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t a k i n
r e e n t
o t h e e x t r e m e
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
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
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.
for making beautiful clay plasters are non-toxic, low embodied energy,
inexpensive, and often available at or near the building site.
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 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
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.
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
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 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.
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.
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
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.
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.
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.
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
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.
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.
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
All of the ingredients used to make and finish cob are completely
non-toxic. It wouldn’t taste good, but technically you could eat
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.
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
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.
A false perception remains that durable interior finishes are not
feasible with something as simple as clay.
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.
Clay plasters are subject to erosion from rain or strong winds, so require annual maintenance when used on exterior in most
Earthen Plaster Articles in The Last
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
information on earthen plasters
The Natural Plaster Book by Cedar Rose Guelberth
& Dan Chiras
Using Natural Finishes by Adam Weismann &
Clay Culture by Carole Crews
Down to Earth Design
Sigi Koko, principal
©2000 Sigi Koko & Down to Earth