The Unique & Beneficial Properties of Clay Plaster

Clay plaster base coat application in process

Clay soil is one of the most ancient, rudimentary, yet effective building materials on the planet. Besides being naturally abundant across much of the earth, clay has unique properties that make it not only suitable but very desirable as a natural building material. Clay is used for the production of fired brick, sun-baked adobes, cob, and natural plaster finishes.

In this very basic introduction to clay soil, I will highlight some of the unique water-loving features and other useful attributes of clay when it’s used as a natural building material, particularly as a plaster finish.

Soaked clay is pushed through a fine screen to make a finish clay plaster

What Exactly is Clay?

Clay is soil composed of particle sizes with a diameter of less than 0.005 millimeter across. Essentially, it’s extremely finely pulverized particles of stone. Individual particles, which are naturally plate- or hexagonal-shaped cannot be seen by the naked eye. Clay soil is plastic when wet, and coherent (stable) when dry. What makes clay an especially compelling building material is its relationship to water. Let me quote from James Henderson here, who says it very succinctly in an excellent article worth reading:

The disks have an electric charge that make them function like magnets. When dry, all the negatively charged sides of the disks cling to positively charged sides of another clay disk next to them. This function gives the clay a binding force. The addition of water separates the disks far enough apart to weaken the electric charge. When the water dries, the charge is re-activated, binding the clay back together. Thus clay is in a constant state of flux and never sets. The miracle is that when sand and straw are added to clay it maintains a solid state that can function as a high performance building material for centuries.

The Relationship Between Clay & Water

Clay has a special relationship to water in both liquid and vapor form. This time I’ll quote Clayworks, who says it neatly here:

Clay plasters have a thirst for moisture. Its most prized characteristic is its ability to readily attract and take up moisture from the atmosphere and retain this moisture within its pore structure. Earth plasters are therefore hydrophilic (water loving) and hygropscopic (can safely hold onto this moisture). Furthermore, as it incorporates this moisture into its structure, it causes the clay molecules to expand, blocking the further passage of moisture through its structure. This forms a water resistant barrier. This self-sealing ability can prevent moisture from wicking into the wall substrate beneath, while still allowing moisture to move back out.

In summary, clay is hydrophilic, hygroscopic, and vapor permeable. A clay plaster finish allows the free passage of water molecules (in the form of vapor) to pass through the skin of a building. This is particularly important for natural (a.k.a., biodegradable) wall systems including straw bale and light clay straw. Natural wall systems function (and only function) successfully with the free passage of moisture, so that water is not trapped within the wall structure, which can lead to decay. Clay has the highest permeability rating of all possible plaster finishes.

Clay plaster applied to a light clay straw wall

Interestingly, clay is also capable of balancing indoor humidity levels. Contrary to popular belief, clay finishes are perhaps especially beneficial in humid climates, where it can absorb moisture from the air and hold on to that moisture until the air dries. When the air is above 50% humidity, clay absorbs the excess moisture present in the air. Later, when the humidity levels drop below the 50% threshold, the clay will release that stored moisture.

Healthy indoor air humidity levels are between 40-50%. If humidity levels cross the 60% threshold and remain there for extended periods of time, mold and mildew growth become a concern. You can now appreciate how useful a clay plaster finish is in humid climates. (I’m not sure if there are any specific studies, but it would be very interesting to know more specific figures about the performance of clay plastered homes in humid climates that do not have mechanical ventilation.)

Clay is Flexible and Repairable

Unlike lime plaster or cement stucco, clay dries mechanically — that means it simply loses water to the atmosphere. There is no chemical curing process. Ultimately, this means that clay can be repeatedly reused, repaired, and re-adapted. It’s possible that plaster can be repaired with a light misting and retouching. (Whether it is burnished or sponged will affect how simple the repair process is).

Previously used material can be re-wetted and used over and over again with no loss to quality, and without any significant energy inputs. In fact, there is a tradition of re-using a certain percentage of earth in new construction in Japan. (See Japan’s Clay Walls by Emily Reynolds for more on this subject.)

Clay plaster is a soft finish and highly flexible, especially compared to cement, which is brittle and prone to cracking. This makes it the perfect companion to soft substrates, including straw bale. It has the ability to flex (to a degree, of course!) with the movement of the wall itself without cracking.

In Summary…

In summary, clay soil has incredibly useful binder properties and a beneficial relationship to moisture. However, there are limits to its use. For example, when clay is used as an exterior finish, it must be very well protected so that it does not erode — driving rain is not a friend to clay finishes. It’s also prone to shrinkage as it dries, but with proper detailing potential issues can be avoided.

There’s much more to talk about than what I can cover in a basic introduction. But now you know more about the unique properties of clay and why it is such an appealing natural building and plaster material.