Lime plaster was used to protect the adobe from moisture, while still allowing it to breath.  Tons of raw material in the form of 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.  Like the timbers in the roof, limestone had to be transported some thirty miles from the Santa Rita Mountains to the east. This was most likely accomplished by means of ox carts, traveling through some thirty miles of the most dangerous Apache country in the world.

Once on site, the limestone was loaded onto a heavy metal grate that rested on a shelf 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.  Once the powder was "hydrated", it was made into a paste, sand was added to make plaster, and it was spread onto the walls to dry.  Though coarse and lumpy by today's standards, it was - and still is - the best protection possible for sun-dried adobe.

Much the same process is followed today by Park Preservation Specialists as shown in this photograph.  When cracks develop or when lime plaster falls off, workers fill the cracks and then replaster the walls using the same materials and techniques as the early church builders.