The temperature drops dramatically as you step through tall church doors into the cavernous interior. The ancient five-foot-thick walls have the dignity of living ruins. Where plaster is missing, you can see graying adobe bricks, and the painted decorations on the whitewashed walls have faded. Yet the Tumacácori mission still seems to breathe, and it still provides sanctuary from the shimmering desert heat.

The tower bells toll, and 10 gray-haired men and women enter, singing in Latin as they make their way to the altar past the dozens of people in the transept. A man in a black cassock and wide-brimmed hat addresses the audience with a heavy Spanish accent. He introduces himself as Father Eusebio Francisco Kino, the Jesuit priest who, beginning in the 1680s, founded a chain of Spanish colonial missions in the Pimería Alta -- modern-day southern Arizona and northern Mexico.

From the 16th to the 18th centuries, Jesuit, Dominican and Franciscan priests established hundreds of missions as far north as San Francisco. The missions served as bases for the Spanish effort to colonize and convert Native Americans and secure the sprawling empire of New Spain. They typically included crops and livestock as well as living quarters (conventos) and a church. Father Kino visited this spot in Arizona a few times, and Franciscans began building the current church in 1800 near the site of a previous Jesuit church. Construction was never finished, but the church is now the centerpiece of Tumacácori National Historical Park, 60 miles south of Tucson. The man in the cassock is really Don Garate, the park's chief of interpretation, who portrays various historical characters to highlight the mission's cultural value.

Soon, Garate changes into a red outfit with a blue cape and a jaunty black hat: He has become a Spanish captain, Juan Bautista de Anza. He poses for photographs, his white handlebar mustache dancing above his smile. "I can bring history to life inside that building a lot easier than I can in an auditorium somewhere," he says, pointing to the church. "That's the beautiful thing about a place like this -- you do have the ruins, the actual building they were in. So that's why you have to do preservation."

Yet despite their importance as tangible symbols of a thorny history, the Southwest's missions survive today in a wide range of conditions. Some are just decaying piles of melted adobe. The National Park Service and other preservation experts are still learning how to take care of them, using earthy materials such as mud and cactus glue. And there's an ongoing philosophical argument: Do we honor history more by arresting its progress or by letting it run its course -- even if it means the loss of historic artifacts?

Slap! Whap! Pedro Tanori throws handfuls of mud into a rectangular wooden frame and carefully presses the mixture of clay, sand and silt into it. You have to keep it flush with the ground to make clean edges on the adobe bricks, he explains. Once he removes the frame, he'll let the bricks dry in the sun for three or four weeks. "Two guys can make about 200-300 adobes a day if they're working pretty quickly and the mud is being made for them," he says.



Images from the restoration of the San Xavier mission.

Tanori is narrating a video about making adobe bricks. It was filmed during a 2008 workshop at Tumacácori -- a fitting venue, since more than 90,000 adobes went into building the original church. Missions  Initiative -- a group of academics, Park Service staff and craftspeople who support using traditional materials to preserve missions -- ran the workshop. They're working on a comprehensive list of all Spanish Colonial mission sites in the borderlands as well as a manual of the best preservation practices and instructional videos like this one, all posted on a bilingual website. Other hands-on workshops have been held over the last 15 years or so on both sides of the border.

"Adobe preservation is so specialized -- there are really no written manuals," says Pat O'Brien, a National Park Service cultural resource specialist in Tucson. "One of our biggest concerns is losing traditional methodology," says David Yubeta, Tumacácori's former exhibit specialist. "(We're) trying to get people to remember how to do it like they did in the old days."

Since Tumacácori became part of the Park Service system in 1916, it has served as an experimental site for preservation techniques. Its walls record the changing philosophy. Early on, the agency believed that structures had to be fully restored -- even rebuilt if necessary -- for visitors to understand and appreciate them. But over the years, a different approach has emerged. It holds that historical authenticity is better served if ruins are stabilized and preserved, but not completely rebuilt.

Preservation crews constantly search for better ways to extend the lifespan of ruins. From the 1930s through the 1970s, they used synthetic resins, latex paints and Portland cement. But those are not compatible with natural materials like adobe mud and lime plaster, which expand and contract from moisture changes in what Yubeta calls a rhythm of "breathing and moving and twisting and turning and dancing and doing the hula-hoop." In many places, cement-hardened adobe walls eroded internally to such a degree that they became little more than hollow shells. The church at Tumacácori suffered from those methods; rebar was even drilled into walls to keep the plaster in place. "Things the NPS did come back to haunt you," says Yubeta. "At the time they were done, that was the best method, but what they didn't realize was the incredible damage it caused."

The Park Service began to take an even more hands-off approach -- simply stabilizing ruins with mostly natural materials -- in the late 1970s. That's apparent at Tumacácori's sister missions -- Calabasas and Guevavi -- which share the 360-acre site along the Santa Cruz River and were brought under Park Service management in the 1990s. At Guevavi, the roof fell in long ago and has not been replaced.

"We view buildings as artifacts," says Jeremy Moss, chief of resource management at Tumacácori. The value lies in the oldness, he adds. "With every restoration project, you're masking the original. ... It's hard to impart to people the importance of the places in the past when the material isn't from the past."