Organic materials being indirectly heated via hot air having the temperature required for their treatment, the process consists in the recycling of the vapours and gases evolving from the organic materials into a hot air circuit by forcing them into contact with the flame of a burner in order to burn the organic gases contained therein. The apparatus for executing the process comprises a double wall sealed tank; the outer wall being insulating and the inner wall being heat-conducting, the space, created by the walls, defines the hot air circuit.