Binders? That sure sounds technical, complicated and potentially boring. You might be afraid to ask “what exactly is a binder?” Fortunately there is a simple, interesting explanation: binders are the ingredient in plaster that, as the name implies, “binds” everything together. No doubt, you remember building castles on the beach as a child. Those towers of sand and water weren't all that durable. After the first wave came along, your castle was mush. If only you had added a binder, the “mineral glue” that holds everything else together!
As you might imagine then, selection of the binder is very important for a plaster. More than any other component the binder determines how a plaster will perform, how breathable or hard it will be. In fact, most plasters are named after their binder such as a “gypsum plaster” or a “lime stucco”. In this article we'll give a brief introduction of the three traditional mineral binders: clay, gypsum and lime. For thousands of years, until the middle of the 19th century, these were the only binders used for plaster in traditional building.
Clay is the original binder and the oldest material associated with construction. How is that so? The primary reason being that clay is extremely abundant and easily accessible. Millions of years of erosion has ground hard minerals like feldspar and quartz into the mass of fine, sticky particles that we identify as clay. Typically, all one has to do is dig two or three feet through the topsoil to encounter thick beds suitable for construction. It is the only traditional binder ready to use straight out of the ground, no manufacturing required. Clay or earthen plasters are easy to make, simple to apply and look beautiful.
Living in North America and Europe where the use of concrete and cement dominates construction you may find it hard to believe that even in the 21st century clay is still by far the most prevalent building material worldwide. Adobe, rammed earth, cob, wattle and daub are all examples of raw, unfired clay based construction. And why not? Clay is both an inexpensive and practically inexhaustible resource having little to no embodied energy in its production. Furthermore, clay is the most ecologically friendly material you can imagine, boasting a completely non-toxic life cycle.
Gypsum is a salt of calcium and occurs naturally as a relatively soft stone. Its formation can mostly be attributed to the decomposition of limestone exposed to sulphuric acid from volcanic activity. Its properties were likely discovered many thousands of years ago around a campfire. By chance gypsum rocks were collected, forming the outer ring of the pit containing the fire. Gypsum “calcines” or changes its chemical state at a relatively low temperature, the typical heat from a campfire being more than enough. Just as we might do today, water was tossed on the fire the following morning when breaking camp. The gypsum rocks would have fallen to powder, absorbing the excess water to form a putty. Within minutes the putty would have hardened again into solid gypsum.
No doubt some clever camper (maybe a plasterer?) found the properties of this rock potentially very useful. That hardening process was more than just rapid drying. Gypsum plasters undergo a chemical change or “set” that distinguish them from clay plasters which simply dry out. Due to its unique chemical properties, plasters made from gypsum are “self-binding”, in other words they don't shrink. This makes gypsum plasters not only excellent wall plasters but particularly useful for thick casting applications such as mouldings and ornament.
The Family of Limes
Lime is unquestionably one of the most versatile and enduring building materials available to mankind. Lime originates principally from the burning of limestone, the compressed accumulation of millions of years of marine skeletal remains. Similar to gypsum, it has been conjectured that the discovery of lime occurred quite by accident. Limestone calcines at a much higher temperature, a level of heat associated with terracotta and ceramic production. Ancient potters might have been disappointed that their limestone kilns fell apart but their plaster and mason fellows became the beneficiaries of an incredible discovery, the ability to burn and reconstitute limestone.
There are “pure” limes, composed almost entirely of calcium compounds that make beautiful white plasters perfect for everyday use or as the grounds for frescoes like those of Michelangelo. Dolomitic limes are a combination of calcium and magnesium compounds that have a long history of use as mortars for masonry. Some limestones have very specific impurities that have leached into their porous structure. When burnt they form “hydraulic” limes, meaning that they “set” or harden when mixed with water. Depending on the nature of these impurities the set can be quite rapid and very hard. A few limestones with high clay infiltration known as “marls” produce a natural cement. The Romans became experts at exploiting hydraulic limestones and marls in Celtic Gaul, modern day France, to construct aqueducts and ports. Back home they discovered that adding volcanic ash from Mount Vesuvius to pure lime would produce a similar hydraulic reaction, the famous “Roman” cement responsible for the greatest unreinforced architectural work in human history, the dome of the Pantheon.
We'll return to examine these binders in far greater detail, breaking down their chemical and physical properties as well as highlighting practical plaster applications. However, in my next blog, I'm going to take a close look at what exactly these minerals bind together: aggregates and fibres.