Natural Hydraulic Lime

Traditional French NHL stucco, mortar

As we considered in a previous post, lime is perhaps the most prized and exceedingly versatile building material of the modern world. Early civilizations such as the Egyptians, Greeks and Romans used lime extensively. Many of their works in lime have survived to the present day testifying to its durability and intrinsic beauty.

Today, lime is processed into plasters, stucco coatings, paints, mortars and cements. Chemically defined, pure limestone is a carbonate of calcium or calcite having the formula CaCO₃. However, there exist several categories of impure limes (dolmitic, magnesium, natural cement i.e.). The subject of our post today focuses on one of these: natural hydraulic lime or NHL. As we shall discover, sometimes impurities result in interesting and very useful properties.

Hydraulicity

What makes a lime “hydraulic”? As you might guess it has to do with water. Perhaps we can start by considering a non-hydraulic example, pure lime*. When water is added to pure lime it forms a putty. As long as the mix is kept covered the lime will stay in a putty state. Pure lime only reacts chemically when exposed to air, reabsorbing carbon dioxide and returning to its original state of calcite, CaCO_3.

Hydraulic materials however, exhibit a chemical change with water or in the presence of water. Moulding plaster and Portland cement are common examples of hydraulic materials. NHL’s mixed with water quickly transform from a putty to a hardened state, even underwater.

Geology

Where do hydraulic limes come from? Limestone is a sedimentary rock that forms from skeletons of marine creatures that have accumulated on the sea floor. With time and pressure these skeletons are pressed together in beds of stone. Nevertheless, limestone remains relatively porous and under certain geologic conditions impurities can leach into or infiltrate the stone over time. A valued impurity for NHL is silica.

Common silica’s like quartz are very prevalent, highly crystalline and non-reactive. Amorphous, chemically active silica’s on the other hand don’t tend to last very long in nature because they are very reactive, especially with lime. The most useful limestones for producing NHL’s have a high amorphous silica content. These limestones are cooked a little hotter than pure limestone, approximately 1100 to 1200 °C, to drive off the carbon dioxide. Once the carbon dioxide is driven off the lime is available to react with the amorphous silica, just add water!

History

Tadelakt objets d'arte, Marrakech souk

The Romans were famous for their great works of architecture. They were the first to have a level of understanding and to make widespread use of hydraulic limes for ports, aqueducts and monumental architecture. Many of these works were accomplished with additions to lime to make them hydraulic. Pozzolanic lime is a subject we will consider in a future post. However, there is also evidence to support that the Romans exploited limestone deposits in the province of Gaul, modern day Languedoc and Provence regions of France, which produced limes that were inherently hydraulic without additions.

Eddystone Lighthouse, 1756

The Romans brought their lime technology in the conquest of North Africa. The tradition of using natural hydraulic limes continued for water cisterns, stucco and objets d’arte. During the Renaissance Palladio makes mention of hydraulic limes in his architectural treatise. By the 18^th century English and French engineers were hard at work identifying quality mineral deposits and exploiting them for public works. Advances in modern chemistry led to the 1807 discovery that lime was not an element but an oxide of calcium. With this knowledge established, French engineer Louis Vicat conducted an exhaustive study and published a landmark, comprehensive paper in 1818 classifying limes on the basis of hydraulicity and compressive strength.

Contemporary Use

With the advent of Portland cement in the 19^th century, NHL production decreased dramatically. The faster set, harder compressive strengths and impermeability of Portland cement were considered superior qualities that allowed buildings to be constructed faster and cheaper. However, with the passage of time and a large inventory of buildings using both materials, advantages of NHL have become clear and production is once again on the increase.

The lower compressive strength of NHL is now appreciated as a good quality for mortar and stucco. The flexibility of natural hydraulic lime reduces cracking, allowing wall assemblies often to bend rather than break when subject to typical settling over time. The increased porosity of NHL stuccoes facilitates water that penetrates the coating to readily escape through the surface. This same porosity is also of great benefit to masonry work permitting soluble salts to slowly deteriorate the mortar (which can be re-pointed), protecting the more valuable brick or stone supports. NHL’s thus preserve many of the benefits of pure lime mortars and stuccoes whilst allowing masonry and stucco work to be conducted at a faster rate and under a greater range of weather conditions.

*Technically there is a chemical reaction of quicklime, CaO, with water to form slaked lime Ca(OH)_2.  However, this article refers to the common definition of a hydraulic lime reaction, the formation of calcium silicates.


Contributed by Patrick Webb 

Buon Fresco

Villa dei Mistieri, Pompeii

The Villa dei Misteri, Pompeii. Raphael’s Villa Farnesina. Michaelangelo’s Sistine Chapel.  All are iconic classical and renaissance examples of decorative art closely allied to architecture. The enduring vibrancy of these masterful works of antiquity is attributable in no small measure to the nature of their shared medium: the buon fresco.

True frescoes are the result of painting mineral or oxide pigments into a still moist or “fresh” lime surface. The pigments penetrate into the lime and become integral to the coating as it dries and cures. Protected from the elements a buon fresco can last indefinitely.

History

Support for such a claim can be found at what is thought by many to be the oldest discovered human settlement, Çatalhöyük circa 7500 B.C. Much of what we know of this ancient civilization is preserved in fresco depictions of hunting, husbandry, maps and geometric motifs of apparently purely artistic expression. Subsequent great civilizations such as the Egyptians, the Indus Valley Harappan, Classical Greece & Rome continued to make widespread use of frescoes in their respective cultures.

Our Western cultural heritage of art and architecture has been heavily influenced by ancient Rome. The Romans left behind many well-preserved examples of buon fresco such as those found at Pompeii, Herculaneum and Nero’s palace, the Domus Aurea. Furthermore, we can be very grateful that Roman engineer and architect Marcus Vitruvius Pollio dedicated an entire book in his multi-volume work De Architectura on preparation of supports, lime and pigments for fresco painting along with application guidelines and colorful, opinionated commentary on what he considered the overly decadent aesthetic of his day.

Vitruvius’ writings would experience publication and circulation in the early 15^th century at the dawn of the Renaissance. High Renaissance artists such as Rafael, Michelangelo and Leonardo da Vinci would study and expand upon Roman examples to become great masters in their own right. This article serves as an introduction to basic fresco techniques: plastering the grounds and painting of the fresco.

Plastering the Grounds

Interestingly, ancient examples of wall plasters were primarily earthen. In the dry climates of Egypt and the Fertile Crescent, fuel for burning at the high temperature production of lime required was limited. Lime was undoubtedly considered a very precious material and was often only used as a finish coat into which the fresco was painted. In Greece and Rome it became common practice to plaster in lime directly over masonry walls for important works, the Romans sometimes opting for wood lath on ceilings. Fortunately, the principles recorded by Vitruvius were utilized in the Renaissance and still apply today. The basic method to achieve a good ground always entails building up initially with a rough, aggregated coat of plaster, applying subsequently finer sand coats and finishing with a thin, smooth finish coat. The process should maintain enough moisture in the system to give the artist sufficient open time to paint into the finish coat.

Trullisatio. Vitruvius describes in great detail this rough base coat composed of lime, sand, and larger pieces of broken terracotta. A similar function is achieved with what we call a scratch or render coat. It serves as a rough intermediary between the masonry or lath support, providing good mechanical key or adhesion for the following coat.

Aricciato. Vitruvius recommended 3 coats using lime mixed with increasingly finer sands. This directly corresponds to our brown or float coat. Each coat is floated with a wooden trowel to compress and harden the surface. Because lime needs time and moisture to cure each of these coats should be wet down daily and allowed to stand for approximately a week before the next coat. Following the precedent of 15^th century architect Leon Battista Alberti, Renaissance and contemporary frescoists have reduced this to one or two coats.

Intonaco. Once again Vitruvius recommended 3 coats using lime this time mixed with increasingly finer marble powders. There is archeological evidence that quality Roman works adhered closely to these advices resulting in total thicknesses of as much as 2 inches. Finish coats varied in the Renaissance as they do today. One Italian contemporary method is to apply a cocciopesto arenato coat (terra cotta sand added to marmorino), compress with a sponge float and immediately apply a finish marmorino. The application firms up in about 2 hours leaving about 6 to 8 hours of working time to paint the fresco.

Painting the Fresco

Following the final coating of plaster, the painter must quickly begin the work of transferring and applying the design before the moisture evaporates from the plaster and locks out the pigments. Working time in buon fresco varies with the humidity of the air and the subsurface, but can generally be assumed to be around 8 hours.

Preparation for painting is key; the design must be understood well, the colors should be prepared and ready for use. Painters of fresco in the past usually had a small crew of assistants to do the plastering, grind and mix colors, and be on hand in case an area had to be redone. It is essential to have done test samples of colors and their tints to judge what they will look like dry, as many colors will change dramatically, especially if they have been mixed with “lime milk”, which goes on clear but dries bright white.

After the plaster has been applied and allowed to slightly set, the first step is transferring the design. Sometimes the design is applied by brush under the final coat of plaster (a sinopia); in other cases it may be scribed into the soft surface or be transferred via a pounce, a drawing that has been pricked with holes to allow charcoal dust to be rubbed through it.

Once the design has been applied, the painter begins to lay in color deliberately and economically, in the manner of watercolor or egg tempera, using the color of the ground as the lightest value, although diluted lime milk may also be used as white and as a mixing tint for colors. The palette for buon fresco is limited to colors that are immune to the chemical severity of lime; several popular colors are not available for fresco, but the palette is nonetheless quite varied, as the ancient murals at Pompeii will attest to. Colors are generally mixed by hand grinding pure pigments with water using a muller and plate, thus pigments that are toxic, such as the cadmium colors, must be used with great caution. There are also newly available pre-mixed colors that can be used, though some purists would reject them. As the plaster dries, the colors are actually drawn down into the surface and a very thin coating of calcium carbonate rises to the top to encapsulate and protect the pigments in a very strong and non-yellowing bond.  Protected from water, the painting of a fresco will last indefinitely. There are techniques for painting after the plaster has dried (called “a secco”) but that will have to be a topic for another day.

Looking to the Future

Traditional frescoes using pure lime as practiced in the Renaissance require a lengthy process taking several weeks due to the necessity of waiting for each layer of lime to cure. A practical alternative is to utilize natural hydraulic lime for the scratch and brown coats. Unlike other setting plasters or cements, natural hydraulic limes do not contain harmful compounds that could later cause efflorescence or otherwise damage the fresco. Natural hydraulic limes achieve a partial cure sufficient to receive a subsequent coat overnight. The entire buildup of grounds can be accomplished in 3 or 4 days at a total thickness of ½”.

Yet another system of grounds being researched for use directly over drywall is a blended mortar of clay, gypsum and lime. Gypsum provides good adhesion whereas the lime and particularly clay components retain moisture for many hours, ideal properties of a base plaster receiving the Intonaco finish and fresco painting.

Conclusion

Intrinsic, enduring beauty has always been a motivational reason to consider buon fresco. However, advances in plaster grounds, safer pigments and reduced expense make the buon fresco not just a whimsical curiosity of the past but a contemporary, vibrant medium accessible to every artist, including you! 

This article was coauthored by Patrick Webb and Steve Shriver

The History of Plaster in Classical Architecture: The Ancient and Classical Periods

Çatalhöyük fresco (ca. 7500 BC)

The art of plastering is as old as civilization. In fact, stated more emphatically, without plaster there is no civilization.  Mankind’s ability to leave the cave, raise a shelter of stones or reeds and coat that shelter with an earthen plaster enabled him to create the “cave” wherever he desired. Building permanent dwellings close to fresh water, upon a fortifiable position or adjoining arable land allowed extended families to gather and the first cities to be born.

The very first plasters were earthen. Being simple mixtures of clay, sand and straw they required no furnaces and dried with the sun. The mixture was cast as bricks and the same basic formula was used as the mortar and stucco. Earthen plasters such as cob and daub are still the most commonly used plasters worldwide.

Calcium plasters such as gypsum and lime were likely discovered through the process of pottery making. By chance, rocks of gypsum or lime were selected to form the crude kiln for firing pottery. The heat of the fire drove off water (gypsum) or carbon dioxide (lime) leaving friable rocks quickly falling to powder. With water thrown on the embers to quench the fire it was soon discovered that this powder formed a paste that quickly hardened.

The Ancient World

One of the earliest archeological examples of both civilization and plaster is that of Çatalhöyük (ca. 7500 BC) located in present day Turkey. A densely populated town, Çatalhöyük‘s dwellings had mud brick walls and floors coated with a locally available clayey marl that made a suitable plaster. What little we know of this ancient civilization survives in lime frescoes depicting numerous scenes of hunting, volcanoes and geometric patterns of purely decorative expression.

Nefertiti

The best preserved examples of plasterwork in the pre-Classical period are found in the monumental architecture of ancient Egypt dating from the 3^rd millennium BC.  Practical construction uses include the pyramids of Giza containing gypsum and lime mortars, the exteriors of which originally received smooth lime stucco. Countless surviving works of frescoes and ornament such as the renowned gypsum bust of Nefertiti attest to the parallel artistic development of plasterwork. In fact, the lime and gypsum plasters produced in Egypt were in many cases of superior quality than commercially available today. This gives testament to the fact that the empirical refinement of plaster manufacture extended many generations further back in time.

The Minoan civilization emerged in the 2^nd millennium BC on the Mediterranean isle of Crete. The Minoans were greatly influenced by the still flourishing Egyptian culture as evidenced by the architecture of the palaces at Knossos and Phaistos. However, the Minoans were to distinguish themselves by the extensive use of plaster in their interiors. In contrast to formalized Egyptian motifs typically carried out al secco, the Minoans had an exuberance of colored decoration realized al fresco. Although maintaining the profile view and stark outline typical of Egyptian art, the buon fresco techniques employed by Minoan artisans obligated a faster pace and improvisation resulting in a fluid, vibrant aesthetic.

The Classical Period

The Mycenaeans would succeed as the dominant culture of Crete and the Greek archipelago maintaining and refining the Minoan architectural style. However, as Rome would fall centuries later to the barbarians plunging Europe into a Dark Age, a similar fate befell Mycenae primarily at the hands of the Dorian and Ionian conquering tribes. During this Greek Dark Age much of the knowledge of construction and architecture was lost for a period of centuries. Finally, in the 8^th century BC, the two rival groups would join to form the Hellenes and establish a culture that left an indelible mark on human civilization.

Although the use of plaster never ceased entirely, it too would experience a renaissance in Hellenic Greece. Thanks to the Greeks we have the English word “gypsum”, directly derived from the Greek gypsos (γύψος). Similarly, it is easy to see the correlation between our word “plaster” with the Greek emplastron (εμπλαστρον) meaning “to daub on”. Beyond our debt of vocabulary, we owe the very foundation of our Western architectural heritage to the Greeks. The highest expression of ornament and representation of the Doric, Ionic and Corinthian Greek architectural orders to this day continues to be realized in plaster.

The Greeks were conquered militarily by the Romans in 146 BC. Yet culturally the Romans were simultaneously enthralled by Greek culture adopting and incorporating their philosophy, architecture and art. The Romans continued the tradition of temple architecture; however, they extended their monumental architecture to include secular basilicas, imperial monuments and palatial villas. Emperor Nero’s Domus Aurea or “Golden House” and similar discoveries in Pompeii and Herculaneum are well preserved examples of how lime plastering was brought to an artistic zenith for the Roman elite. These sites offer a glimpse into a bygone era of opulence, of lavish interiors realized in fine plasterwork, entire rooms painted al fresco and barrel vaults coffered with sumptuous ornamentation in bas relief.

Pompeiian Thermae

The Romans produced not only great artists and architects but formidable engineers. A treasure remains to us in the exhaustive architectural treatise, De Architectura, by 1^st century BC Roman military engineer Marcus Polio Vitruvius. In this work known commonly known as the Ten Books on Architecture, Vitruvius dedicates three chapters of Book II to the selection of sand, lime and pozzolans for stucco and concrete works. He further devotes the majority of Book VII to proper lime stucco preparation, application and fresco work.

The greatest civilization of the ancient world coincided with the greatest understanding and development of plaster. The Romans expanded upon a significant discovery made by the Greeks: the additions of pozzolans to lime would create a plaster that sets in water. Concrete was born, architectural engineering was ascendant and the Romans would go on to construct roads, aqueducts and ports that endure to this day. Roman engineering prowess and the discovery of concrete culminated in their unparalleled architectural achievement, the Pantheon. Having an interior diameter of 142 feet at its base the Pantheon remains the largest unreinforced concrete dome ever constructed.

The Pantheon, Rome

Vitruvius treatise began to achieve widespread publication in the early 15^th century. By the late 15^th century there is written and archaeological evidence of Vitruvius’ hydraulic stucco recipes being utilized in Venice and Murano, 300 years before the advent of modern cement. Later we will explore how his writings together with archeological discoveries at the Domus Aurea would inspire creative geniuses such as Da Vinci, Michelangelo and Rafael to attain to dizzying heights of artistic expression in buon fresco and the modeling of stucco during the Italian Renaissance.

Contributed by Patrick Webb

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

In our previous post we familiarized ourselves with the concept of varietals and how they exist in wine and plaster. For wine we learned that each variety of grape possesses unique characteristics and that a few of these grapes varieties actually produce a good wine without blending, known as a varietal.

Varietals are important; however, for the French there is one contributing factor in making a truly great wine or plaster that is absolutely fundamental. That essential component is the one we have little to no control of. It is summed up in a small yet tricky to pronounce word called terroir (ter-whah).

This blog post is the second of a five part series on French traditions of wine and plaster making framed in a very wine oriented vocabulary:

• · Varietals
• · Terroir
• · Viticulture
• · Viniculture
• · Pairing

Terroir in Wine

Terroir comprises all the geologic, atmospheric and climactic conditions found within a wine-making region that give grapes the foundation they need to develop the characteristics we enjoy in our favorite wines. Just like it sounds, terroir is an extensive topic. For our glimpse into this world; however, the following overview addresses three key aspects of terroir: temperature, moisture and soil composition.

All of the world’s major wine growing regions are situated between 30˚ and 50˚ latitude in both the northern and southern hemispheres meaning that cool to moderate temperatures are optimal for wine grape growing. In cooler climates with less sun exposure (think Washington State or Germany) ripe grapes retain enough “greenness” to produce wonderfully crisp, herbaceous wines like sauvignon blanc which pairs nicely with a green salad topped with goat cheese, green beans and basil pesto. In warmer climates (California and Bordeaux) grapes remain on the vine longer sunning themselves and developing more color, sugar and complexity in the process. The resulting big, bold, fruit is why cabernet sauvignon is known as King Cab.

California and Bordeaux are world-class wine producing regions with oceans bordering both regions providing a moderate and stable climate with minimal risk of damaging frosts. Rainfall in both regions averages a healthy 29 to 34 inches annually. In general, when grapes receive less than optimal rainfall, the vines will produce fewer grapes, though often of superior flavor. In contrast, if heavy downpours occur, particularly close to the harvest time, the grapes absorb too much water, which dilutes the flavors and produces low quality wine. So balance in moisture be it rain, fog or dew is crucial to superior grape production. However, while moisture is important for the vineyard, of equal importance is the vineyard’s drainage system. This leads to our third topic of terroir: soil composition.

As a rule the best wines come from grapes that have suffered a bit; where nutrients are available, but the vine must work for them. According to The World Atlas of Wine writers Hugh Johnson and Jancis Robinson state that best soil (for vineyards) is not particularly fertile, it drains quickly thus forcing the roots downward in search of a water supply. Vineyards in the Medoc region of Bordeaux produces some of the most notable wines because beneath the gravelly topsoils are alternating layers of hard compacted sand and clay. These quickly draining surfaces force the roots to dive deeper until they reach a moist layer of sand and clay near the water table. When nutrients are too easily accessible, vines become complacent and results are one dimensional grapes with underdeveloped structure and complexity.

Terroir in Plaster

For plaster, terroir means geology. It is what nature has provided us. You can’t make something from nothing and you can’t produce a great plaster if the geology is poor. Let’s again take a closer look at the terroir of two of the minerals used to make the plaster blend Terre de Séléné: lime and gypsum.

Lime typically is derived from limestone. Limestone is sedimentary rock formed from the skeletal remains of marine creatures that accumulated on the sea floor millions of years in the past. With time and pressure these skeletons are pressed together in beds of stone.

Some limestone contains contaminants of clay or other materials that affect its chemical properties. When these "hydraulic limes" are baked they will readily react and harden when mixed with water. However, many limestone deposits found in both France and the United States are relatively pure with little contamination from clay or other materials. This type of pure limestone produces a lime that blends well with other plasters. It is the very terroir needed for the lime in Terre de Séléné.

I mentioned in the previous post that gypsum and limestone are geologically related. While limestone was forming on ancient sea beds another phenomenon was occurring in salt marsh lagoons along the shore. Through repeated cycles of seawater infiltration and evaporation gypsum, salts and other compounds precipitated and formed large masses.

Many of these masses have been preserved relatively unchanged, covered by layers of clays that protected them from erosion. In these one can find gypsum rock with many of the original impurities that give the gypsum very interesting properties often useful for construction. In many instances though the original gypsum, being more soluble in water than other precipitates, would be carried off with underground water to recrystallize in successively purer forms in subsequent locations. Many distinct forms of gypsum with diverse crystalline, chemical structures and levels of purity formed. There are gypsum crystals found in underground caves in Mexico for instance that measure over 30 feet long and weigh several tons each, the largest crystals in the world! 

Grapes, gypsum and limestone were here a long time before we arrived on the scene. Terroir is the result of millions of years of geology and current climate conditions we study and benefit from but can’t imagine to control. That being said good wines and beautiful plasters don’t make themselves. Next blog we’ll consider the human touch, Viticulture.

This article was coauthored by Angela and Patrick Webb

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

The French are renowned and appreciated the world over for their many traditions and a unique perspective on life that form the foundation of their culture and have contributed to our culture here in America. Who doesn’t love a French press coffee complimented with a buttery croissant on a lazy Saturday morning? A visit to the Metropolitan Museum of Art to see the impressionist works of Monet and Cézanne? Although we as Americans enjoy our own hustle and bustle way of life, I think we appreciate the fact that someone on the other side of the pond has taken a little more time to perfect a few of the finer things and are willing to share. Vive la différence!

Two of those traditions mean a lot to my family personally and professionally: Wine and Plaster. My wife Angela is a professional chef trained in classical French cuisine and a wine consultant specialized in wine and food pairing. I am a plasterer who has made numerous visits to France to improve my skills in moulding and ornament. Angela and I talk a lot about our respective interests and have perceived a philosophical constant, a sophisticated approach to product development that these completely different industries appear to share. We got excited about the opportunity to do a project together comparing the common themes we see and sharing them with others.

This blog post is the first of a five part series on French traditions of wine and plaster making framed in a very wine oriented vocabulary:

• Varietals
• Terroir
• Viticulture
• Viniculture
• Pairing

Varietals in Wine

As you may know, a varietal is a wine made from a single grape variety. Varietals are very popular in the United States, especially where we live in California. I think we like the simplicity of having a wine from a single grape variety that we can learn about and expect to have certain characteristics. By contrast the French generally prefer wines that are blends. For example, the prized and often very pricey Châteauneuf-du-Pape may contain up to eighteen distinct grape varieties!

Throughout our wine and plaster comparison we are going to concentrate on the popular and much simpler Bordeaux blend. A Bordeaux blend can have up to six grape varieties. Typically though, the following three grapes dominate most Bordeaux wines: Cabernet Sauvignon, Merlot & Cabernet Franc. Interestingly, in the United States we grow and blend these grapes in a similar way. More about that later! For now let’s take a closer look with Angela at a couple of the varietals that she and I love to drink, Cabernet Sauvignon & Merlot.

Native to Bordeaux, the Cabernet Sauvignon grape is the love child of the Cabernet Franc and Sauvignon Blanc grapes. You can recognize the Cabernet Sauvignon grapes because they are quite small, their skins are very thick and are dark blue in color. They cling to each other in very tight clusters and bond tightly to the vine with their strong stems. Many of these physical characteristics express themselves by producing a powerful, complex and masculine wine. The skins, large seeds and stems give the Cabernet Sauvignon wine a dark, almost inky color and strong tannins which can be overwhelming in a young wine, but mellow beautifully with age. With regard to aroma and taste, the flavor profile will vary greatly depending on where the grapes are grown. In new world production, cabernet sauvignon is typified by bold, jammy mouth-filling flavor. Up front you may taste over-ripe blackberries and plums or dried currants gently blended with notes of chocolate or coffee. By contrast, old world productions, particularly French, are lower in alcohol and much less fruit-forward. As a result, earthier and more complex notes of mushroom, leather and tobacco are given their chance to shine.

Merlot, sometimes called “cabernet without the pain” is a perfect foil for its partner. Where Cabernet Sauvignon is bold, powerful and angular, Merlot is round, soft and voluptuous. Merlot grapes are larger than cabernet sauvignon and their skins thinner and almost violet in color. These characteristics produce medium-bodied wine with lower tannins. Although, Merlot is an integral part of the orchestra that is Bordeaux wine, it does quite well on its own with a remarkable range of aromas. In the new world merlot shows notes of plump and perfectly ripened dark-skinned fruit while old world merlot displays deep notes of vanilla or coffee beans and earthy aromas of damp grass and leaves. 

  

Varietals in Plaster

Plaster has its varietals as well. Whereas varietals with wines start with a single grape variety, varietals in plaster begin with a single mineral. A few of the popular minerals that historically have been used to make plaster are: gypsum, clay, limestone, marl and silica. Paralleling our tastes in wine, plasters made from a single mineral are very popular in the United States. We generally manufacture and use clay, lime and gypsum plasters mixed only with sand. I think the American approach to plaster manufacture resembles our wine production. We like the simplicity of having a plaster from a single mineral that we can completely understand and expect to have certain characteristics. It probably comes as no surprise that the French have a long history of developing plasters that are blends of many minerals.

During our wine and plaster comparison we are going to examine parallels with the aforementioned Bordeaux blend with a historic French plaster blend, Terre de Séléné. As with our Bordeaux blend three minerals dominate this plaster blend: gypsum, limestone and clay. In the United States we have deposits of these minerals and mine them aplenty. Now it’s my turn to take a closer look at a couple of these minerals.

Pure limestone is a carbonate of calcium or calcite having the chemical formula CaCO₃. Lime is the main component of many materials familiar from everyday life: teeth and bones, chalk and marble are common examples. It is this type of limestone that is used to make the lime for Terre de Séléné. Plasters made exclusively from pure limestone always have certain characteristics. For example, lime is a very white, reflective material which makes it a great base for creating colored plasters. Lime is highly alkali and inhibits mold growth. Lime plasters such as Venetian plaster, Tadelakt and marmorino are very popular in the United States.

Gypsum and limestone are geologically related. Whereas limestone is a carbonate, gypsum is a sulphate of calcium having the chemical formula CaSO₄. Like its cousin, pure gypsum is a very white, reflective material that is easily tinted with mineral colorants. At the same time it has some properties that are unique. Gypsum plaster can be manufactured at a very low temperature (and corresponding low environmental impact), starting at about 150 °C or 300 °F. It also has a fast set with no shrinkage which makes it very useful for moulding and casting. 

As with all grapes, including our Bordeaux varietals Cabernet Sauvignon and Merlot, geology makes a considerable contribution to the qualities of a wine. Geology makes an even bigger impact in the world of limestone and gypsum. Next blog it’s time to go full French and talk Terroir!

This article was coauthored by Angela and Patrick Webb

Stuc Pierre

Courtesy of Plâtres Vieujot

French Stuc Pierre is a rendered or cast technique for imitation of Ashlar stone derived from a mix of gypsum plaster, hydrated lime (optional) and pulverized aggregate of the original stone to be imitated.

History

House of Sallust
circa 100  B.C.E.

Although there exist examples of the imitation of stone with stucco among several ancient civilizations, it would be the Greeks and Romans who would perfect the art. The Greeks developed stucco techniques to directly emulate their monumental stone architecture. By contrast, Romans would display a more cavalier interpretation in defiance of Greek norms. Romans manifested a preference for its use in interior ornament and would take advantage of the freedom of the physical constraints stucco afforded by creating purely decorative realizations not possible in actual stone.

In medieval Europe the art of Stuc Pierre was to diminish, if not entirely disappear, transcended by the imitation of stone with distemper and limewash paint techniques. During the Italian Renaissance a resurgence began of the imitation of stone with lime stucco, a notable example being the 16^th century Palazzo del Te outside Mantua where cornices, columns, pediments and a variety of ornament were developed to perfection in homage of the prestigious Roman travertine palaces of antiquity.

Palazzo del Te

Courtesy of Plâtres Vieujot

France would soon follow in the 17^th and 18^th centuries. Stuc Pierre based on gypsum plaster would predominate in 19^th century private and public interiors adorning common areas such as entries, halls and stairwells. Not only did it create the illusion of classical stone monumental architecture but provided a comparable durability that has allowed many original installations to be enjoyed until the present time.

Courtesy of Plâtres Vieujot

Due to widespread availability of gypsum throughout France, Stuc Pierre was in common use in regions diverse as the Normandy coast, Provence, Burgundy, the Pyrénées and Côte d'Azur. Particularly in Paris and the Île-de-France it is not uncommon to see extant examples of façades rendered entirely in Stuc Pierre or in combination with Stuc Brique, a similar technique where pulverized stone aggregates are replaced with brick powder. Stuc Pierre was traditionally rendered over a brick or masonry support. With the advent of iron and steel construction at the beginning of the 20^th century, Stuc Pierre would increasingly be used over lath to preserve the appearance of a classical architectural façade.

Mise en Œuvre

The first step is the precise selection of the mix. For restoration works a counter type of the original limestone or mortar is engineered. New construction allows for a great artistic liberty. Unlike lime or cement, gypsum is a self binding material. Aggregates such as crushed stone, brick or sand are not necessary for performance of the coating but are added for decorative effect or to lower the total embodied energy. Similarly materials as diverse as wood chips, glass beads, sea shells or linen fibers can be added for artistic expression.

As with any rendered coating, the cleanliness and stability of the support are very important. When used in exterior several principles associated with classical architectural design are to be respected. Eaves, entablatures and stringcourses are important features in shedding water from the façade and preventing localized streaming. Horizontal and backsplash surfaces commonly occurring at gables, window and door openings should be properly flashed. A water table such as a dense, impermeable stone at the foundation prevents water rise due to capillary action. Adherence to a few, simple, well documented precautions results in a beautiful work that endures generations.

Mixing can be done by hand, drill or machine. Application in exterior can be made in a single or successive coats for a total minimum thickness of 1 ¼”. In interior reduced thicknesses of ½” to ¾” are possible over masonry and lath supports or over drywall substrates as a veneer. In all cases traditional stucco and plaster tools can be used including hawk and trowel, floats, darbies, corner tools etc. Various mix designs are available for render, veneer, run in place, mouldings and ornament. 

The finishing of Stuc Pierre is where the skilled artisan is relied upon to unlock the great artistic potential of the material. A French steel razor or Berthelet is traditionally used to shave and level the surface, exposing the beautiful aggregates contained therein. Further treatments such as washing, brushing, burnishing or sanding can be successively used to achieve desired effects. For the cutting of joints a traditional railroad tool or Chemin de Fer is used. Joints can be left open in an Ashlar pattern or refilled with uncolored material to give a mortar effect.

Sustainability

Courtesy of Wright Architects

Stuc Pierre is increasingly being valued in the sustainability market. In the EU, Stuc Pierre is commonly used in straw bale and hemp lime construction. Gypsum plaster has a relatively low energy of manufacture, produced by heating raw gypsum to about 300°F. Additions of clay binders, hemp fibres and reclaimed or recycled aggregates can reduce the embodied energy even further. Traditionally a small percentage of hydrated lime is contained in the mix contributing an alkalinity and natural mould resistance. All of the materials utilized in Stuc Pierre are mineral or renewable, non-toxic and free of VOC’s. Furthermore the porosity of the coating ensures a breathable assembly that takes full advantage of latent heat transfer and reduces thermal bridging.

Conclusion

I would like to thank Joël Puisais, Les Compagnons du Devoir and my colleague Marc Potin at Plâtres Vieujot for the historical references for this posting. Plâtres Vieujot was founded in 1880 and remains the sole privately held gypsum plaster manufacturer in France. More information can be found on our website: http://www.platre.com/platre/


Contributed by Patrick Webb