My experience "piping" materials was in the context of a rotary furnace. There is no hard set rule on copper and base metals but generally the big enemy is iron. You want the shot to be nonmagnetic. I've tried over the years quartz (expensive but resistant to oxygen and works well if IN the melt and not exposed to corrosive metal oxides, in particular CuO!), alumina (breaks very easily) but the only thing I've ever seen hold up is preheated zirconia blowers like what they use for steel.
I've done with sterling and with copper and it was always blown after the borax or flux was added. The flux is the receiver for the oxides so if it's not there you can't blow oxygen. If the melt is high in iron you will actually generate some heat in the melt pool if doing this in a rotary. At one place I rep material at, it's usually 3000-4000 lbs Cu, half as much borax, roll it around 3-4 hours then check if some of the material is magnetic from iron. Zinc is already out at that point. Then 1" diameter schedule 40 iron pipe and oxygen...
I've done with sterling and with copper and it was always blown after the borax or flux was added. The flux is the receiver for the oxides so if it's not there you can't blow oxygen. If the melt is high in iron you will actually generate some heat in the melt pool if doing this in a rotary. At one place I rep material at, it's usually 3000-4000 lbs Cu, half as much borax, roll it around 3-4 hours then check if some of the material is magnetic from iron. Zinc is already out at that point. Then 1" diameter schedule 40 iron pipe and oxygen...
