I did a boat load of gold and silver refining back in the late 70's.
In fact for several years, The Jewelery section of the art department
was a larger consumer of Nitric acid than the chemistry department,
thanks to my efforts.

The main thing I learned is that it is cheaper in the long run, to pay
someone else to do it.

The problem is that you are burning out the paper filters with the
borax flux. This turns the borax into a glassy substance that seems to
be completely insoluable.

You have to minimize the amount of paper charcoal that mixes with the
flux.

First rinse the filter down well with distilled water until clean and
white and your precipitate is all collected in the bottom of the cone.
Allow the filter to dry completely and snip off the section with the
precipitate. Don't sweat small amounts stuck on the paper, even at
todays gold prices it doesn't amount to much.

Then, when you collect enough filters, get a clean castiron frying pan
with a lid, put the filters into it and roast it until the filters
turn to ash. A little salt peter helps consume the paper.

Then put this residue in your crucible and melt it.

Paul K. Dickman

On Thu, 11 Aug 2005 11:56:56 -0500, Jon Elson
wrote:

Hello, all,

I haven't had much luck on the jewelry newsgroup, so I'm wondering if
anyone here has some goldsmithing experience. I am recovering gold
from electronic scrap, and the current difficulty is smelting the
gold out of the precipitate. I first cook the filters down to ash
in a crucible with a propane torch. Then, I got some cupels to smelt
that ash. I do get gold out of it, but I also get a greenish-black
glassy substance that is impossible to remove from the cupel, and pretty
hard to get out of a crucible, either. What IS this stuff? Is it some
gold-bearing salts that just need to be cooked more to burn it down to
gold? Is it something combining with the flux? (I'm using borax flux
that I got for brazing.) Am I using too much flux, or not enough?
I've already destroyed one cupel, and it is hard to separate the
refractory from the gold and the green glop.

I'm doing the smelting with Oxy-Methylene (generic MAPP).

So, any tricks to make this easier would be greatly appreciated!

Jon