Non-Chemical help on how to produce a residue free gold assay sample

[Anonymous]

Hi-ya everybody,

I am just setting up premises for testing and assaying gold using XRF and fire assaying, ( for my own personal requirements)

i would appreciate any advice on how to produce a good gold bead sample to use on the XRF,

the sample's are raw unprocessed natural gold, nuggets, dust, etc with quartz and black sand thrown in for good measure by the miners,

i am using oxy- acetalyne to melt the samples in some small white crucibles i purchased from a jeweller's supplies in the UK , but the problem is residue on the bead after melting,

at first i thought it was borax, so i melted the sample in a new crucible using no flux at all, and i still get a residue on the surface,

i need the sample to be free from residue's because i also need to determine the melt loss % ,

i tried dropping the red hot bead into cold water to shock the residue off but it's not been completly succesfull,

can any one tell me a brain dead easy solution.

Regards

CongoGold

 

[Harold_V]

Without knowing of what the residue is made, it may be hard to provide good information.

If your button has no silver, or the silver content is very low, you might consider boiling the button in dilute sulfuric acid. If that doesn't work, try HCl, then try nitric. One of them is likely to dissolve the material without dissolving any of the gold.

I'm having a hard time picturing something that comes from the gold alone. Surely, you're not heating the material to the point of melting silica?

Have you considered pouring the material to an assayer's cone mold? That usually makes it much easier to separate the values from any flux that may be present.

Harold

 

[Lou]

It sounds like surface oxides.


Try melting in an inert atmosphere if you don't want those to form.

 

[Irons]

Pickling the bead in acid before XRF would probably give a false result as the surface would be leached of base metals, leaving a surface rich in Gold and PGMs which might not be representative of the bead in total.

 

[Harold_V]

It sounds like surface oxides.

That makes sense. I was assuming something that was thicker.

The lack of oxides was one of the ways I used to judge my refined gold. If it remained shiny after melting, I knew I was close to pure.

Harold

 

[goldsilverpro]

I would try quenching the red hot bead in about 10% H2SO4, or stronger. I agree with Irons if something like nitric is used. I doubt if there's anything in the bead that would be attacked by the H2SO4.

Is the residue glassy or a surface stain? Does it coat the whole bead? What is the color?

 

[qst42know]

I have a question not a recommendation.

If once melted the oxygen was shut off or turned way down sooting the bead until it set. Would that prevent the oxides from forming? The resulting soot could be scrubbed off with a brush and soapy water.

Another question isn't soda ash used in conjunction with borax to reduce oxides? You want it all as metal, contaminates included for the XRF to detect.

 

[goldsilverpro]

qst42know,

Harold and I have battled the question, "Does soda ash reduce the base metal oxides to metal", many times. I say no and Harold says yes. I don't want to rehash the argument. I just don't think what you're suggesting will work.

 

[qst42know]

Even if it did or didn't reduce, a fluid flux puddle that could cover the bead when the torch was removed could prevent the oxides from forming, yes?
Much as when a cone mold is poured. This may be a good reason to melt in a disposable cupel as well. The cupel could be broken to retrieve your button.

 

[Irons]

Unless you know your source very well, many sources of 'Gold' from Africa are nothing but scams.