Abstract: Masonry units are cured in a kiln having a gaseous atmosphere including water vapor. A portion of this atmosphere is withdrawn and mixed with a flowing stream of heated water. This mixture is injected into a body of water disposed within the kiln at a plurality of spaced apart locations and beneath the surface of the body of water whereupon the gaseous atmosphere is heated and rises to and escapes from the surface of the body of water in the form of bubbles. In a preferred embodiment, the apparatus for withdrawal of atmosphere from the kiln includes an aspirator incorporated in a conduit which also circulates heated water to the body of water within the kiln.