Your electric furnace struggles to reach the melting temperature of copper any foreign material on the copper surface will reduce the heat reaching the copper in addition any heat escaping the crucible will lower the performance of your furnace.
The addition of a fluxing agent would be the first to melt once liquid becomes a wetting agent transferring heat to the melt also the flux would pick up any oxides absorbing them into the slag.
This list of flux's is the most commonly used, some like Borax are used alone or in combination with other ingredients to achieve different end goals.
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