The Role of Aggregates and Fibers in Plaster

Rammed Earth
Courtesy of Root Down Designs

Originally posted April 2016 on Traditional Building Magazine Online

“To see a world in a grain of sand and…hold infinity in the palm of your hand.” – William Blake

In my last post, I discussed binders, the “mineral glue” as it were, that holds things together. Now it’s time to consider the “things,” the aggregates and occasionally fibers that typically compose the greater part of the volume and mass of a plaster.

While it’s true that most aggregates and fibers are less costly than our precious binders, they’re not simply cheap fill material. Rather, they play a very active role in the strength and performance of the coating as well as the aesthetic quality of the finish. We’ll approach the subject here by considering various physical properties of aggregates and fibers.

Sharpness

It seems intuitive that if your aggregate is sharp and jagged that it will be easier for the binder to find a “key,” that is to say a means to physically attach. Clays and limes shrink as they dry, threatening to crack or otherwise weaken the plaster. Crushed sand and glass are examples of very sharp aggregates that can help overcome this weakening effect, permitting the application of thicker base coats.
Sharper is not always necessarily better. A disadvantage of sharp aggregates is that they make it challenging to achieve a smooth finish. Often plasterers will switch to a more rounded aggregate such as a river or rinsed sea sand for that final coat. Finish applications are applied relatively thin to minimize shrinkage and the rounded nature of the aggregates allows the plaster to be easily brought to a smooth finish with the trowel. Whether an aggregate is sharp or more rounded can be quickly and simply determined by rubbing it between the fingers.

Granulometry
Another approach to counteract the potential effects of shrinkage is to pay close attention to granulometry, the distribution of aggregates of various sizes within a plaster. Particularly for the thicker base coats it is important to have a small percentage of larger aggregates, a majority of medium size ones and a small portion of fines in the mix. This allows the aggregates to interlock most efficiently with the smallest quantity of binder required to fill the voids. For the thinner coats of finish work a smaller distribution of finer aggregates permits the plasterer to create a smooth, closed surface. Usually a visual inspection in the hand will furnish a good indication if you have a sufficient distribution of aggregate sizes.

Gypsum plaster 1200x magnification Courtesy of Plâtres Vieujot

Hardness

By hardness we’re actually referring to the more interesting quality of softness. Gypsums and certain natural cement binders have the unique and interesting characteristic of being “self-binding,” that is to say they don’t actually depend on aggregates to perform well as a plaster. That opens up a great deal of aesthetic freedom regarding what aggregates can be added to them. Often, softer aggregates such as chalk, porous limestones or crushed gypsum rocks are utilized. It only takes a short while before the binder becomes harder than the aggregates allowing the surface to be cut, sanded or otherwise honed to a smooth surface for a striking resemblance to actual limestone.

Porosity

Silica sand, crushed marble and glass are relatively impervious; however, I personally love the benefits of porous traditional aggregates such as crushed limestone, terracotta and pumice, especially when making lime plasters. I find such plasters more ductile, comparatively smoother to apply. The porous aggregates soak up water like a sponge. They can be particularly helpful for the “grounds” or base coats of fresco work. As water evaporates from the binder it gradually and evenly is replenished from the porous aggregates maintaining the surface at the perfect level of humidity for buon fresco painting, extending the working time significantly.

Density

Being mineral based, most aggregates range from medium to relative high density. However, there are a few exceptional minerals that produce lightweight aggregates. Among these are Perlite and Vermiculite. These minerals have water trapped in their crystalline structure. When heated sufficiently they “exfoliate” or expand dramatically, something akin to a mineral popcorn! The resulting aggregates are correspondingly lightweight, have greater fire resistance and significantly increase the thermal and sound insulation value of plasters made with them.

Traditional lime plastering with exposed stone
Courtesy of Lafarge

Hydraulicity

Certain aggregates create a “hydraulic” or accelerated setting action in lime plasters. We’ll make this family of aggregates, known as “pozzolans,” the focus of its own future essay.


Flexibility

In addition to aggregates, fibers can be a very useful addition to plaster binders. Straw and manure have been used for millennia to help reduce erosion in exterior clay-based earthen plasters. Horse, goat and other animal hairs increase the tensile or flexural strength of gypsum and lime plasters, particularly useful for plasters applied over lath thus subjected to greater shear forces than they would experience over masonry. Burlap woven from hemp, jute, sisal or coconut fibers can be embedded into walls in large webs and are very effective in reinforcing as well as attaching traditional plaster moldings and ornament.

Hopefully this sheds a little light on the importance of aggregates, part of an overall effort to demystify the medium of plaster. In our next essay, we’ll commence our in-depth review of traditional plaster binders with clay, expanding upon its chemistry, manufacture, properties and possible specifications.


Contribute by Patrick Webb

Traditional Plaster Binders

Courtesy of The Academy of Classical Design

Originally posted March 2016 on Traditional Building Magazine Online

Binders? That sure sounds technical, complicated and potentially boring. You might be afraid to ask “what exactly is a binder?” Fortunately there is a simple, interesting explanation: binders are the ingredient in plaster that, as the name implies, “binds” everything together. No doubt, you remember building castles on the beach as a child. Those towers of sand and water weren't all that durable. After the first wave came along, your castle was mush. If only you had added a binder, the “mineral glue” that holds everything else together!

As you might imagine then, selection of the binder is very important for a plaster. More than any other component the binder determines how a plaster will perform, how breathable or hard it will be. In fact, most plasters are named after their binder such as a “gypsum plaster” or a “lime stucco”. In this article we'll give a brief introduction of the three traditional mineral binders: clay, gypsum and lime. For thousands of years, until the middle of the 19th century, these were the only binders used for plaster in traditional building.

Clay

Clay is the original binder and the oldest material associated with construction. How is that so? The primary reason being that clay is extremely abundant and easily accessible. Millions of years of erosion has ground hard minerals like feldspar and quartz into the mass of fine, sticky particles that we identify as clay. Typically, all one has to do is dig two or three feet through the topsoil to encounter thick beds suitable for construction. It is the only traditional binder ready to use straight out of the ground, no manufacturing required. Clay or earthen plasters are easy to make, simple to apply and look beautiful.

Courtesy of Earthen Endeavors Natural Building

Living in North America and Europe where the use of concrete and cement dominates construction you may find it hard to believe that even in the 21st century clay is still by far the most prevalent building material worldwide. Adobe, rammed earth, cob, wattle and daub are all examples of raw, unfired clay based construction. And why not? Clay is both an inexpensive and practically inexhaustible resource having little to no embodied energy in its production. Furthermore, clay is the most ecologically friendly material you can imagine, boasting a completely non-toxic life cycle.

Gypsum

Gypsum is a salt of calcium and occurs naturally as a relatively soft stone. Its formation can mostly be attributed to the decomposition of limestone exposed to sulphuric acid from volcanic activity. Its properties were likely discovered many thousands of years ago around a campfire. By chance gypsum rocks were collected, forming the outer ring of the pit containing the fire. Gypsum “calcines” or changes its chemical state at a relatively low temperature, the typical heat from a campfire being more than enough. Just as we might do today, water was tossed on the fire the following morning when breaking camp. The gypsum rocks would have fallen to powder, absorbing the excess water to form a putty. Within minutes the putty would have hardened again into solid gypsum.

Courtesy of Palladio Mouldings

No doubt some clever camper (maybe a plasterer?) found the properties of this rock potentially very useful. That hardening process was more than just rapid drying. Gypsum plasters undergo a chemical change or “set” that distinguish them from clay plasters which simply dry out. Due to its unique chemical properties, plasters made from gypsum are “self-binding”, in other words they don't shrink. This makes gypsum plasters not only excellent wall plasters but particularly useful for thick casting applications such as mouldings and ornament.

The Family of Limes

Lime is unquestionably one of the most versatile and enduring building materials available to mankind. Lime originates principally from the burning of limestone, the compressed accumulation of millions of years of marine skeletal remains. Similar to gypsum, it has been conjectured that the discovery of lime occurred quite by accident. Limestone calcines at a much higher temperature, a level of heat associated with terracotta and ceramic production. Ancient potters might have been disappointed that their limestone kilns fell apart but their plaster and mason fellows became the beneficiaries of an incredible discovery, the ability to burn and reconstitute limestone.

There are “pure” limes, composed almost entirely of calcium compounds that make beautiful white plasters perfect for everyday use or as the grounds for frescoes like those of Michelangelo. Dolomitic limes are a combination of calcium and magnesium compounds that have a long history of use as mortars for masonry. Some limestones have very specific impurities that have leached into their porous structure. When burnt they form “hydraulic” limes, meaning that they “set” or harden when mixed with water. Depending on the nature of these impurities the set can be quite rapid and very hard. A few limestones with high clay infiltration known as “marls” produce a natural cement. The Romans became experts at exploiting hydraulic limestones and marls in Celtic Gaul, modern day France, to construct aqueducts and ports. Back home they discovered that adding volcanic ash from Mount Vesuvius to pure lime would produce a similar hydraulic reaction, the famous “Roman” cement responsible for the greatest unreinforced architectural work in human history, the dome of the Pantheon.

The Pantheon

We'll return to examine these binders in far greater detail, breaking down their chemical and physical properties as well as highlighting practical plaster applications. However, in my next blog, I'm going to take a close look at what exactly these minerals bind together: aggregates and fibres.


Contributed by Patrick Webb

Roman Architecture: Weeks 7 & 8

Trajan's Column

I signed up last autumn for an online course from Yale University Department of Classics on Roman Architecture. The last couple of weeks we have been focusing on the emperors Trajan and Hadrian, great patrons of architecture at a time when Rome was at the peak of its power. We also received our most challenging assignment yet: To put ourselves in the place of an architect of ancient Rome competing for the major commission of a lifetime, to design and build a newly founded Roman city. To obtain the commission we must submit a proposal to the patron describing our city including drawings and a letter to the patron. I'll share what I submitted in conclusion.

Trajan 98 - 117 C.E.

Grandiosity. The most fitting word I can think to describe the architecture that Trajan would commission for Rome. The world had not seen anything like it. His first order of business was to have a forum built as large as all of the existing imperial fora combined. Though much of it was later destroyed and pillaged for marble, remarkably the famous Column of Trajan stands virtually intact. What a masterpiece! A visual "scroll" unfurls as a spiraling frieze recounting in relief the conquest of the Dacians, a people from modern day Romania who fiercely and nobly opposed the Roman conquest. There is a stairway inside that can still be climbed to the very top, a height of 125 feet, marking the amount of soil removed from the Quirinal Hill for Trajan to build his forum!

An extension of the forum and a remarkable project in its own right were the Markets of Trajan. The shear size and complexity of them makes it hard to believe it approaches 2,000 years. Serving as an ancient shopping mall it would have accommodated hundreds of vendors, the Via Condotti or 5th Avenue of its day.

Markets of Trajan

Hadrian 117 - 138 C.E.

Canopus, Hadrian's Villa

Rome could not have asked for a better architectural patron than Hadrian. He was an architect himself! Both contemporary critics and those of his day try to diminish him as an amateur; however, the attributed Temple of Venus and Roma as well as several of the projects at his sprawling villa give testimony of a gifted sense of design, combined with formal studies. Notice the use of the "Serliana" or "Palladian" opening, the original design informing these Renaissance architects who rediscovered this form some 1,400 years later.

However, the undisputed champion of Roman architecture, representing the triumph of Roman concrete and engineering skill is the Pantheon. What a remarkable accomplishment. Aside from a myriad of remarkable features there is the signature unreinforced concrete dome 142 feet across, the oculus 27 feet across and 7 1/2 feet thick at the opening. I can imagine as the barbarians descended on Rome burning and pillaging centuries later they stayed their hand from the torch at the sight of the Pantheon. Even they could appreciate the excellence, the sheer majesty of her.

The Pantheon

Design Your Own Roman City

It is with utmost humility before your divine majesty, Imperator Augustus, that I present before you these plans for a new Forum for the town of Luna. As I bear witness, your great building program has turned Rome into a symbol of greatness for the world to observe and you have instructed humble servants such as I to share the greatness of Rome throughout the provinces. Luna indeed, is a fine beneficiary of your benevolence for our fellow citizens diligently oversee the quarrying of the finest marble in service of the Republic.

 

The selection of a very healthy site is proposed, elevated from the river Macra, away from the marshes, closer to the marble quarries. Much thought was also given to the dangers of unfavorable winds from the sea and mountains. The Decumanus Maximus has been laid from Solanvs (east) to Favonivs (west), granting access to the forum through the precinct wall whilst the Cardo Maximus runs from Septentrio (north) to Avster (south) and lead to Suburbia. Concerning materials for the building works the province is richly blessed with cypress and fir producing timbers felled in autumn, suitable clays for brick making and the finest marble for the cooking of lime. Just the pit sand of Baiae is lacking for the Opus Reticulatum and

Caementicium (concrete) works should your excellence deems these plans worthy of your patronage.

Mercury has indeed blessed our people with a fine economy so a prostyle, tetrastyle, pseudoperipteral Templum to honor him is raised upon a grand Podium against the precinct wall of the Forum manifesting the symmetry of god and man, duly proportioned in its whole as in its members, of the Ionic order as befits the statesman of the gods. I have taken note in its placement the wise observations of your servant Vitruvius, "the temple and the statue placed in the cella face the western quarter of the sky. This will enable those who approach the altar with offerings or sacrifices to face the direction of the sunrise in facing the statue in the temple, and thus those who are undertaking vows look toward the quarter from which the sun comes forth, and likewise the statues themselves appear to be coming forth out of the east to look upon them as they pray and sacrifice."

The Basilica adjoins the Forum, has its breadth exposed to the southern sun and its entrance situated to avoid a direct wind, a suitable orientation so that the discomforts of inclement weather will in no way hinder the matters of the Republic. For the citizens both a Theatrum and Thermae has been included. The theater follows the Greek model of concentric circles as the wave of the voice like the wave of water emanates in similar fashion while a vaulted passage allow the Decumanus to pass through. The Palaestra and Natatio provide for vigorous exercise, Apodyteria for changing, and the requisite Tepidaria, Caldaria, and Frigidaria.

May the gods continue to bless your valor and noble policies.
Your humblest servant Marcus Julius Patricius

DCCLXVI  (14 C.E.)


Contributed by Patrick Webb 

Roman Architecture: Weeks 1 & 2

Temple of Portunus

I signed up last autumn for an online course from Yale University Department of Classics on Roman Architecture. To my pleasant surprise its really fabulous. There is either a free option or $50 if you want to receive a certificate upon completion. Basically, it is an opportunity to virtually participate in a 3 credit hour Spring semester course, including some fantastic forum discussions with on site students and teaching assistants as well as moderators and online students around the world. There are lectures, homework, exams, the full experience of which I would like to share some of it with you!

Early Rome

When Rome was founded in the 8th century B.C.E., the Etruscans were still the most dominant culture on the peninsula, based out of Tuscany to the north. Before long the Greeks would begin to establish colonies to the south in Nea Polis (Naples) and on Sicily. The Romans were influenced by both cultures yet established themselves as unique. One way the Romans distinguished themselves was in their early temple architecture. We'll take the well preserved 1^st century example of the Temple of Portunus on the Tiber river to illustrate this point.

Temple of Portunus plan

Etruscan temples were typically a masonry structure fronted by deep porticoes, sitting on high podiums, with a single stair oriented to a dominant façade. These features are often preserved by the Romans; however, the tripartite cella and a rather simple 'Tuscan' order of wood construction that allowed for wide intercolumniation typical of Etruscan design was eventually rejected.

Tetrastyle façade, Ionic order

Greek temples used the Doric or Ionic order, featured a single cella, and had narrower intercolumniation that was peripteral, in other words having supporting columns going around the entire temple. The Romans worked out a compromise, incorporating elements of both styles resulting in a temple design that was more Etruscan in plan but featured peripteral, engaged columns or pilasters resembling Greek prototypes in elevation.

Does New Technology Lead to Revolution or Revolution to New Technology?

Our initial assignment was to write a short essay considering whether the discovery of a new technology leads to the creation of new forms or conversely whether the desire to express something in a different way leads to the invention of a new medium. Below was my response:

I would venture that the modern Western perspective of the role technology in architecture and culture is fundamentally different than that of the Ancient Romans.

Illinois Institute of Technology
S.R. Crown Hall

A principal leader of the Modernist architectural movement, Mies van der Rohe, emphatically declared at an address in celebration of the addition to the Institute of Design to Illinois Institute of Technology that “Architecture depends on its time”, going on to state that his real hope would be that architecture and technology would “grow together, that someday the one be the expression of the other”. Certainly in his own work and teaching, Mies was a proponent of using technology (specifically modern concrete and steel) as a driving force in pushing the limits of structural possibility.

Sanctuary of:
Jupiter Anxur
1st century B.C.E.

 Everything I have studied and come to  understand of Roman architecture and culture has led me to the conclusion, that by contrast, the Romans understood architecture not as a temporal phenomenon but a locational one. That “locus” was Rome, in the sense of an actual place, likewise in the sense of a powerful culture. The use of technological innovations such as the arch, vault, dome and Roman concrete served as a physical representation of their profound sense of history and identity. If there was a revolution, it did not manifest itself as a radical rejection of historical precedent. Rather, what we see from the Romans is a deliberate, measured, incremental sophistication expanded upon over centuries, of which we today can appreciate as a harmonious legacy.




Contributed by Patrick Webb

Plaster Word of the Day; 21 - 30

CLAY

The rebel Titan Prometheus was said to have fashioned man from clay. The Jewish, Christian and Islamic tradition all attest that the first man Adam (whose name means red earth) was likewise a divine result of breathing life into clay.

Mastery of clay is in fact integrally associated with civilization itself, at the heart of what separates the man from the beasts. Clay bricks and plasters permitted man to raise his first shelters and establish his first cities. Clay and earth continue to be the primary building materials worldwide.

Modern, Western society has eschewed clay and earth as a primitive medium in favour of high embodied energy materials and industrialized systems. However, we may share father Adam’s punishment for our transgression, “from dust you came…unto dust you will return.”

BINDERS

Photo courtesy of Plâtres Vieujot

‘Binders’ are the fine materials in a plaster that undergo a physical and usually a chemical change to ‘bind’ the plaster together in the setting.

Clay binders arrange themselves in tetrahedral sheets or ‘platelets’ a purely physical change.
By contrast, for gypsum and lime based plasters the binding action is accomplished by interlocking crystal formation, both a physical and chemical change.

COMPO

‘Compo’ really short for “Composition Plaster’, is a material and technique developed by the French in the late Baroque and very much associated with the Rococo period.

Various proprietary recipes are typically some mix of pine rosin, plaster of Paris, linseed oil, turpentine and the key ingredient rabbit skin glue. The key properties of the resulting mix is a flexible yet strong medium that captures fine details and will adhere to both plaster and wood.

LATH

Photo courtesy of Plâtres Vieujot

A latticed background of wood or metal for the support of plaster, most often over a timber frame backgroun. The penetrations or separation of the lath are so spaced to allow a portion of the plaster to fall behind and form a ‘key’ upon setting.

Although expanded steel is the most common lath system today because of ease of manufacture and installation, a quality wood lath such as chestnut produces a system that can endure for centuries, even through several plaster replacements.

 

DOUBLE-BACK, SWEET COAT


This technique of applying fresh plaster onto still moist plaster can be very useful for controlling the total thickness of a finish coat.

Because the initial coat retains moisture there is less suction thus reducing movement, avoiding ‘cold joints’ and facilitating a smooth finish. Very little plaster is used for this coat; instead the 'sweet coat' plaster acts more as a lubricant to level and close the surface.

SOAP

As plasters remain absorbent, topcoats are often used to provide some stain resistance especially in interiors. Traditionally in Europe natural soaps were preferred over waxes. The main reason is that unlike a wax which rests as a film on the surface, these soaps penetrate into or ‘impregnate’ the pores of the plaster.

If freshly applied over a lime plaster a chemical reaction takes place, ‘saponification’. The resulting reaction of the alkali soap with the fresh hydrated lime produces a thin protective layer of calcium stearates (i.e. soap scum) which is highly resistant to liquids and at that concentration remains transparent.

BLACK SOAP, SAVON NOIR

The traditional natural soap of North Africa based on raw olive oil and potash (potassium salts, mostly potassium carbonate).

WHITE SOAP, SAVON DE MARSEILLE

The traditional natural soap of Southern France based on palm kernel oil using lye and soda ash as the alkalizing agents to maintain its whiteness.

HEMP LIME

The woody inner core of the hemp plant, called the shiv or hurd, has excellent binding as well as thermal

Photo courtesy of Vicat

insulating properties. About 20 years ago the French began mixing Hemp shiv as an aggregate with NHL or Natural Cement and pouring into forms to create monolithic substrates.

After a quick setting, the low embodied energy, insulating building substrate is ready to plaster inside and out. The British and Irish soon followed suit and the technique is gaining notoriety in the US and Canada. 

NATURAL CEMENT, ROMAN CEMENT

‘Natural Cement’ is an eminently hydraulic lime that is baked from clayey marls that have an high alumina content in addition to active silicas. The Romans exploited hydraulic lime deposits and would sometimes combine them with pozzolans to increase the hydraulic setting action even further.

With properties of a rapid set, self binding and high resistance to salt attack, natural cements are used for exterior statuary, run mouldings and as an accelerator for natural hydraulic limes. 

Photo courtesy of Vicat

Contributed by Patrick Webb  

Plaster Word of the Day; 1 - 10

Image courtesy of Plâtres Vieujot

PLASTER

One of the foundations of a good trade education is understanding the vocabulary of that trade. So for a plaster education we’ll begin with the most fundamental of terms, Plaster.

In some languages and localized areas of the English speaking world plaster is ‘mineral specific’, such as a gypsum derived material, or ‘use specific’ such as a coating applied in interior. However, our English word “plaster” ultimately derives from the Greek ‘emplastron’ (εμπλαστρον) meaning “to daub on”.

So the term “plaster” can rightly describe a variety of interior or exterior coatings that are daubed, thrown or trowel applied.


SCREED

Finally we have an English word that has its roots in…English!
Screed is an Old English word that originally meant a strip of cloth but eventually came to mean a strip of anything. In fact, ‘shred’ has the very same etymology.

In plastering it refers to strips serving as a termination and guide for the larger infill areas. Often if wood, metal or non-plaster materials are used it will be referred to as a ‘ground’. However, if the strip is made of plaster it maintains the name screed.

Also, a screed is a generic term for the straight edge used to level the infill surface and the act of leveling is called ‘screeding’. There are a variety of screeds that have specialty applications with names such as rods, darbies, slickers etc.


RENDER, FLOAT, SET

This grouping of vocabulary comes from across the pond, common terminology in the UK and Ireland.

It is most descriptive (but not exclusively) of the traditional 3 coat lime plastering system used over monolithic substrates such as brick or stone.

The RENDER is the first coat of plaster, the ‘coarse stuff’ with large, sharp aggregates. It is applies rough and and gets scratched up to provide a mechanical key or bonding surface for the next coat.

The FLOAT is the next coat with the same material. However, the surface is scoured, compressed and leveled with a wooden hand float, hence the name.

The SET, is the final coat. This is the final mix with lime putty and fine aggregates with Plaster of Paris (gypsum plaster) added to initiate a 'set' or firming up of the lime.


GYPSUM PLASTER

Image courtesy of Plâtres Vieujot

The Greek work ‘gypsos’ (γύψος) described the mineral that we today identify as gypsum.
Gypsum plasters may contain sand and small percentages of other binders such clay or lime.
Many useful gypsum plasters are in fact a blend of different types of gypsum:

Gyp – Pulverized gypsum with the chemical designation hydrated calcium sulphate (CaSO4•2H2O). Often used as a natural accelerator in gypsum plaster.

Beta Gypsum – Gypsum baked at a low temperature to form calcium sulphate hemi-hydrate (CaSO4•~0.5H2O). Also, known as Plaster of Paris.

Alpha Gypsum – The hemi-hydrate form baked under higher barometric pressure. The resulting material can have up to 10 times the compressive strength of beta gypsum.

Anhydrous Gypsum – Gypsum baked at a higher temperature to form pure calcium sulphate (CaSO4). Often used as a natural retarder in gypsum plaster.

BLISTER BRUSH

Also known as a ‘felt brush’ or just a ‘mop’.

This is an effective tool for moisture management when finishing clay, lime or gypsum plasters.

The release of water is directly related to pressure applied. Over-saturated areas common with sponges and pump sprayers are easily avoided with a blister brush.

 
SGRAFITTO

Image courtesy of Plâtres Vieujot

This technique of carving into a plaster to reveal a contrasting colour underneath is very much associated with the Italian tradition from whom we have directly borrowed the word sgrafitto meaning ‘scratched’. However, the Italians in turn had borrowed the term from the Greek ‘gráphein’ (γράφειν) meaning to write.

From the same origins the Germans first coined the common designation for the form of carbon useful for writing, ‘graphite’.

 
BENCH RUNNING

Plaster mouldings can either be formed 'in place' or on a flat table or 'bench' to be later applied.

When formed on a bench typically Plaster of Paris, moulding plaster is used which has a rapid set. You have to move quickly when using pure moulding plaster hence the term 'running'. 


GYPSERIE

The technique of chiseling into fresh plaster in deep relief in situ (called ‘gebs’ in Arabic) is a craft that was developed to perfection by the Moors. After the expulsion of the Moors from the Iberian peninsula Muslim artisans were given opportunity to convert and some continued their craft in Christian motifs developing a very rich tradition in Provence.

Gypsum is the ideal medium for this technique. While fresh gypsum plaster is soft enough to easily carve it has great plasticity and will not easily crumble. This makes it possible to achieve deep undercuts with sharp edges and thin details that soon develops hardness and durability as the plaster dries and fully cures. 


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