I did not realize that a 15% brazing bar is high in copper. Generally for cupellation materials high in copper (as in copper based bullion) benefit from an assay process called scorification. Whereby a high copper sample is dissolved in a bath of molten copper in a melting dish with a little borax sprinkled on top. This isn't as difficult as it sounds, just add granulated lead, your sample, some borax and heat it, usually for 45 minutes to an hour while driving the lead by keeping the door ajar. This process is effective in driving the copper into the slag layer that forms. The remaining molten lead which holds the PM's is poured into a cone mold and cupelled normally after it has had any slag removed.