The metal cylinders filled with investment material, so-called "flasks", are burnt overnight in an oven. It is important to follow certain heating curves to prevent damage to the investment material. As explained in the previous steps, the wax parts inside the flask are a replica of the customer's model. When the flask is heated, the wax starts to liquefy and runs out of the flask. What remains is a plaster cast, which now surrounds a cavity instead of the wax. This creates a negative mould of the model, which can hold the precious metal to be poured in.
If the customer wishes to use wood-like natural products, for example, instead of wax, liquefaction by heat is not possible. These models then burn and the desired cavity is also created. However, ash residue may remain which has a negative influence on the casting result. The same applies to poorly burning plastics or stereolithographs from 3D printing. Residues can also result from the ashes here. In extreme cases, the model may even corrode and damage the investment material during burning.