CLAY PLASTER

Down to Earth Design
design | educat ion | consult ing
with an exclusive focus on eco-sensible buildings
CLAY PLASTER
Down to Earth Design
Sigi Koko. Principal
admin @ buildnaturally.com
202-302-3055 DC
215-540-2694 PA
www.buildnaturally.com
WWW .BUILDNATURALLY.COM
WHAT IS CLAY PLASTER?
Plasters made with clay are beautiful, durable, and
made from inexpensive and non-toxic ingredients. Clay
is unique in that it is pliable & sticky when wet with a
strong internal structure as it dries. This makes it an
ideal binder for sculptable plasters.
T HE MAIN I NGREDIENTS
Clay: sticky binder that holds the ingredients together;
can use clayey site soil or purchase dry bagged clay
Sand: aggregate reduces shrinkage and minimizes
cracking; use course concrete sand for base plasters
and fine mason’s sand for finish plasters
Straw: finely chopped reinforcing fiber helps further
minimize cracking and adds bulk for sculpting
OTHER ADDITIVES
Numerous additives can enhance the qualities of clay
plaster or adjust the appearance of the finish.
I
recommend experimenting with any additives before
committing to a large surface finish.
Minimize Dusting: wheat paste (purchase a powder
or make with flour & water), casein, lime putty (be
careful, lime is highly caustic), prickly pear juice, glue,
oils (linseed, tung, etc.), laundry soap & alum, egg
Straw Alternatives: hemp, manure, paper pulp
BENEFITS
Completely non-toxic. All ingredients in clay plasters
are completely non-toxic.
Moisture control.
Clay naturally absorbs excess
moisture in the air, helping air to feel more comfortable
on humid summer days.
High thermal mass. Clay plasters have high thermal
mass, which stores heat or cooling energy that helps to
maintain constant air temperatures.
Local, Indigenous. Often soil dug from foundation
excavation contains sandy clay that is ideal for making
beautiful clay plasters in a variety of colors.
Low-Tech & Easy to Learn. Techniques for mixing
and applying clay plasters are extremely easy to learn
and fun. Perfect for the plaster novice!
CHALLENGES
RESOURCES
WEBSITES
www.thelaststraw.org/bonus-articles/earthplaster.html
www.networkearth.org/naturalbuilding/aliz.html
en.wikipedia.org/wiki/Earthen_plaster
www.buildingwithawareness.com/earth-plaster-walls.html
strawbale.pbworks.com/Mother-Earth-New-Earth-Plaster-Article
www.buildnaturally.com/EDucate/Articles/ClayPlaster.htm
B OOKS
Clay Culture: Plasters, Paints and Preservation by
Carole Crews
Using Natural Finishes: Lime- $ Earth-Based Plasters,
Renders & Paints by Adam Weismann and Katy Bryce
The Natural Plaster Book: Earth, Lime and Gypsum
Plasters for Natural Homes by Cedar Rose Guelberth
and Dan Chiras
Earth Plasters for Straw Bale Homes by Keely Meagan
Perception. A false perception remains that durable
interior finishes are not feasible with something as
simple as clay.
Exterior durability.
Clay plasters require annual
maintenance when used on exterior in most climates.
Timing. Clay plasters are not generally a standardized
material, so additional time is required to experiment
with specific recipes and techniques. If paying for all
labor, this additional time translates into higher cost.
Aesthetic Additives: pigments, mica
APPLICATION
Your substrate must provide firm backing to support the
plaster, and the surface should always be wet down
before applying clay. Burlap dipped in clay slip and
imbedded into the plaster allows you to plaster over
surfaces that clay will not adhere to (such as wood).
Clay plasters are very forgiving, so experiment!
First coat: goal is covering the straw, not making it
look pretty. Increase the clay content and eliminate
chopped straw in this coat; best results when plaster is
worked into strawbales by hand; use ½” screen
Second coat: goal is shaping the wall. This is the thick
sculpting layer, so add more straw to sculpt and apply
thicker flattening coat by hand or with trowel; use ½”
or ¼” screen
Finish coat: goal is to create a beautiful texture. This
coat is usually thin, so screen soil through 1/8” screens.
Finish can be burnished for super smooth texture and
can be pigmented, or covered with an alis, breathable
paint, or clear sealer.
SHAKE TEST
A simple shake test determines
relative percentages of clay and
sand contained in your soil.
It
works
because
clay
remains
suspended in water, whereas sand
and silt sink in water.
1. Fill approximately ¼ of a cylindrical-shaped glass jar
with crumbled soil (free of visible stones).
2. Fill to the top with water, close the lid, and shake
well, until all of the clay is dispersed.
3. Set the jar down on a level surface and watch for 10
seconds. All of the sandy solids will settle to the
bottom. Draw a line on the jar at the top of the
sand. The water remains cloudy with clay.
4. When the water becomes completely clear, draw
another line at the top of the settled clay. The ratio
between the height of the sand and the height of the
clay represents the ratio of sand to clay in the soil.
Note: it is difficult to differentiate silt in this test, as
silt is similar to sand, only smaller and spherical.
HANDS-ON WORKSHOPS
Down to Earth Design teaches workshops on
how to make clay plaster and apply it to strawbale walls
and other substrates.
WORKSHOPS INCLUDE:
• overview of clay plaster, including properties of each
ingredient & how to analyze site soils for clay content
• demonstration of mixing & application techniques
• guided hands-on experience mixing & applying clay
plaster to strawbale walls and other substrates
Check our website www.buildnaturally.com
additional information.
for