Glazing silica dishes is important, as metals tent to stick to the silica surface. Mainly ones with low melting point, as metal does not have that much room to shrink besides silica. Two different coeficients of thermal expansion (metal vs. silica) cause the bead to naturally separate from the dish in many circumstances. Like melting PGMs in silica - it is relatively OK and if you does not fill the crucible too much, it can be easily pried out with tweezers or dull knife.
You don´t want borax glazing when you are going to high temperatures, as it will significantly dissolve silica and create very thick viscous "slag", and eventually turn the bottom of the dish to jelly like material, rendering it essentially useless for melting.
Opposed,
you want to glaze dishes with borax for melting common metals (gold, silver, copper, lower melting base metals and alloys) as it is much easier to coalesce the molten metal to one bead/puddle, as drops of molten metal can slide better on the walls of the dish.
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For melting, insulation is crucial. Propane/air torch physically cannot reach that high temperatures, so the heat transfer to the melting metal isn´t that great - temperature gradient is very small. So it often took ages to heat up, despite you are pumping big power into the furnance. That is why
it is important to retain as much heat in the crucible as you possibly can. Kaowool is perfect for this, and some types of firebricks can also do the trick - "foam" ones, which are not solid, but made of refractory clay/cement foam.
Solid firebricks have significant coeficients of heat conduction, so they suck out the heat from the molten metal very quickly.
Minimal insulation requirement is from the bottom, you need to place the dish/crucible on some refractory material with good insulation properties. Then comes the sides of the dish to minimize cooling from the sides. Last is the insulation from the top, as radiation heat loss from infrared spectra is very significant at issued temperatures. This is sometimes hard to do, as you need to place burner somewhere etc... But essentially, with good bottom and sides insulation of the dish, you can melt gold relatively easily even with just propane/air torch.
It is also good to consider that still air is one of the best insulators you can have, so minimize direct contact of the heated object with any worse insulators like bricks, tongs, dish holders etc..
And just make sure it isn´t windy outdoors
As slightest wind can ruin all the work when heating the metal to the temperature
Good luck melting
