Heating in normal atmosphere. This method is low cost, high throughput and well controlled, also larger pieces can be made.
Heating in controlled atmosphere, can be in inert gas including Ar, He and Xe, as well as reactive gas including Methane, Benzene and Hydrogen.
Very rapid heating by inducing electric eddy currents in the metal. Usually used in continuous production.
Heating in high vac or ultra high vac ensures no atmosphere interaction. This method makes sputter cleaning and deposition possible.