Tumacacori national historical park - Lime kiln

Lime plaster was used to protect adobe buildings from moisture. Tons of raw material (limestone boulders) had to be brought to the mission for processing so that a coat of plaster, often more than two inches thick, could be applied to the walls. Limestone had to be gathered from distant mountain drainages or, like the timbers in the roof, transported some thirty miles from the Santa Rita Mountains to the east. Once here, the limestone was loaded onto a heavy metal grate that rested on the shelf you see about halfway down the wall of the lime kiln. Fire was placed underneath the grate and the rocks were "cooked" until they began to swell and break open. At that point they could be readily hammered into a powder. The powder was then "slaked" by putting it in water for a day or two.

After the powder was "hydrated," it was made into a paste, sand was added to make plaster, and it was spread onto a building's walls to dry. Though course and lumpy by today's standards, it was (and is) the best protection possible.