Whilst I think the "borak" explanation sounds close to the mark, thought I might cloud the issue while I still could.

A major use of borax is (as Jackie says) in mining or, to be more correct, as a flux for the smelting of mineral ores, particularly gold. It's not impossible to imagine a hopeful prospector poking the borax and mineral concentrate in the crucible to get an idea of how much gold he'll have once it's poured into the mould to make an ingot.

Another idea is that this may have some reference to an old time laboratory process that uses a bead of borax, a blow pipe and a bunsen burner to identify minerals. As I recall from high school chemistry (25 years ago now), a sample of the finely ground mineral is placed on a carbon block next to some borax powder. Air is blown through the flame via the pipe to melt the borax, which is then poked onto the mineral sample, engulfing it. The composite bead thus formed is collected in a loop of fine wire, held to the flame and subjected to an air blast. The colour of the flame that comes off the bead is indicative of the mineral present - the basis of spectroscopy and Fire Assaying (I think).