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"Be the change
you wish to see
in the world..."
            -M. Gandhi

t a k i n g   g r e e n   t o   t h e   e x t r e m e

CLAY PLASTER

what is clay plaster?
the main ingredients
other additives
surface preparation
application
tips
benefits
challenges
additional resources
related articles

WHAT IS CLAY PLASTER?

Plasters made with clay are beautiful, durable, and made from inexpensive and non-toxic ingredients.  Clay creates a breathable finish material with a natural capacity to regulate moisture and temperature in the surrounding air.  Clay naturally absorbs excess moisture in the air, helping air to feel more comfortable in summer, while its high thermal mass creates a battery heat (or cooling) storage that helps to maintain constant air temperature.  Materials can often be found on or local to the building site, making the ingredients dirt cheap...literally.  Techniques for mixing and applying are simple and the plaster is extremely forgiving, even for beginners of any age.

Clay plasters are especially suitable as finishes for strawbale walls, where breathability of the wall system is essential. Clay plasters are suitable for interior surfaces, and provide additional thermal mass to interior spaces to help improve energy performance.  They can also be used on well-protected exterior surfaces, as they are more prone to weathering.  If maintenance is not a bad word, clay plasters can be used as an exterior finish, but generally will require annual resurfacing.

Clay plasters over strawbale walls are usually applied in 3 coats. The first coat is used to completely cover the straw. The second coat is used to shape/straighten the wall as desired. And the final coat is used to create the desired finish texture.  The final step is usually to apply a clear sealer or, if color is desired, breathable paints or clay alis can be used.

THE MAIN INGREDIENTS

The ingredients for making beautiful clay plasters are non-toxic, low embodied energy, inexpensive, and often available at or near the building site.

CLAY
Clay absorbs water...lots of water.  When wet, clay becomes sticky and pliable, creating a fabulous sculptable binder for plaster.  Clay for plaster can come from sifted clayey soil or can be purchased in bulk as bagged powder.  If using site soil, sifting through wire mesh helps eliminate rocks and breaks up clumps of clay for easier mixing.  I use 1/2-inch wire mesh screens for base coats of plaster, and 1/4-inch or finer wire screen for finish plasters.  If using clay powder, your clay will generally become more pliable if you let it soak in water before use.

Do not use top soil!  Organic top soil adds inert fill with no benefit.  Top soil shrinks over time, potentially creating voids in your plaster.  Dig below the top soil to explore for clay below and keep the top soil for gardening.

See Building with Cob for information on how to test soils for clay content.

SAND
Sand provides strength and minimizes cracking as the clay plaster dries.  I use course sand (such as concrete sand) for base plasters and fine mason's sand for finish plasters.

Note that silt differs from sand, in that silt is spherical, like a marble, whereas sand is very jagged with lots of surface area.  Silt will feel smooth to the touch, but will not help with cracking control in the plaster, since it has so little surface area to bond with the clay.  Silt will also result in a weak and often dusty plaster.  So avoid silt in favor of jagged sand with lots of surface area.

STRAW
I use straw as an insurance policy in clay plaster, or where the aesthetic of chopped straw is desired in the finish.  I make test areas that are just clay and sand, in varying proportions, and select the mix that was easy to apply but doesn't crack as it dries.  Then the chopped straw goes in the mix to provide additional strength and resistance to cracking.  Often manure can also provide the same benefits, though it tends to adversely impact the color of the plaster, so I recommend it for bottom coats of plaster.

OTHER ADDITIVES

A variety of other ingredients can be added to finish plasters to increase workability, stickiness, durability, or color.  Below is a list of some common additives and what they do.  For a more comprehensive list plus recipes, I recommend Carole Crews' book Clay Culture.

WHEAT PASTE
Wheat paste can increase stickiness and help prevent dusting when added to clay plasters.  It is especially useful when site clay is poor.  Wheat paste can be purchased as a dry powder or you can make your own by boiling flour & water.

CASEIN
Casein is milk protein and can be used as an additive to clay plasters or various paints to strengthen and prevent dusting.  Casein can be purchased as a powder or you can make your own.

LIME PUTTY
Non-hydraulic lime putty helps reduce shrinkage of the clay particles as they dry, thus reducing cracking and creating a stronger finish surface.  However, lime is extremely alkaline, which means you must use protective gear to protect from contact to skin & eyes.  This makes applying plasters less fun!  The tougher finish of clay with lime means that the surface texture of the plaster cannot be reworked after it dries the same way that a pure clay plaster can be.

SURFACE PREPARATION

Be sure to wet down any wall surface prior to applying plasters, and keep the walls damp as you work.  A dry substrate will pull moisture from your plaster very quickly, and can cause a poor bond between layers or excess cracking.

Clay plasters get more pliable as they fully absorb water, so plaster can be made ahead of time and allowed to sit (covered) until you are ready to work with the.

Be sure your finish texture has ample surface area for the first & second coats of plaster, so each subsequent coat of plaster keys in with a good bond to the layer below.  When plastering on strawbales, the surface of the straw provides ample bonding for the plaster.  For each subsequent coat, be sure the surface has good texture for the new plaster.  The surface can be very lumpy, for example, if the previous coat was applied by hand.  Or you can used a notched trowel or plaster comb to create surface texture.

When applying clay plaster onto drywall, you need a bit of extra texture to keep the plaster sticking to the smooth surface.  One option is to paint the drywall with a sanded primer and add wheat paste to the clay plaster.  Keely Meagen paints the gypboard with 10 parts wheat paste, 1 part fine sand, 1 part clay - she lets it dry completely  (see her article "For Love of Mud" in The Last Straw)  In either case, do not wet the drywall prior to plastering.

If you have a lot of cracks in a previous coat of plaster, I find it helpful to patch those cracks with a thin coat of clayey plaster and burlap integrated fully into the wet clay.  The trick is to use a very open-weaved burlap (like that used for landscaping erosion control) and to really make sure the wet patch clay comes through the holes in the burlap.  This helps prevent cracks from resurfacing in a new coat of plaster.

FIRST COAT - goal is covering the straw, not making it look pretty
For the first coat of plaster, it helps to have higher clay content to help bind to the straw surface.  It also helps to NOT add straw to this plaster, since it can make the plaster more difficult to bond with the straw.  I find it easiest to apply this coat by hand, not with trowel, that way you can feel when the plaster is bonded to substrate. You can also apply this coat by spraying clay slip onto the straw; some people also dip their bales into clay strip before installing them, though this makes the bales very very heavy.

SECOND COAT - goal is shaping the walls
This is a thick sculpting coat used to straighten strawbale walls, for relief sculpting and sculpting niches.  In this coat, chopped straw in your plaster increases strength and allows you to install thicker sculpting layers.  Often the second coat is really multiple coats, especially if you have deep reliefs or very uneven strawbale walls that you want to straighten.

FINISH COAT - goal is the final finish texture
This coat provides your surface texture after you've shaped the walls.  The finish plaster is usually applied thinner than the previous coats.  The finer you want your finish, the finer you should sift your ingredients.  Chopped straw, mica, or other ingredients may be added for aesthetic purposed as desired.

TIPS

MAKE TEST SAMPLES
For each coat of plaster, plaster test areas for various different proportions of clay & sand.  I make my tests at least 3-feet by 3-feet and then write the ratio of the mix right in the plaster.  Wait for your test mixes to dry completely before assessing which is the best one to use.  If your test plaster is extremely crumbly or dusty, there is probably not enough clay in the mix.  If cracks appear in your test mix, there is probably too much clay.  You generally want the mix to be the highest ratio of sand that is still easy to plaster on the wall.  Note also that if your clayey soil changes in appearance as you dig, it is likely the clay has changed.  This is a signal to take time for another round of test plasters.

PROPERTIES of DIFFERENT Clays vary 
Note that different types of clay have varying capacity to absorb water.  Absorptive clays tend to swell and shrink a lot, which makes these clays more prone to cracking.  Non-absorptive clays do not swell very much, and therefore do not shrink much as they dry, resulting in less cracking.  Different clays can be more or less sticky as well.  A very sticky clay means very little binder (clay) is needed in your plaster, which means maximum amounts of sand can be added.  Clays that are not very sticky cannot bind very much sand and often need an additive to improve them.  The bottom line is that no two clayey soils will act alike, so test, test, test your soil before you commit to entire walls.

KEEP YOUR WALLS DAMP AS YOU PLASTER
I think the number one mistake I see with clay plaster is applying wet plaster onto a dry surface.  The dry surface quickly sucks the moisture out of the fresh coat of plaster, which can prevent the old and new from bonding and can cause excessive cracking in the new plaster.  I wet my walls down at least once or twice the day before plastering, then again the morning of, and then throughout the day as I go.

BENEFITS

Completely Non-Toxic
All of the ingredients used to make and finish cob are completely non-toxic.  It wouldn’t taste good, but technically you could eat them.

Local, Indigenous Materials
Often the soil dug from foundation excavation contains sandy clay and can be used to build with.  If you do not find clay soil locally, dry bagged clay works as well, however it should hydrate for several days to make the clay sticky enough.

Low-Tech & Easy to Learn
Techniques for mixing and building with cob are extremely easy to learn and fun.  Tools needed are few and inexpensive:  shovels, tarp, and buckets.

CHALLENGES

TIMING
Clay plasters are not a standardized material (with the exception of American Clay or similar expensive processed clay plaster products).  This means time is required to experiment with specific recipes that work well with your specific materials.  Clay plasters can be sprayed on walls, though are most often applied by hand.  Each coat must dry completely before additional coats are applied.  Construction timing needs to take the additional time for application and drying of each coat into consideration.

SKILLS NEEDED
Clay plaster are very forgiving, even to a novice plasterer.  However, very fine smooth finishes require lots of practice or professional experienced help to perfect.

Perception
A false perception remains that durable interior finishes are not feasible with something as simple as clay.

COST
If you are paying for all labor, you will probably pay more for a finely finished clay plaster.  However, the first two coats require only basic skills and so afford a great candidate for sweat equity.

EXTERIOR DURABILITY
Clay plasters are subject to erosion from rain or strong winds, so require annual maintenance when used on exterior in most climate conditions.

ADDITIONAL RESOURCES

Earthen Plaster Articles  in The Last Straw Journal
www.thelaststraw.org
includes For Love of Mud by Keely Meagan, Plasters & Strawbale by Cedar Rose, Earth Plasters and Aliz by Carol Crews, and The Straw Bale Earthen House by Athena and Bill Steen

Earth Plaster in Wikipedia
en.wikipedia.org/wiki/Earthen_plaster
information on earthen plasters

The Natural Plaster Book by Cedar Rose Guelberth & Dan Chiras

Using Natural Finishes by Adam Weismann & Katy Bryce

Clay Culture by Carole Crews

RELATED ARTICLES

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