number of apertures 9 which provide open communication between the plenum chamber 5 and the treatment chamber 3 the plenum chamber 5 is adapted to receive gases at a gas receiving port 11 and exhaust them at a gas discharging port 13 the plenum chamber 5 has a radially outward insulated wall 15 which spirals inwardly from the gas receiving inlet port 11 to the gas discharging outlet port 13 as can be seen in the drawings the spiral of the outside wall 15 provides a width wi adjacent the gas receiving port 11 and a width wo adjacent the gas discharging port 13 fig2 shows an insulated cover 17 which may be raised and lowered by means not shown to provide access to the treatment chamber 3 to put in and take out work pieces circulating gases are exhausted from the interior of the treatment chamber 3 through centrally located exhaust ports 19 which in turn communicate with exhaust conduit 21 a work table 23 is located within the treatment chamber to support work pieces as here illustrated the work table 23 is mounted for rotation which can if desired be spun to provide centifugal forces for rotational or centrifugal casting of work pieces undergoing treatment in chamber 3 in operation the cover 17 is lifted and work pieces are positioned on the work table 23 the cover 17 is then closed and air at elevated pressure which has been heated or cooled to a desired treatment temperature is introduced into the gas receiving port 1.