El Tadelakt Marroquí

Esgrafiado hecho por el maestro "Mâalem"

Enigmático, exótico, artesanal, rústico, seductor... Estos son solo algunos de los adjetivos que intentan describir la esencia del Tadelakt, el revestimiento de cal de Marrakech que en los últimos años ha fascinado el mundo del diseño occidental, primero en Francia (lógicamente por su habla francesa y ser Marruecos un destino popular de vacaciones. Además, tengo que admitir que muchos diseñadores franceses tienen un gusto impecable) y, posteriormente, en toda la UE y EE. UU.

Hace algunos años viajé a Marruecos con un grupo organizado desde Venecia por el verdadero maestro italiano, Franco Saladino. Fuimos a estudiar la aplicación y la fabricación del Tadelakt. Inmediatamente me enamoré de la comida, la música, el "terroir", la cultura, y sus revestimientos. Uno de mis compañeros del sector en los EE. UU. Ryan Chivers, realizó una visita similar y también quedó cautivado. Ryan y yo haremos nuestro mejor esfuerzo para compartir lo que hemos aprendido sobre su historia, fabricación y aplicación.

Historia

Aljibe cubierto en Tadelakt, Palacio Al Badi

La palabra Tadelakt es una transcripción del árabe " تدلاكت ", que significa "masajeado" o, alternativamente, "frotado", "amasado". El nombre es significativo y un gran indicio de lo que se trata Tadelakt. Sí, es un tipo de enlucido que tiene ciertas propiedades, pero es una práctica muy importante que involucra un poco de palizón en su proceso. La calcinación de la cal para hacer mortero ha estado ocurriendo durante mucho tiempo en el norte de África. Sabemos que en Egipto, por ejemplo, estaban utilizando variosas revestimientos de yeso y cal para la construcción de las pirámides y otras estructuras que datan de hace 5.000 años. Sin embargo, fue bajo la dinastía "beréber". Almorávide del siglo XI, con sede en Marrakech, cuando los artesanos comenzaron a usar el método Tadelakt para dar una acabado a la cal producida localmente para hacer los aljibes reales impermeables.

La piedra caliza utilizada para producir Tadelakt proviene de la zona este de las montañas del Alto Atlas. La piedra caliza es arcillosa, lo que significa que contiene un porcentaje relativamente alto de arcilla. Además, hay una pequeña infiltración de sílice amorfa que hace que la cal Tadelakt sea ligeramente hidráulica. Combinando las propiedades naturales del material con los métodos tradicionales de aplicación del tadelakt, se consiguen unas cualidades impermeables que a se usaron simultáneamente a modo decorativo en fachadas exteriores, para hacer vasijas, y en las paredes interiores de los conocidos "hammams" o casas de baños públicos.

Elaboración

La fosa abierta

Las "fábricas" de Tadelakt se encuentran en las faldas de las montañas del Alto Atlas. Las fábricas son poco más que un terreno plano de tierra con una serie de fosas cilíndricas forradas con ladrillos de adobe. Cada fosa tiene un túnel estrecho de entrada que conduce al suelo de la fosa para que se pueda agregar combustible durante la cocción. Entonces, se levanta un bóveda bien hecha a mano sobre la fosa. La piedra caliza se descompone en trozos más o menos del tamaño de las piezas de un pomelo. No hay ningún tipo de mecánica, por lo que el trabajo se realiza todo a mano, rompiendo cada piedra una a una. Es muy importante que el tamaño de las piedras a cocer sea el mismo para una cocción uniforme. Si las piedras a quemar son demasiado grandes, el carbono no se eliminará completamente. Si son demasiado pequeñas habrá una vitrificación de los silicatos. De cualquier modo, siempre puedes terminar con muchos pedazos inservibles.

La bóveda erguida, listo para hornear

Este es precisamente el proceso descrito por Vitruvio y utilizado por los romanos hace dos milenios para producir el mortero de cal. Los árabes conservaron muchos de los antiguos textos romanos y durante su edad de oro, tradujeron muchos documentos del latín y el griego al Árabe. Tadelakt, es esencialmente un cemento romano que utiliza la piedra caliza local de Marrakech. El combustible muy eficaz para el horno proviene de los campos circundantes, un arbusto abundante con alto contenido en aceites naturales, este se colecta unas semanas antes y se deja secar. La temperatura dentro del horno se eleva a aproximadamente 950 ° C (1750 ° F), y se mantiene durante al menos 24 horas bajo la supervisión cuidadosa de un hornero experimentado. Por supuesto que no hay termómetro. Se determina la temperatura observando el color de la llama y el olor del humo.

el hornero inspeccionando la bóveda

Después de un día completo de enfriamiento, la bóveda se desmonta. Si la piedra caliza se ha cocido correctamente, las piedras se convertirán en polvo rociando ligeramente con agua. La reacción es rápida produciendo una buena cantidad de calor, aunque no tanto como con la cal viva. Finalmente, se somete a una serie de tamices para eliminar los trozos demasiado cocidos o sin cocer. Esto se hace típicamente al aire libre por dos hombres que sostienen y sacuden un tamiz manual grande de aproximadamente 0.75m x 2m, y con un ayudante que tira la cal en él. La cal Tadelakt permanecerá altamente activa si se empaqueta y se almacena rápidamente para evitar su carbonatación con el aire.

Aplicación

La aplicación de Tadelakt es un proceso con varios pasos. La esencia de Tadelakt hábilmente aplicada es el tiempo. Comprender cuándo realizar cada paso es algo que es difícil de describir y requiere experiencia. Con el Tadelakt, hay muchas maneras de lograr el mismo resultado, dependiendo de las herramientas y los materiales que se utilicen. Esta descripción pretende describir el proceso de Tadelakt como se hace tradicionalmente en Marrakech.

Herramientas

Cortesia de Franco Saladino

Las herramientas tradicionales utilizadas en Marruecos, son muy sencillas. Una paleta grande de albañil se usa para mezclar. Se utiliza un cubo de agua y un cepillo grande para mojar las paredes. La paleta de albañil se usa en combinación con el "Taloche", un fratás de madera que se puede usar como superficie para sostener o aplicar el material. Una vez que se ha aplicado el material, el fratás de madera se utiliza para aplanar y rellenar la superficie. La paleta de albañil se usa para alisar inicialmente la superficie. El "Galet" es una piedra de río dura que generalmente es plana y lisa por un lado. Se utilizan varias piezas de plástico para un alisado final. Una de ellas, es una pieza plana rígida con un borde pulido. También se utiliza una lámina de plástico plegada para suavizar formas redondas.

Mezcla

El Tadelakt es tradicionalmente mezclado a mano. El material se tamiza a través de un tamiz fino para eliminar las piezas más grandes de agregado. El polvo se agrega al agua y se mezcla bien con la paletade albañil. El material se mezcla sorprendentemente aguado teniendo en cuenta la succión relativamente alta de los sustratos tradicionales. El pigmento se agrega después de mezclar la cal. El pigmento seco, primero se rocía con un poco de agua, se mezcla completamente y entonces se añade a la mezcla.

Aplicación

El primer paso del proceso es comprobar la absorción del sustrato. El tadelakt se aplicaba tradicionalmente en sustratos altamente absorbentes como paredes gruesas de tierra, cal o cemento. Para reducir la succión, se lanza una pequeña cantidad de agua a la superficie. Para la aplicación a gran escala, se aplica una pequeña cantidad de material en la pared para garantizar que el fondo absorba a una velocidad adecuada. El Tadelakt se aplica en varias capas delgadas hasta un espesor final de aproximadamente 4-6 mm. La paleta de albañil se utiliza para aplicar, y el fratás de madera se utiliza como una superficie para sujetar el material. Alternativamente, el fratás de madera se puede usar como una llana y entonces usar la paleta para colocar el material en el fratás.

Durante la aplicación, se tiene especial cuidado en allanar bien la superficie de manera uniforme. Después de un corto tiempo, el fratás de madera se frota sobre la superficie para rellenar y alisar. Los puntos altos se muelen y los puntos bajos se rellenan. Otro aspecto importante de este paso es aplastar todos los granos de arena más grandes y traer los finos a la superficie. El siguiente paso es suavizar la superficie del Tadelakt con la paleta de albañil, una vez que el grosor del enlucido se ha secado justo lo suficiente para poder trabajar la superficie. Nuevamente, se puede rociar un poco de agua si la superficie está demasiado seca. También se puede utilizar una espátula de plástico para suavizar la superficie.

Ryan Chivers puliendo con el "galet"

Después de un período adicional de secado, el Tadelakt está listo para ser pulido con la piedra. Este proceso puede empezar cuando la superficie es apenas móvil. El pulido debe continuar hasta que la superficie está completamente seca y hasta que se logre una superficie lisa. En este punto, cualquier orificio pequeño o imperfección puede rellenarse con la paleta o la espátula de plástico y luego nuevamente frotar suavemente con la piedra. Cuando se completa el pulido de la piedra, y la superficie se ha secado con una ligera adherencia, se pule con la espátula de plástico. Esto se hace horizontalmente, luego verticalmente. El plástico le da a la superficie un alto brillo y es el paso final hasta que el jabón se aplique al día siguiente.

El Tadelakt se deja secar durante 12 horas o más, generalmente durante la noche. Entonces se cubre la superficie completamente con agua jabonosa y se pule inmediatamente con la piedra. En este punto, se utiliza una presión firme para consolidar la superficie cuando se pule con la piedra. Después de que se haya pulido toda la superficie, y el jabón haya penetrado hacia dentro, se pasa un paño suave y seco para limpiar el exceso de jabón. Es común aplicar varias capas de agua jabonosa con un pincel los días posteriores de la aplicación. Este paso ayuda al curado y a la carbonatación y agrega una capa de brillo al Tadelakt. También es común que se aplique un recubrimiento de cera después de un período de carbonatación de 30 días.

Conclusiones Propias

Gran parte del entusiasmo que rodea al Tadelakt no proviene solamente de su belleza intrínseca, sino también de sus características de impermeabilidad. Siempre advierto a mis compañeros de que Tadelakt es una capa de enlucido y podría no ser impermeable por sí misma. Más bien, forma la última superficie expuesta de un sistema impermeable. En Marruecos, e incluso en la UE, los bloques de terracota, ladrillo o cemento con una capa de cal hidráulica o cemento podrían ser un soporte típico. Sin embargo, en las construcciones de estructura de madera, tan comunes en los Estados Unidos, es importante asegurarse de que la superficie a aplicar sea segura, sin flexión ni movimiento y que el sustrato sea el adecuado. Alterntivamente, se pueden usar tablas de cemento o listón de metal como substratos para la aplicación del Tadelakt.

Solamente puedo decir cosas positivas sobre el "Savon noir" marroquí, el jabón negro. Es un jabón hecho con aceite de oliva natural y que se vende a granel en los bazares, se usa para la higiene personal (mi champú) y también para todo tipo de limpieza del hogar. Se presenta como una pasta espesa, pero se emulsiona fácilmente en agua si se mezcla y se deja reposar durante la noche. Como Ryan menciona, es muy importante aplicar el jabón antes de que el Tadelakt comience a carbonatarse. La acción de la piedra frotando el jabón en el Tadelakt impregna profundamente el enlucido, el jabón no forma una película superficial como la cera. La alcalinidad de la cal reacciona químicamente con el jabón para formar otro mineral, el estearato de calcio. La superficie se vuelve altamente resistente al agua líquida, resistente a los arañazos y más dura de lo que normalmente es un acabado de cal, y aún así todavía sigue siendo permeable al vapor.


Escrito por Patrick Webb  y Ryan Chivers
Traducido por Anna Castilla Vila 

Decorative Plastering

Students at the
American College of the Building Arts

Without question the craft of plastering has always held widespread practical utilitarian value to our built environment. Stuccoes rendered in exterior provide a sacrificial function, protecting vulnerable substrates from erosion and water damage. Plaster applied inside insulates, attenuates sound and provides a sanitary, durable wall surface. Extrusions of profiles in plaster create mouldings that add architectural interest, helping to delineate space by means of shade and shadow. However, among the many craft specializations of the Decorative and Applied Arts, plaster is by far one of the most expressive mediums. We'll take a quick overview of the Art of plastering via some of the traditions still practiced in Decorative Plastering.

Color and Texture

Clay plaster with osyter shells

Fortunately, two of the most commonly used minerals to produce plaster, lime and gypsum, are inherently very white and accept color readily. A few clays are also a light grey and can be tinted to produce a broad, if muted range of colors. Other clays are naturally occurring in a variety of earthen colors such as sienna, umber and ochre that most of us love just as they are.

Marmorino, meaning "little marble", is an Italian tradition of integral colored lime putty plastering inherited from the ancient Romans. Enjoying a 20th century Rensaissance in the Veneto it soon was popularized once again in Italy and now throughout the world.

The French have there own long standing tradition of adding colors and aggregates to plaster. The French plaster is based on gypsum which is naturally more matte than lime. So, instead of marble  the French tradition emulates limestone, called Stuc Pierre, meaning "Stone Stucco". The surface of Stuc Piere is typically shaved with a "Berthelet", a hand held plaster razor, and often scored to create joints in imitation of ashlar masonry. Virtually every culture has developed its own artistic flare using color and texture with plaster: Shikkui in Japan, Tadelakt in Morocco, Enjarre in Mexico to name a few additional examples.


Ornamentation

Moroccan "gebs" or Gypseries

There are two principal approaches to creating ornamentation in plaster. The first is reductive. Morocco has cultivated master artisans of  "gebs", otherwise known as Gypserie, a wonderful tradition of carving into gypsum plaster that is very akin to wood carving, using similar chisels and gouges.

A more widespread reductive method applied to a variety of different plasters around the world is Sgraffito, carving plaster in very low relief. Sgraffito relies on contrast of color between plaster layers for the effect and is a relatively inexpensive way to add a lot of visual punch.

Of course there are the additive forms of ornamentation for which plaster is famous. The finest ornamented stucco is done by hand, in situ. Lime is the preferred medium although sometimes a quantity of gypsum is added to speed up the work and create higher relief. The most awe inspiring work left by the ancient Romans and emulated in the Renaissance was all painstakingly carried out by hand by armies of sculptors. These must have been very exciting times to be a plasterer! As mold making technologies increased in the 18th century, in situ ornamentation became largely displaced by pre-cast ornamentation in gypsum plaster. Gypsum has a rapid set, just a few minutes, so once time has been invested in a master model, many copies can be made quickly.

Enrico Trolese, contemporary Venetian Stuccotoro

Scagliola and Buon Fresco

There are a few really special applications of decorative plastering that could easily take a lifetime to master. One of those is Scagliola. Scagliola is a technique of emulating marble, typically with gypsum plaster. The artistry required is tremendous. Just matching colors as they occur in marble or developing your own color palate is a challenge in itself. As the technique requires cutting, folding and stacking loaves of plaster in various orientations repeatedly, you must continually visualize what is happening inside, how all of those layers are coming together in a natural way, recreating the subtle variegation of color, veining, stratification and fracturing that occur in marble are all separate skills.

Perhaps the highest artistic expression of plaster, one that blurs the line Buon Fresco. Painting mineral pigments into lime plaster while it is still fresh takes incredible understanding of materials. The plaster must be prepared in a way so that it maintains a consistent level of dampness for as long as possible. Fresco can be as simple as brushing two or three coats of a mineral wash into a completed wall to give it a soft, cloudy, parchment effect to most elaborate works of fine art and trompe-l’œil.

What is important to recall about all of these various decorative plastering traditions is that many of them can and are used in combination. Scagliola might be pressed into moulds to make ornamental pieces that resemble carved marble. Marmorino or a similar fine lime putty plaster is the grounds for painting Buon Fresco.

The art of plastering really has not changed much in thousands of years. We use the same commonly available materials and techniques we always have. And although to become truly expert at the various arts of decorative plaster requires patience and practice, the truth is they are quite humanistic endeavors, appreciable and accessible to most everyone.



Contributed by Patrick Webb

Moroccan Tadelakt

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

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 11^th 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

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.

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.

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

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.

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

A New Beginning

We approach the close of the year, a time of repose, reflection. My thoughts have drifted towards the many friends I have made across the country and the globe due to my curious profession. Plaster is an ancient, noble craft, virtually unchanged, whose practitioners form part of a continuity of human culture as old as civilization itself. Personally, I must attest to the fact that I have been the beneficiary of a wealth of knowledge from many generous colleagues. What is there to do then? For myself there is a resolution for 2013: Share, Teach, Diffuse traditional plaster trade knowledge.

State of the Art

I have to ask myself, why do I feel impelled to make such a commitment? The simple answer: there is a need. We have to be honest. The plaster trade is far from its zenith. The guilds, academies and unions that traditionally shouldered the responsibility of passing on trade knowledge have collapsed or are severely diminished. The atelier system of master, journeyman and apprentice in place for centuries has been replaced by a typically divided corporate structure of management and labor.

Yet, as my grandmother used to say, “if it’s not dead don’t bury it”. Actually, plaster is far from dead. There is a renewed appetite to learn the trade by artisans, to specify plaster by architects, to live in plastered homes by everyday people. The restoration, preservation movement in the US and EU began to gain momentum in the mid-20^th century. Loads of painters and artists have taken up decorative veneer plasters, particularly Venetian plaster and marmorino in the past 20 years. Natural builders are going back to basics plastering rammed earth, cob plasters and straw bale homes inside and out.

What Is There to Learn?

Materials. There are a number of modern materials and systems that have supplanted traditional plastering: EIFS, drywall, Portland cement stucco to name a few. No need for my support there. I’m more interested in what we’ve been using for plaster for the previous 12,000+ years: clay, gypsum and limes. Studying their individual chemistry, physical properties, interaction with each other, compatibility with various building assemblies and highest and best use in diverse climates goes a long way in understanding the traditions surrounding them.

Traditions. In plaster these are as diverse as humankind. I could not pretend to achieve expertise in them all. Nevertheless, there are two basic categories that all plastering can be classified under. Primarily plaster is used as a coating or render. Traditional plaster is used as a coating over a solid substrate or a lath typically to protect and finish the structural supports of a building. However, plaster also can be modeled. Running cornices, coves and other profiles in place or on a bench as well as casting ornament, modeling or carving in situ explores an entirely different art in which plaster excels like no other medium.

How to Share?

Those who know me also know that I believe in sharing plaster knowledge openly. My figurative door is always open to discuss any technical or artistic inquiry. If I know a best practice or can direct someone to reliable information I will share it, freely. There are three ways to learn plaster best practices. 1) Read technical information. 2) Study well done examples of plasterwork. 3) Plaster yourself. By far the best of these is the latter, the physical act of plastering. I am focusing my efforts this year on how I can share, physically. 

I have been working with talented, experienced colleagues to organize hands-on plaster workshops. French, Italian and Moroccan coating traditions are scheduled for January in association with Prima Terra Plasters (http://www.primaterraplasters.com/). A workshop teaching running and casting of plaster is being organized at the same facility for later in the year. Two onsite consultations are similarly being organized to help individual artisans set up their own studio for running and casting plaster.

Plaster is a trade with a rich history and has given me a lot. I’m always enjoying learning more, meeting more people. Hopefully, we’ll have a chance to roll up our sleeves together in 2013. Until then, Happy Holidays everyone!!

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Contributed by Patrick Webb

EN 459-1:2010 European Standard for Building Lime

Monsieur Louis Vicat

Europe has a continuous, documented history of building with lime dating from the Roman Republic, a period of well over 2,000 years. The above referenced standard is the UK implementation of the European Committee for Standardization (CEN) Cement and Building Lime Technical Committee 51. The European Committee for Standardization is an international non-profit providing a similar function to the American Society for Testing and Materials. The standard is a comprehensive document for defining and distinguishing manufactured limes used in construction. Parts 2 and 3 of the standard refer to Testing Methods and Conformity Evaluation respectively.

Whereas lime enjoyed only a brief history of widespread use being largely displaced by Portland cement in the US, in Europe the tradition is maintained and is inclusive of a wide range of limes. This post is a follow up on a previous article on Natural Hydraulic Limes. Hopefully it will serve to dispel some of the mystery and confusion surrounding the classification of NHL’s and highlight the balanced rationale based on experience, science and practical use reflected in the European standard.

Louis Vicat

In our previous article we briefly discussed the long history of hydraulic limes from Roman times, through the Renaissance and culminating in a scientific understanding in the early 19^th century. Frenchman Louis Vicat began his career as an engineer conducting research into limes used for hydraulic works. Although hydraulic lime works had been already been underway since the mid-18^th century Vicat was the first to consolidate the research and publish a comprehensive paper in 1818. Ten years later he would revise and expand upon his original work, publishing Résumé des Connaissances Positives Actuelles sur les Qualités, le Choix et la Convenance Reciproque des Matériaux Propres et la Fabrication des Mortiers et Ciments Calcaires, mercifully abbreviated to Mortars and Cements in Captain John Thomas Smith’s 1837 English translation.

Vicat’s testing procedures and classification index was to be the standard until recent times. The principles they were based on still are. His determination for classification was primarily twofold. The first was the chemical composition and percentage of argillaceous (clayey) infiltration in the given limestone under test. Second, was the reactivity and hydraulicity of the quicklime produced from said limestone. A summary of typical characteristics with approximate ranges for which Vicat himself acknowledged and documented exceptions to strict classification:

Rich limes

Containing less than 6% of inert impurities

Very reactive with a swelling during slaking exceeding 2 times in volume

No set with water

Lean or poor limes

Containing less than 30% of inert impurities

Significantly less reactive with minimal swelling during slaking

No set with water

Feebly hydraulic limes

Containing less than 12% of active* impurities

Reactive with minimal swelling during slaking

Set in 15 to 20 days

Moderately hydraulic limes

Containing less than 18% of active* impurities

Significantly less reactive with minimal swelling during slaking

Set in 6 to 8 days

Eminently hydraulic limes

Containing less than 25% of active* impurities

Almost unreactive with little to no swelling during slaking

Set in 2 to 4 days

*Vicat does explain that by active he is referring to silica not alumina

Vicat’s developed a precise method so he could consistently define when his tested limes achieved a set. However, he also furnishes his readers with a useful explanation that a “set” approximately corresponded to the hardness reached when the mean or average pressure exerted by the arm would resist an impression by the fingertip. I appreciated reading his book that he always provides both scientific, laboratory methods and results as well as practical tests that would be useful in the field for prospectors or workmen.

EN 459-1:2010

Despite our focus in this post on NHL’s, I will say the EU standard provides useful information for a broader range of building limes. For example, there are concise definitions for quicklime and hydrated limes, high calcium and dolmitic limes with impurity percentile categorizations roughly corresponding to Vicat’s rich, lean and poor classifications. Also, there are additional classifications for “Formulated” and “Hydraulic” limes that have additions of pozzolans, fillers, cements, fly ash etc.

Natural Hydraulic Limes fall under three classifications: NHL 2, NHL 3.5 and NHL 5. Not unlike Vicat the classification is based primarily on two factors: chemistry and set. However, the calculations are arrived at differently and I would argue more useful for construction.

NHL 2

Available hydrated lime, Ca(OH)₂ ≥ 35%

Compressive strength at 28 days, ≥ 2 to ≤ 7 MPa*

*A megaPascal (MPa) or Newton (N/mm²) is a metric unit of pressure roughly corresponding to 145 psi

NHL 3.5

Available hydrated lime, Ca(OH)₂ ≥ 25%

Compressive strength at 28 days, ≥ 3,5 to ≤ 10 MPa

NHL 5

Available hydrated lime, Ca(OH)₂ ≥ 15%

Compressive strength at 28 days, ≥ 5 to ≤ 15 MPa

Courtesy of Lafarge Natural Hydraulic Limes

How do the NHL classifications compare with Vicat’s? The chemical requirements are a bit different. Vicat’s tests were based on setting underwater whereas the NHL testing is determined by compressive strength. So we can say they don’t compare exactly. 

Nevertheless, at least in regard to compressive strength, an average NHL 2 generally corresponds and tests within range of what Vicat had classified as Moderately hydraulic limes. NHL 3.5 overlaps between the stronger Moderately and weaker Eminently hydraulic limes. An average NHL 5 corresponds to the stronger Eminently hydraulic limes and stronger NHL 5’s exhibit compressive strengths corresponding to what Vicat might have classified as a Natural cement. Although there are other requirements under the NHL designation such as water demand and retention, bulk density, whiteness etc. this does provide an overview of how the two classifications bear some relationship to one another.

Practical Implications

Why the broad range of allowable compressive strengths for each NHL classification? I’ve yet to read any published documentation addressing this question; however, there appears a general consensus among those involved in manufacturing. Testing requirements for manufacturers as prescribed by EN 459-2:2010 are designed to achieve optimal compressive strengths under laboratory conditions. The mortar has a proportion of one part freshly baked NHL to 3 parts of specified sand by weight (approx. 1:1 by volume). Only enough water is added to the mix to vibrate and compress. 

Lafarge NHL 3.5

Typical field use NHL to sand ratios from 1:1.5 to 1:3 by volume, additional water (unpurified) for workability, lack of vibration/compression are just some of the factors that make it highly unlikely anything near a manufacturer’s published compressive strengths will be achieved in the field at 28 days. The various designated manufacturing requirements of NHL 2, 3.5 and 5 refer to minimum compressive strength requirements in MPa under lab conditions to ensure that mortars reach a practical compressive strength in the field. For sensitive restoration work it is best practice to perform tests on actual mortars under consideration for use in the field rather than rely exclusively on a published manufacturer’s compressive strength.

Average compressive strengths of the classification are as follows:

NHL 2 – 4.5 MPa

NHL 3.5 – 6.75 MPa

NHL 5 – 10 MPa

A significant requirement of the NHL classification is that almost no additions are allowed. The single exception is 0.1% of a grinding agent helpful in the manufacturing process. Two important components result from the baking and subsequent slaking of limestone utilized for NHL: hydrated lime, Ca(OH)₂ and belite, a dicalcium silicate that forms in the baking process. The belite is the component responsible for the hydraulicity of the NHL. During the baking some of the belite agglomerates forming small pebbles. Manufacturers often retain these in the screening process. Manufacturers are permitted to grind these and add them back into the NHL to increase the hydraulicity. This is not considered an addition as it is a component of the original limestone. This is how some manufacturers are able to produce multiple NHL designations from a single limestone source.

There is some controversy over whether it is acceptable practice for an engineer or craftsman to add hydrated or putty lime to lower the compressive strength of NHL mortars in the field. As shown above hydrated lime is already a main component of NHL so there is no fundamental incompatibility. Extensive testing of the effect of high calcium hydrated lime mortars in NHL mortars have been conducted in the UK and results published in Hydraulic Lime Mortar for Stone, Brick and Block Masonry. Estimates for reduction in compressive strength from the addition of lime putty are more difficult to predict as factors such as length of slaking and water content can vary results considerably. 

Historically, pozzolans such as microsilicas  have been added for the occasional need to increase compressive strength, accelerate the set or otherwise alter the properties of NHL mortars. It would be advisable to consult with an expert in the potential long term effects of any such additions.

Contributed by Patrick Webb