Racks of concrete block are placed in the steam-curing kiln at normal temperature for a holding period of 1-3 hours. This "presetting" allows the blocks to harden slightly. Steam is then gradually introduced, increasing the kiln temperature incrementally, until peak temperature (usually 165-170 degrees) is reached. The blocks are then allowed to "soak" in the steam for 12-18 hours. Finally, the moist air is exhausted to allow the blocks to dry.