The main reason behind heating the powdered gold is to ensure complete elimination of:

Powdered gold must be heated in a flame to ensure the complete elimination of the wax. When heating in bulk on a tray, an excessive amount of gold should be avoided because of the difficulties that prolonged and repeated heating can create. Care should be taken to handle pieces with stainless steel wire points or similar instruments that will not contaminate the gold. Ref:Phillip’s Science of Dental Materials ed 12 pg 414