The reduction and oxidation can be defined by the relative electron change or by the change of the oxidation state of the starting material. During reduction, electrons are added to the starting material, which lowers the oxidation state, i.e. the "willingness" to accept unbound electrons. Since the amount of electrons is determined by the conservation of electrons, oxidation and reduction (redox reaction) always take place simultaneously. Consequently, a reducing agent is required for the reduction, which itself undergoes oxidation. In a rotary kiln, the reduction process is usually carried out in a hydrogen (H2) atmosphere.