To correct Ian:
Borax was routinely used in the fire assay of gold and silver between 1973 and
1989 when I worked with Capper Pass. we conducted something like 200
commercial assays a month using flux containing borax. During that time I have
visited assay houses and other smelters some of whom have incorporated borax
into fluxes. It was a matter of established practice so I don't know the
specific reason but would guess that it gave a cleaner pouring slag when dealing
with samples from a wide variety of sources and where you need a much more
versatile flux that lead silicate.
Richard
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