In our previous article, we were introduced to binders, the “mineral glue,” that holds our plaster together. Now it’s time to consider the “things,” the aggregates and occasionally fibers that typically compose the greater percentage of the volume and mass of a plaster. While it’s true that most aggregates and fibers are less costly than our precious binders, they’re certainly not cheap fill material. Rather, they play a very active role in the strength and performance of plasters, stuccos and mortars. Furthermore, they contribute significantly to the aesthetic quality of the finish, that is to say, how pleasing to the eye the plaster will be. Let’s approach our consideration of aggregates and fibers by considering their various physical properties.
It seems intuitive that if your aggregates are sharp and jagged, it will be easier for the binder, such as clay or lime, to find a “key” or “bond,” which are terms we use to describe how plasters develop strength internally and on the surfaces they are applied. Aggregate sharpness can be very important for clays and limes, which shrink as they dry, threatening to crack or otherwise weaken the plaster. Crushed sand and glass are examples of very sharp aggregates that can help overcome this weakening effect, permitting the application of thicker base coats.