Natural degradation of YBa₂Cu₃O_7−δ powders leads to trapping of CO₂ by the powder. The presence of residual carbon in ceramics or textured materials can strongly affect the superconducting properties. Optimal conditions for carbon elimination during thermal treatment are defined. Amounts of carbon lower than 500 ppm are easily obtained when thermal treatment is performed in temperature, gas flowing rate, and heating ramp conditions allowing CO₂ departure before pore closure. Liquid phase formation and the presence of BaCuO₂ are limiting factors. An oxycarbonate formed by reaction between this latter compound and evolved CO₂ is implied in the last step of the decarbonation process.