from o=xiding and allows the piece to soldered or in the case of castings, assists in the flow of molten metal, because there is no surface oxide when molten. The best flux when casting is simple Borax powder, which I used to buy form a chemist.....it's also used to remove smell from shoes.
To release a melted thing from the flux - in a crucible for instance - is quite a different thing than the heat required to cast. You'll find the metal wont even change colour the heat is so mild, and no it wont create more oxide. You will find that there is still some flux on the metal and this can be removed with the pickle. It's hard to explain without showing you in person, but I think you're getting confused with the different heats etc. The remaining flux will stay in the crucible and that which is on the metal can be removed in the pickle.
As far as trying to polish off flux........don't even think about it. You'll never achieve it.
You wont remove the metal under it, but you sure polish away the metal around the flux and thus have an uneven surface. Whilst it is lower in melting point, the flux is harder than your metal.
You'll find sulphuric acid at a chemical supply house.
I cant direct you to one because I don't know where you are, but in Australia, the best one for me has been Ace Chemical Company.
Alternatively you can buy pickle solution from a jewellers supply house.
Hope that helps.
Cheers, P
