I think I'm leaving the roof on, Johnny!
About using soda ash when melting pure gold.
Long ago I advised a method for cleaning badly coated and sticky melting dishes. It involves the use of soda ash, melted, which then alters the state of the coating on a dish, returning the dish to a useful condition, albeit at the price of some thinning of the dish. Soda ash has a strong appetite for dissolving silica.
As everyone knows that reads my ravings, I am not an educated person, and I certainly have no credentials where chemistry is concerned.
Having stated the above, one of my observations when cleaning melting dishes is that the flux coating, which had darkened considerably and is very sticky, slowly forms beads of metal---beads that are never evident prior to treatment. From this I have drawn a conclusion, perhaps wrongly, that soda ash reduces oxides of metals. GSP says that's not true----and I respect his knowledge----but something has to explain where the button of metal comes from-----and the fact that the color of the flux slowly lightens as the bead forms is pretty damning.
For the record, I am well aware of prills, none of which are ever evident prior to the soda ash treatment.
OK, now that I've gone through all that---my point is that pure gold requires no flux. If you need flux, your gold is questionable in quality.
I readily admit that adding a trace of borax is helpful, for pure gold never really is----it's just that the percentage of contamination is so small that it ceases to be troublesome. It is for that reason, along with allowing the molten gold to flow freely, that I advise a thin coating of borax on a melting dish, with none used in addition aside from an occasional light sprinkling when the dish starts getting sticky. I followed that routine for years and had excellent results.
Back to the use of soda ash when melting pure gold. I never used it, and I still speak against the idea.
If my observations of metal forming when soda ash is applied to dirty flux is correct, what soda ash does is reduce oxides, returning them to the values, which is exactly opposite that which is desired. Borax alone will absorb the oxides.
Until I have conclusive evidence that I am wrong, I will maintain that soda ash has no place in melting pure gold, that if it improves the appearance, it's strictly by recombining the oxidized base metals with the gold, lowering it's quality while making it have the appearance of being pure.
Edit: If you want a thin flux, use some fluorspar. It makes water thin flux, and it takes very little to accomplish the task. Be advised that it is correspondingly hard on crucibles and furnace lining.