Lou said:
Soda ash isn't serving as the reductant itself, in any event. It might serve as the precursor to thermally self-reducing species. I do not think soda ash is effective for cleaning gold any more than would be lime or sodium hydroxide, which work well in the removal of refractory elements from gold, as well as such elements as aluminum, gallium, and silicon. These fluid fluxes often need viscosity adjustments.
It has always been my stance that soda ash should not be included when melting gold, as, if it does have the least bit capability of reducing oxides, by what ever means, that's exactly what one doesn't want to happen, as it then adds anything that's reduced to the metal being melted. In the case of pure gold, that's simply counterproductive.
I'm of the opinion the only flux for gold and silver is borax, and only for lubrication purposes on the crucible. This is for the pure metals, i.e. silver out of a cell, gold out of a cell or right out of a properly done aqua regia process.
Again, due to my years in the lab, I agree, but I also consider a borax coating indispensable when inquarting. If there was enough present to pour with the melted material, there was clearly too much. All I wanted was a thin coating, which behaved as a lubricant, as you alluded. However, that thin coating was quickly contamined with all manner of garbage, as the materials being melted were far from being clean. As a result, my melting dishes slowly became dark, resulting in a sticky coating that would withhold the last bit of metal instead of allowing it to flow freely. It also was a source of prills, as the molten material required a good stirring to ensure a homogenous mix. As a result, I had a policy of adding fresh borax, which I did not try to blend with the old borax, but allowed to form a new surface instead. When I had done this a few times, I would then clean the dish, restoring a clean surface.
I can't iterate enough times the fact that I did NOT allow prills to remain in my dish. There was never anything visible that was allowed to remain in the dish. If I was trapping prills, they had to be microscopic. I think the experience that Rick related parallels those of mine, by the way.
Too often I see (commercial) refiners use a "more is better" approach to fluxing melts of even quite pure metals.
Agreed, and I've posted numerous times in that regard.
A good rule of thumb on the pure metals is that no more flux should be added than is required to 1.) see the surface of the melt bright and shiny, 2.) never add so much that you cover the melt with flux. At the most, a crescent moon of flux. The flux should pick up no real color from the melt
That parallels my experience. In fact, aside from needing lubrication, I've often commented that no flux is required to melt pure gold, and if there is need, it isn't pure. That's precisely why I stared refining mine a second time, as I always had to flux the surface to get a nice color. That something was being removed (oxides of base metal) was evident by the change of color of the flux. That all changed when I started refining a second time. I spite of the fact that my gold contained an elevated level of silver (I now understand that very well, thanks to Lou), the resulting flux was never discolored aside from picking up a purple (more like pink) hue, most likely from colloidal gold. I expect that minute particles were superheated and evaporated, then trapped in the flux. I melted with a Hoke torch, but a very large one, almost identical to a rosebud.
Slags should always be kept to a minimum and examined critically.
Any slag I generated was included in my run of waste materials, where they were subjected to prolonged heating and thinning (I used fluorspar). An assay of the flux that resulted from this operation revealed that the values remaining were too low to warrant re-running with litharge. I was quite happy with that finding.
On the topic of slag, I poured to cone molds, and always checked for prills. I did that for a reason, as I wanted to know that my fluxing was not a problem, but I was also aware that they might go undetected. It was for that reason I remelted my slag when I processed my many years of accumulation of waste material.
Harold
