Abstract: A blast cabinet having at least one cabinet door opening into the blast chamber includes a pneumatic interlock between the door and a valve through which air is supplied under pressure to a gun within the chamber. The valve is actuated to an open position by air under pressure admitted under the control of an operator. Incorporated in the tubing system through which the air under pressure is directed to the valve is a header which is left in communication with ambient air whenever the door is open, to depressurize the supply of air to the valve and thus prevent it from opening under these conditions.