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| Sgrafitto by a "Mâalem", master plasterer |
Enigmatic, exotic, artisanal, rustic, seductive…just a
few of the adjectives that attempt to capture the essence of Tadelakt, the
plaster of Marrakech that has in recent years fascinated the imagination of the
Western design world, first in France (logically as Morocco is French speaking
and a popular destination for holiday and vacation homes. Also, many French
designers I must admit have impeccable taste) and subsequently throughout the EU
and US. A number of years ago I accompanied a team from Venice organized by a
true master of the Italian tradition, Franco Saladino, to study Tadelakt
application and manufacture. I immediately fell in love with the food, the
music, the “terroir”, the culture and of course the plasterwork. One of my
plaster colleagues in the US, Ryan Chivers, made a similar visit and was
likewise enthralled. Ryan and I will do our best to share what we’ve learned of
its history, manufacture and application.
History
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| Tadelakt lined cistern, Al Badi palace |
The word Tadelakt is an English transliteration of the
Arabic “تدلاكت”,
meaning “massaged” or alternatively “rubbed”, “kneaded”. The name is meaningful
and a big hint to what Tadelakt is all about. Yes, it is a type of plaster that
has certain properties but more significantly it is a traditional practice of
application that involves a bit of elbow grease. Lime burning for plaster has
been going on for a long time in North Africa. We know Egypt for example was
using gypsum and lime plasters for the construction of the pyramids and other
structures dating back 5,000 years ago. However, it was under the Almoravid
“Berber” dynasty of the 11th century based in Marrakech that
artisans began to use the Tadelakt method of finishing the locally produced
lime to waterproof the royal cisterns.
The limestone used to produce Tadelakt comes from the
High Atlas Mountains to the east. The limestone is argillaceous, meaning it contains
a relatively high percentage clay. Also, there is a small infiltration of
amorphous silica making Tadelakt lime slightly hydraulic. Combining its natural
properties with traditional application methods Tadelakt’s waterproofing
qualities were subsequently put to decorative use in exterior façades, small
drinking vessels and famously the “hammams” or public bath houses.
Manufacture
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| The open pit |
The Tadelakt "factories"
are found at the foothills of the High Atlas Mountains. The factories are
little more than a flat stretch of land with a series of cylindrical pits lined
with adobe bricks. Each pit has a tight tunnel entrance leading to the floor of
the pit so fuel can be added during the baking. The limestone is broken down to
more or less grapefruit-sized pieces. There is no mechanical equipment so all
the work is done by hand breaking one stone against another. Then a vault is carefully arranged by hand over the pit. It is
very important that the size of the stones are the same so that there is an
even baking. Too large and the carbon will not be completely driven off. Too
small and there is a vitrification of the silicates. Either way you can end up
with a lot of useless chunks.
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| The vault constructed, ready for baking |
This is precisely the same process
outlined by Vitruvius and utilized by the Romans two millennia ago to produce
lime plaster. The Arabs had preserved many of the old Roman texts and during
their golden age translated many Greek and Latin documents into Arabic.
Tadelakt is essentially a Roman cement utilizing limestone local to Marrakech. The
very effective fuel for the furnace comes from the surrounding fields, a plentiful
brush high in natural oils that are set aside and dried weeks in advance. The
temperature is raised to about 950°C (1750°F) which
is maintained for at least 24 hours under the careful supervision of an
experienced baker. There of course is no
thermometer. He determines the temperature by observing the color of the flame
and the smell of the smoke.
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| The baker inspects inside the vault |
After another full day of cooling
the vault is broken down and carted away. If the limestone was properly baked,
the stones will fall to powder by lightly spraying with water. The reaction is
quick producing a good amount of heat, though not as much as a pure quicklime.
Finally it is put through a series of screeds to remove
over or under cooked chunks. This is typically done in the open air by two men
holding and shaking a large manual screed approximately 3' x 8' with an
assistant throwing plaster on it. The Tadelakt lime
will stay highly active if quickly packaged and stored to prevent carbonation
with the air.
Application
Application of Tadelakt is a multi-step process. The
essence of skillfully applied Tadelakt is timing. Understanding when to do each
step is something that is difficult to describe and requires firsthand
experience. With Tadelakt, there are many ways to achieve the same result,
depending on the tools and materials that are being used. This description is
meant to describe the Tadelakt process as it is done traditionally in Marrakech.
Tools
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| courtesy of Franco Saladino |
The traditional tools used in Morocco, are very simple. A
large masons trowel is used for mixing. A bucket of water and large brush are
used to wet the walls. The masons trowel is used in combination with the “Taloche”,
a wood float that can be used as a hawk or a trowel to hold or apply the
material. Once the material has been applied, the wood float is used to flatten
and fill the surface. The masons trowel is then used to initially smooth the
surface. The “Galet” is a hard river stone that is usually flat on one side and
polished smooth. Various pieces of plastic are used for a final smoothing. One
is a stiff flat piece with a polished edge. Also used is folded up sheet
plastic that is used for smoothing round shapes.
Mixing
The Tadelakt is traditionally mixed very simply by hand.
The material is screened through a fine screen to remove the largest pieces of
aggregate. The powder is added to the water and mixed well with the mason’s
trowel. The material is mixed surprisingly thin to account for the relatively
high suction of traditional substrates. Pigment is added dry after the Tadelakt
is mixed. The dry pigment is sprinkled with a little water then thoroughly
mixed in.
Application
The first step in the application process is to test the absorption
of the background. Tadelakt traditionally was applied to highly absorbent
backgrounds such as thick earth, lime, or cement walls. To reduce the suction,
a small amount of water is dashed onto the surface. Before large scale
application, a small dab of material is applied to the wall to ensure that the
background is taking up the water at the proper rate. The Tadelakt is initially
applied in multiple thin layers one after another to achieve a final thickness
of about 4-6 mm . The masons trowel is used to apply and the wood float is used
as a hawk to hold the material. Alternatively, the wood float can be used as a
trowel with the masons trowel used to scoop the plaster onto the float.
During application care is taken to try and get a fairly
even and level surface. After a short time, the wooden float is scoured over the
surface to fill and flatten. High spots are ground down and low spots are
filled. Another important aspect of this step is to crush in all of the bigger
sand grains and to bring the fines to the surface. The next step is to smooth
the surface of the Tadelakt with the mason’s trowel after the thickness of the
plaster has dried until just the surface is workable. Again a little water can
be sprinkled if the surface is too dry. The plastic skimmer can also be used to
smooth the surface.
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| Ryan Chivers polishing with the "galet" |
After a further period of drying, the Tadelakt is ready
to be polished with the stone. This process can begin when the surface is just
barely movable. Polishing should continue as the surface dries until a smooth
surface has been achieved. At this point, any small holes or imperfections can
be filled with the trowel or the plastic skimmer and rubbed smooth with the
stone. When the stone polishing is complete, and the surface has dried enough
to have a slight tack, it gets burnished with the plastic skimmer. This is done
horizontally, then vertically. The plastic gives the surface a high gloss and
is the final step until the soap is applied the next day.
The Tadelakt is left to dry for 12 hours or more, usually
overnight. The surface is thoroughly coated with soapy water and immediately
polished with the stone. The surface is polished again with the stone. At this
point, firm pressure is used to consolidate the surface. After the whole
surface has been polished, and most of the soap has been rubbed in, a soft dry
cloth can be used to wipe off any excess soap. It is common to apply several
subsequent layers of soapy water with a brush in the days following the
application. This step aids curing and carbonation and adds a layer of luster
to the Tadelakt. It is also common to apply a coating of wax after a 30 day carbonation
period.
Concluding Thoughts
Much of the excitement surrounding Tadelakt stems not
only from its intrinsic beauty but also its waterproofing characteristics. I
always like to caution folks though that Tadelakt is a veneer plaster application
and can’t possibly be waterproof by itself. Rather, it forms the last exposed
surface of a waterproof system. In Morocco and even in the EU terracotta, brick
or cement block with a brown coat of hydraulic lime or cement stucco might serve
as a typical support. However, timber frame construction is the norm in the US so
it is important to make certain that the framing is very secure with no flex or
movement and the substrate is appropriate. Cement boards or metal lath are well
supported solutions adopted for tile and stone that can also be useful for
Tadelakt.
I can’t say enough good things about the Moroccan “Savon
Noir”, black soap. It is a natural olive based soap sold by the kilo in the
bazaars and used for everything from personal hygiene (my shampoo) to all
manner of washing in the home. It comes as a thick paste but easily emulsifies
in water if mixed and left overnight. As Ryan mentions it is very important to
apply the soap the next day before the Tadelakt begins to carbonate. The action
of the galet rubbing the soap into the Tadelakt deeply impregnates the plaster.
The soap does not form a film like wax, however. The alkalinity of the lime chemically
reacts with the soap to form another mineral, calcium stearate (think soap
scum!). The surface becomes highly resistant to liquid water, scratch resistant
and harder than cured lime yet still breathable to water vapor.
This January Ryan Chivers will be taking the lead in
instructing a two day workshop at Prima Terra Plasters who are importing authentic Tadelakt
from Morocco (
http://www.facebook.com/events/485838111460505/).
I’ll be there as an assistant and hope to see some of you as well!
This article was coauthored by Patrick Webb and Ryan Chivers
The innovative,
original American system of straw bale construction has been warmly and
increasingly embraced as a beautiful and ecologically sustainable construction
alternative for France and other countries of the European Union.
This same chemical
property of raw gypsum allows the manufacture of gypsum plaster to be achieved
at comparatively low temperatures. Typically gypsum is baked in a horizontal
kiln at the relatively low temperature of under 500°F for a couple of
hours under autoclave conditions to create plaster. This is considerably less
than the embodied energy expended in lime (1500°F) or Portland cement (2700°F)
stuccos. Unlike hydraulic limes and cements, gypsum coatings can be removed and
recycled as a plaster.
