Preheating furnaces gradually heat invested casting rings or flasks before metal casting, burning out wax patterns and bringing the investment mold to the correct temperature to receive molten metal. Controlled preheating prevents thermal shock that could crack the investment, ensures complete wax elimination to avoid casting porosity, and helps achieve the dimensional expansion needed to compensate for metal shrinkage on cooling. This step directly precedes casting in the conventional lost-wax metal framework workflow and is essential to producing accurate, void-free copings, crowns, and removable partial denture frameworks.