Understanding Materials: Clay

clay plaster

We applied a lot of lime plaster in 2016. But we’re just as amped about clay and clay plasters. Curious about why?

Clay is a fascinating material with many unique properties. Clay is made of very tiny particles of rock which have been broken down over millennia. Because of the small particle size of clay, it has a very large surface area. The surface area of the clay is important because it creates a lot of space for the clay to bond with minerals and hold water. The water holding, or hydrophilic property of clay means that water is drawn to clay and absorbed into its structure.

For building purposes, clay is used as a binder. Some clays are great for building, and others not so much. When you moisten clay, it becomes sticky and pliable. That’s why clay is so useful in making sculptures, pottery, tiles, and bricks. For our purposes, we use almost exclusively unfired clay instead of heating it to set its shape.

When you mix clay with sand and straw, everything gets held together by the sticky clay, but can still be molded into different shapes. Depending on the texture of the mix, it can be spread into a thin plaster, sculpted into a cob bench, or lightly coated onto straw to tamp into a wall cavity. As it dries and the water evaporates, the material hardens and holds its shape. The sand and straw ensure that the finished product is strong and that it won’t crack as it dries.

Benefits of Clay as a Building Material:

adobe house
A traditional adobe home with proper roof and foundation detailing is a durable and affordable home in a warm climate, with a long and rich history.

Clay is a time-tested building material, having been used in some of humanity’s earliest structures. When protected from direct rain and wicking moisture out of the ground, clay-sand-straw mixtures dry into a strong and durable material. Additives like wheat paste can be used in plasters to increase durability and hardness.

cob clay balls
Cob is just clay, sand, and straw and could be used in the yard if you have leftover material.

Clay can be found on-site or nearby almost anywhere that you are building, making it a low embodied energy material. Unlike many other modern building materials, if you have too much on hand, you don’t need to send the extra to a landfill because it is safe and simple to dispose of.

clay plaster window
Clay finish plaster drying around the laundry room window.

Again, unlike many building materials, clay is naturally free from volatile organic compounds (VOCs) and other unhealthy chemicals which can cause poor indoor air quality and adverse health effects. Clay’s ability to absorb and release humidity is also beneficial for improving the indoor air quality and comfort of a living space, particularly in high humidity areas like basements, kitchens, bathrooms, or laundry rooms.

cob oven
A cob oven during a repair process, ready for the final plaster over a recently finished intermediate clay layer.

Unfired clay can be easily rehydrated with water and then reworked. Most bricks and tiles are examples of fired clay, which has chemically changed from its original state due to the heating. But unfired clays can be moistened and will then bond again. This makes it easy to fix cracks, gouges, or other damages, or to replaster or add a layer of clay paint to walls to change color or texture.

Thermal Mass
earth floor clay
An earthen floor being installed. The earth will absorb heat on sunny days and regulate the temperature of the home.

Clay holds onto its temperature longer than the environment around it. This means that clay walls can hold the nighttime coolness in your home during the summer, even after the day begins to heat up. Conversely, a sunny earthen floor will continue to radiate heat into the room even after the sun has gone down in the winter.[divider]

clay plaster kitchen
Clay plaster in a kitchen adds some texture and shine to the walls.

Clay is easy to sculpt into curves or fun shapes to create a unique look. You can also tint clay to a variety of colors.

If this information has piqued your interest in clay or how we can incorporate clay building or clay finishes into your home remodel or backyard design, get in touch.

clay samples
Cheryl labeling clay plaster samples.
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