Overworked Plaster: Breaking the Second Commandment
In a previous column I touched on the importance of aggregates in the success of earthen plasters. The upshot was that in order to fill the spaces between particles to achieve a durable render, the aggregate must have a range in size from the very fine to the very coarse. I consider this the most crucial aspect of plastering with mud; it is the first commandment of mudding.
That's one of several purely technical aspects of plastering. Another important component is technique, the methodology of applying the mix. Where the mix is a specification, able to be derived very accurately through measurement of physical attributes, the application is subjective and varies with personal style. Nevertheless, there are principles that should be used to guide the process. Chief among them is maintaining that nice mix of aggregate that you have designed into the mud. That is the second commandment of plastering with earth.
For some reason it has become "traditional" in the Southwest to trowel mud onto the wall. Yet one only needs look at photographs from Taos or Jemez a mere 50 or 60 years ago to see the ejarradoras (yes, they were mostly women) throwing wads of mud onto a wall, pressing it smooth with the heel of their hands and finally finishing it out with the swipe of an open palm. Three steps: casting, smoothing, finishing.
Europeans do something similar. A British craftsman will cast the mud (or lime) onto the wall with a Harling trowel that looks like a little, one-handed coal shovel. The Italian and Spaniard (and by derivation the Mexicans, Salvadorans and Guatemalans) will cast it on with the flick of an 8-inch, pointed masonry trowel. The Brit will wait until it sets up a bit and then go back with the same Harling trowel and "scrape it down," taking off the high spots.
The southern Europeans (and southern Americans) will use a screed made of a straight board about a yard long to accomplish the same task, but before it gets quite as stiff as the Brit would like. Depending on the desired appearance, either technique will then progress to the finish, which may be another, thinner coat of plaster, or a wash.
Unfortunately, the state of the art in industrialized, Santa Fe-style adobe production is to mix the mud with no particular care, then smooth it out in a few strokes as you would a Portland cement-based plaster. All with a polished, flat, rigid, steel, plastering trowel.
So what's the problem with that? Well, if you spent any time designing in and locating a nice range of aggregates for your mix, you just screwed it up. Each time the hard face of the trowel is passed over the damp plaster, suction pulls water to the surface. You can actually feel it; the trowel will firmly adhere by surface tension of the moisture in the mix. That is why the trowel, in the hands of a fast plasterer, is slightly cocked during the stroke: the tension is broken.
But by then the harm is done. The water does not come to the surface alone; along with it come the fines of both the binder -- the clay -- and the aggregate. Displacing both is a double whammy since the glue has been drawn out of the core of the mix, and the aggregate has lost the fine filler in the little spaces between particles. The plasters you have seen that erode requiring that infamous once-a-year maintenance, are likely those that have not been designed with the aggregate in mind, or that have been overworked before they even got to the job.