Aggregates: The Key to Successful Mud Plastering
Once a month, on average, someone calls to ask for The Recipe for mud plaster. Sometimes the questions are general and simply inquire after the ingredients and the basic proportions. More often they refer to a particular plaster job that has been admired that we may or may not have done. Whether we have done the work or not, the answer is the same: a "recipe" will do you no good.
A plaster is more than a mix of ingredients. It is a process that includes preparation of the wall, selection of materials, mixing and application. We have done exterior, parapeted walls in mud plaster that we have successfully warranted for ten years. (These are walls, mind you, with no snake oil weather-resistant chemical additives; just well designed muds.)
When folks hear about these muds they want to know how it is done. More accurately, they want to know how to do it themselves or have their contractors replicate it. I have absolutely no problem with that and have never considered knowledge proprietary. I have had difficulties giving free advice and getting a call a few years later to report that, following my prescription to the letter, the plasters that resulted were a dismal failure. The fact that the advice was free saves the day. Had I charged for the service I would really get an earful.
There are many reasons that no single recipe or approach exists for mud plaster. These are not materials that you buy in a bag, just add water and apply. And they are seldom, contrary to local myth, soils directly from the site upon which the building sits. Even historic plasters in the majority of instances contain one or more components that were imported. The source may have been only a few arroyos over, but nevertheless it is a rare day when you can dig a basement and make your adobes and mud plasters from the spoils.
Since books have been written on the subject, and the subject is complex enough to warrant them, I cannot in this short space provide more than one fundamental to help the do-it-yourselfer get under way.
Perhaps the most intriguing aspect of mud plasters is the aggregate. The durability, or weather-resistance, of a mud plaster does not come from the clay, which is comprised of very fine particles, susceptible to erosion. The only purpose of clay is to serve as a binder. The strength is in the aggregate which is to say, the sand.
I seldom see a plasterer, even those who consider themselves "traditional," use anything but clean No. 8 masonry sand straight
from the materials supplier. Using such stuff is, in my mind, the single most egregious mistake in mixing mud for exterior use. The aggregate should have a range of sizes evenly distributed, more or less, from silt-sized particles to 3/8 or even half-inch gravel. The reason is that you want to fill the spaces.
Imagine, or better yet do the experiment with, two fishbowls. Fill one with marbles; shooters from the toy store, all the same size. Fill the other with roughly equal amounts by volume of shooters, boulders and pee-wees and then mix in various sizes of beads and a handful of sand. Then try to stick your fist into the "aggregates."
You will have no difficulty displacing the uniformly sized marbles and reaching the bottom of the fishbowl. The last time I did this in real life, the 18-year-old stud who accepted the challenge for fishbowl number two (stubbornly) broke the bowl and sent my multifarious collection of spheres spinning across the parking lot.
Imagine those aggregates in plasters and judge intuitively which will be the more durable.