Upon this rock. . . .
Twenty or twenty-five years ago, we in the business of preserving old, adobe buildings realized that the greatest threat to our favorite building typology was that, 50 years prior, Portland cement plaster had been applied over the traditional mud. It took a few years and a few collapses to get the point across that the hard plaster was letting moisture in but not letting it out. It is an immutable fact that when an adobe block in the base of a wall reaches 14 percent moisture by weight, it will fail.
The loss of historic adobe churches in El Valle and Peña Blanca drove home the heart-rending truth about hard plasters and did much to inspire the drive to preserve our earthen heritage. One of the first points that conservators had to address with owners and caretakers was the necessity of stripping impermeable renders off walls and replacing them with mud or lime.
That process involved two difficult challenges: first, we were asking for the repudiation of two or three generations' worth of commitment to "modern" materials, and, second, we had to provide an alternative that worked. It was during that stage that I performed my most dazzling feats of pseudo-scientific experimentation with lime plasters which, with sheer bravado and borderline incompetence, I convinced a number of victims to apply to their buildings. It was an almost hypnotic time; I could sell the idea of lime plaster as "breathable," durable and good for the buildings, which it is, and convince people to try it despite my lack of credentials.
It speaks volumes about the good people of New Mexico that I was never lynched and, as far as I know, never lost a friend when my Frankensteinian formulae peeled off the walls in sheets.
The problem was the attachment mechanism. Lime and adobe are moderately compatible materials. But unlike mud as a plaster, which is absolutely compatible with the adobes because they share an identical makeup, lime requires a physical link to bond it to the wall. The old-timers knew this, and solved the problem in a brilliantly simple way. Alas, as you will gather upon learning the process, simple does not mean inexpensive -- at least in a time when labor is at a premium.
The term rajuelar is a Spanish verb connoting the application of stone, or rajuela. The application in this case is of small, spatulate, preferably angular and permeable pieces of rock in the mortar joints between the adobes. With the rajuelas protruding, say, three-quarters of an inch beyond the vertical plane of the wall, an effective lath was created. The embedded stones supported the lime plaster.
In my view, the last 20 years of work re-introducing lime plasters has yet to reach the shadowline where it passes from speculative to successful -- interior renders excepted. We simply can't afford to hire people to break big rocks into little rocks and then embed them in a wall. And metal lath is unacceptable because lime is permeable and the moisture demolishes even galvanized wire in three or four years. I have tried synthetic materials as lath, but Mary Shelley's apparition is there to take them down.
I'm actually quite satisfied with 20 years' effort to make lime a successful render. I haven't succeeded by a mile, but the experience has grown my appreciation for those who came before and solved a problem within the means of their time.