By the use of a closed chamber and a continuous flow of gas through this chamber a controllable welding atmosphere has been provided for experimental work. The atmosphere may be varied from 100% argon, 99.95% pure, to 100% 0(2), N(2), or any combination of these or other gases. The chamber provide a fresh gas shield in an atmosphere of the same composition as the shield. Using the controlled atmosphere chamber, welds in carbon deoxidized arc cast molybdenum are shown to be subject to hot short cracking when the oxygen content in the argon atmosphere around the arc exceeds 0.2%. The ductility of these welds was drastically reduced by the presence of more than 0.05% oxygen in the welding atmosphere. Oxygen in sintered molybdenum is shown to produce porosity and hot short cracking. However, crack and porosity free welds have been produced by using deoxidizing agents in sintered molybdenum.