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ideally you want to cupel at 1750°F and your melt furnace exceeds that temperature with little ability to control it closely. You need more lead to effectively cupel the brazing rod for a 1 gram sample try 15 grams of lead.
But the roaring sound tells me you have an oxygen rich environment in the furnace and you are losing more Lead to the air than is being absorbed into the cupel. And the lead oxide is what absorbs the base metals and drags them into the cupel.

Too hot, too much air turbulence in the furnace, and not enough lead.

[QUOTE="4metals, post: 317146, member: 6914"] [I]ideally you want to cupel at 1750°F and your melt furnace exceeds that temperature with little ability to control it closely. You need more lead to effectively cupel the brazing rod for a 1 gram sample try 15 grams of lead. But the roaring sound tells me you have an oxygen rich environment in the furnace and you are losing more Lead to the air than is being absorbed into the cupel. And the lead oxide is what absorbs the base metals and drags them into the cupel. Too hot, too much air turbulence in the furnace, and not enough lead. [/I] [/QUOTE]

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