Assuming that you are asking about an instructable, the best place to ask is in the comments section at the end. That way the people will know what you are talking about : )
That said, a well built waste oil furnace can melt iron, so over 1700 C/3000 F (1538 °C/ 2800 °F is the melting point of iron, pouring temp is several hundred degrees over that).
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don't get it above the melting point of the material it's made ofjust keep it under 5000C
unless it's made of aluminum, which melts at 660oCso, if it's made of aluminum, keep it under 300oC