Abstract: Apparatus for blending dry or slightly moist steam with water to produce wet steam of 20% to 70% moisture content by weight for treating logs or other objects in a closed steam treatment vat or vault. The apparatus includes a blender having a mixing chamber, to one end of which comparatively dry steam from a suitable source is supplied at constant pressure and flow rate through a pilot-operated pressure reducing main valve and a metering orifice. The blender also has a port through which water at a constant pressure and volume is supplied to the mixing chamber by a positive displacement pump maintained under a constant gravity head of water in an elevated supply tank. The outlet of the blender is connected to a back pressure regulator for controlling the outlet pressure of the blended mixture of water and steam to be supplied to the vault. The pilot valve is connected across the main steam valve and has a solenoid valve connected in the line supplying operating fluid to said main valve.