A process for treating a piece of tantalum or of a tantalum alloy, which consists in: placing the piece in a furnace and heating the furnace under vacuum at least at 1 400° C.; forming a carbon multilayer in the peripheral part of the piece, by injecting, in the heated furnace, a gas carbon source at a pressure ≤10 mbar, the multilayer comprising at least one layer C1 of tantalum carbide, which is located at the surface of the piece, and two layers C2 and C3 comprising a carbon content lower than the carbon content of the layer C1; stopping the formation of the multilayer by cooling the piece; placing around the piece a device capable of trapping carbon, oxygen and nitrogen to protect the piece from carbon and oxygen and nitrogen traces present in the furnace; causing the diffusion of carbon present in the layer C1 towards the layers C2 and C3, by heating the furnace under vacuum, the piece being held in the protecting device; and stopping the diffusion of carbon in the piece by cooling the piece under vacuum before the carbon present in the multilayer reaches the center part of the piece. Thus, a piece the surface of which is free from TaC, the center part of which is free from carbon and the part of which located between the surface and the center part comprises tantalum and carbon is obtained.