Molybdenum and graphite are the most popular choices as hearth materials in vacuum furnaces. Throughout the last 40 years, the heat treating industry has experienced a significant shift in the type of hot zone most commonly used in furnaces. In the 1970s, most of the heat zones installed were all-metal hot zones. Today, however, the vast majority are graphite-based hot zones. But, what is the most efficient design in order to minimize hot zone losses and reduce power consumption?