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

An American Couple’s Perspective on French Wine and Plaster Traditions: Viticulture

Château de Chambert

Nature. Culture. Perhaps these seemingly disparate aesthetics were no better reconciled than by the French Renaissance tradition of the formal garden.

“In the Renaissance taste the garden was an extension of the main design. It was a middle term between architecture and Nature. The transition from house to landscape was logically effected by combining at this point formality of design with naturalness of material.” – Geoffrey Scott, The Architecture of Humanism

To this point we have considered Varietals and Terroir…learning about grapes and minerals…exploring soils, weather and geology…recognizing all of nature’s generous contributions. All that we have hitherto discussed is most fundamental; however, wine and plaster are uniquely products of culture. The balance of our five part series will consider the human touch.

Viticulture in Wine

Although located in what is considered the “old world” of wine production, Bordeaux is squarely in the forefront with regard to wine-making technology.  So in this segment we are going to discuss an aspect of the Bordeaux wine industry that receives nowhere near the attention it deserves. We are talking about viticulture. Viti is latin for vine therefore viticulture roughly translates to vine cultivation.  In this article, we will examine two methods of viticulture that are essential to making a great wine; vine manipulation and pest control.

Vine leaves contain chlorophyll cells that absorb sunlight enabling the plant to extract carbon dioxide from the air and convert it to sugar. The nutrients imparted by the sugar feeds the vine roots, grape clusters and leaves ensuring the entire plant receives exactly what it needs, when it is needed.

Allowing too much foliage shields the grapes from the sunlight they need for the last stage of their healthy development, so pruning is crucial to producing a quality wine. However caution must be exercised with cutting, because every cut is an entry point for pests to enter and attack the vine.  On the other hand, if too many leaves are pruned, the plant does not have the means to absorb sufficient sunlight to sustain the entire vine.

Wine grapes emerge at the end of the growing season so the plant’s nutrients must further be shared with the new grape clusters. If there are too many clusters, the sugar and acid levels will likely be undeveloped and/or unbalanced resulting in a poor showing as a wine.  Too few clusters negatively affects potential profits from wine sales.

Pest control is another very important aspect of viticulture. In the 1870s a small, deadly phylloxera louse made it’s way to Europe and all but wiped out all wine production. Phylloxera destroys the grapes, rots the vines and often leaves its larvae in the root, eventually killing the vine completely.  Although Bordeaux and Europe at large have regained their wine producing capabilities, phylloxera and other lice, along with viruses, bacteria, fungi, mites and insects are still among the many threats to healthy vines.

In an effort to eliminate ongoing threats to their vineyards and livelihoods, many late 20^th century wine growers often used chemical fertilizers and pesticides indiscriminately.  Thankfully much has changed since then with most of the region’s winegrowers using more environmentally conscious, natural pest control methods.  For example, Bordeaux wine growers are currently and constantly experimenting with root grafting in order to find the genetic combination that is naturally resistant to harmful bacteria and viruses.  Scientists and wine growers are also experimenting with sea algae as a natural deterrent to gray rot. 

There is no doubt that viticulture is both science and art.  Winemakers must have intimate knowledge of their vineyard’s terroir as well as which viticulture methods will work best within its parameters. It is with this intricate knowledge and dedication to quality that winemakers are able to extract the best wines from the best grapes.

Viticulture in Plaster

France is a geologically, minerally rich country. Correspondingly rich in culture, the French have been very successful in exercising their influence over a number of raw mineral materials to produce some of the finest plasters in the world. The plaster equivalent to Viticulture is baking. Let’s now take a closer look at how 3 minerals are prepared for our blended plaster, Terre de Séléné.

Clay is the primary mineral used for plaster in Terre de Séléné. It is an abundant mineral worldwide, the result of millions of years of erosion. In parts of France a relatively pure form is available just under the topsoil, just a few feet below ground. It is easy to excavate and is still traditionally dried by the sun. Later, with minimal effort, it is ground into a powder ready to be used for plaster. While there are a variety of clays in France, clay with a low shrink-swell capacity such as Kaolinite is desirable for Terre de Séléné.

Historically, the French were enamored with this type of clay for additional uses. The word “Kaolin” comes to us directly from French. They in turn inherited the term from China. In the early 18^th century the French were obtaining an extremely pure form of clay useful for porcelain, “China”, from a deposit near a mountain the Chinese called Kao “high”, Ling “hill”.

Gypsum is the secondary mineral used in Terre de Séléné plaster. Gypsum is plentiful in France and particularly so in Paris. Gypsum plaster is almost synonymous with the expression “Plaster of Paris”. Paris in fact sits on a “massif” or deposit of mineral gypsum that is among the largest and finest in quality on earth. Naturally occurring gypsum is a type of salt that precipitates through cycles of evaporation from lime or other calcium compounds, typically in lagoons or inland seas.

Preparing gypsum plaster requires a little more effort and energy than clay. It is usually mined from underground deposits. Relatively soft as a stone, it is easily pulverized to a coarse sand ideal for baking. Most of the gypsum plaster useful for Terre de Séléné only needs to be baked at under 350° F for less than an hour. In general, considerable influence can be exercised in the baking process. Adjustments to the grind, temperature, length of baking and even barometric pressure can produce an amazing range of properties in gypsum plaster such as fast setting plasters good for casting or extremely dense, hard plasters appropriate for floors or countertops.

Limestone is the third mineral used for our plaster blend. In abundance in the South of France, limestone is a sedimentary stone, the result of millions of years of marine skeletons accumulating on ancient sea beds. The lime most useful for Terre de Séléné plaster is very pure, having little contamination from magnesium or silicates. By itself, limestone is very useful as a building material; however, to produce a plaster requires considerable fuel and labor.

Limestone is found underground but is plentiful and easier to extract from surface mines. Much harder than gypsum or clay, extraction is laborious. For baking limestone is broken into golf ball size pieces. Traditionally, it was baked for 24 hours in vertical kilns at an extremely high temperature of 1500° F. Modern production methods utilizing crushers and horizontal kilns have reduced the time considerably.

The resulting “quick” lime is highly caustic, potentially hazardous to handle. At this point of production enough water is introduced to cause a partial reaction that reduces reactivity and danger. The slaked lime, also known as dry hydrate, is now ready to be blended with the clay and gypsum plaster to make Terre de Séléné.

As you have read, the French traditions of Viticulture and plaster preparation are very sophisticated. The usefulness of our modern scientific, chemical understanding still lags behind the practical experience gained through centuries of empirical observation and practice. This is especially evident in our subsequent, fourth segment considering the art of the blend, Viniculture.

This article was coauthored by Angela and Patrick Webb