Abstract: In the production of silicon articles at an elevated temperature, a stream comprising a controlled mixture of an oxygen-containing first gas and a second gas is admitted to the processing chamber. The first gas is one which partially dissociates under the conditions in the chamber to form both oxygen and the second gas. The second gas is one which is not harmful to silicon at the conditions in the chamber. Substantially equilibrium conditions are established in the chamber so that the dissociation of the first gas to oxygen occurs reversibly. The partial pressure of oxygen (P.sub.O.sbsb.2) is sensed in the chamber during processing of the article. In response to the P.sub.O.sbsb.2 level, the ratio of the rates of flow of the oxygen-containing gas and the second gas is adjusted so as to maintain the P.sub.O.sbsb.2 at a level less than about 10.sup.-6 atmosphere, and usually no greater than about 10.sup.