The ravages of Time

Wind, rain, summer heat, winter cold all take their toll on the walls of the San Xavier del Bac Mission. But unlike many other structures that have long passed into oblivion, San Xavier del Bac is renewed through constant renovation. To give you an idea of the passage of time, here is a photo by C.R. Savage from the late 1800s:

The photo is in the form of a stereoscopic view and comes from the New York Public Library, Stephen A. Schwarzman Building / Photography Collection, Miriam and Ira D. Wallach Division of Art, Prints and Photograph. If you click on the old photo, you can that even in the 1800s the buildings were old and in need of repair. The building has survived two earthquakes and thirty years of abandonment during the 19th Century. The restoration efforts make for interesting reading. See Conservation at Mission San Xavier del Bac by Jason L. Metcalfe. Here is a quote summarizing the efforts to preserve the building taken in 1989:

In 1984, a whitewash of portland cement was applied to the exterior walls and a coat of white elastomeric emulsion paint was applied to the entire roof to seal it. The paint was supposed to be impervious to water; it worked for awhile, however, with the intense southern-Arizona heat and sunlight, the acrylic eventually deteriorated. The paint film shrunk, developing small fissures which allowed rainwater to penetrate, trapping the moisture under its surface. In 1988, when Robert Vint (the architect in charge of the exterior conservation) examined the roof vaults, he "peeled up" a small section of the impermeable roof emulsion to reveal thriving mold growth in the dark, damp, and warm environment beneath.
In 1989, to remedy the situation, Vint had the acrylic emulsion and layers of cement removed. Once the original masonry was exposed, he employed a traditional technique learned from his mentor, Mexican architect Jorge Olvera. Two layers of lime and sand plaster (free of soluble salts), bound by mucilage extracted from the nopal or prickly pear cactus, was applied as a "breathing" protective coating. The mucilage was prepared by boiling pads of prickly pear cactus in large kettles and mashing them to obtain the gooey extract used to mix with the mortar. The mortar was also applied to the exterior surfaces of the mission, which were then burnished with smooth river cobbles (also a traditional technique) to compact the surface and reduce the rate of water absorption.³. In addition to the burnishing, a hydrophobic material (aluminum stearate) was added to increase the water repellence. There are problems, however, with the use of a hydrophobe on top of a plaster. Cracks that develop in the plaster as a result of thermal movement become drainage channels when the hydrophobe and gravity convey water across the damaged areas. Consequently, the use of a hydrophobe has been discontinued in the current repairs. Notwithstanding, it is now felt that, overall, the traditional method is best for the long- term benefit and structural integrity of the mission. The lime and sand combination is most compatible with the original brick; however, the finish will need frequent reburnishing and whitewashing: the key to protecting a traditional preservation system for clay bricks is maintenance.