Clayfest 2019

As a traditional plasterer based in the United States, I've been tangentially involved in natural building for many years, in both projects as well as gatherings. Of interest to traditional plasterers straw bale, light earth, adobe, etc. all involve coatings which typically include lime, clay or some combination thereof. However, natural building, contemporary traditional building and historic preservation (or what is called in the UK heritage and conservation work) remain parallel developing but largely separate sectors in the US. I've made a point in my writing and speaking to nudge these movements towards what I see as a natural convergence. To that end I was quite interested to see the state of affairs in the UK and to some extent the EU by attending Clayfest, an event sponsored by Ebuki and hosted by CAT, the Centre for Alternative Technology near Machynlleth, Wales. What I experienced was a bit unexpected.

The Encouraging

The aforementioned convergence between heritage, traditional new construction and natural building seems already well integrated in the UK and a number of the lectures were on this very topic. We heard form an heritage officer involved in organising training in traditional craft skills, a couple of traditional plasterers who work in and provide training to both the heritage and natural building communities as well as an academic from the University of York who's lecture was explicitly entitled, "Rethinking Sustainability as Heritage". Drawing from the vast inventory of traditional earthen buildings, stone masonry and timber framing in the UK and EU, I was left with the impression that the technical as well as embodied craft understanding of these materials and systems combined with the cross discipline communication puts these folks well ahead of where we are presently in the US.

The Disappointing

Day one was a lecture day. We sat in a lecture hall and listened to lectures...all day. There was supposed to be time for Q&A at the very end of each session but of course, as these things are wont to do, the speakers went overtime and hardly any audience participation occurred. I've no doubt that there were hundreds of years of collective wisdom within that room but the format did nothing to release it. I was really looking forward to the practical workshops of day two. However, the way these were structured also inhibited too much participation and there ended up being a lot more standing around than I'm used to seeing. These were nothing more I believe than structural impediments that could be easily addressed with some reorganisation. In that respect I think there have been some advances in the US with what is called the unconference or open schedule conference that allows attendees to think about what they might want to speak or hear about and participate in setting the agendas for smaller group discussions that reassemble later for strategy sessions.

The Troubling

You say you want a revolution 
Well, you know 
We all want to change the world - JL

The first thing I'll say is a bit intangible: I didn't feel welcome. That may not sound like much but it was jarring for me. I've always felt welcome in the natural building community. If I had a people, these would be them. Yet, something felt off from the moment I arrived. Where to begin...well, they're rolling out this train the trainers initiative and a common sentiment, practically an unofficial theme that I kept hearing over and over was along the lines of, "we have to train people to think like we do". I lack their certainty. I know I'm wrong (I strongly suspect they are too). I parade my ideas out there with the expectation and hope that I'll be told how wrong I am. Even if I'm right today, that answer will be wrong tomorrow; the world is dynamic, it's a living process. Oddly enough, none of the individuals seemed arrogant when talking with them one on one, yet this hubristic spirit hung over the entire proceedings.

You say you got a real solution 
Well, you know 
We'd all love to see the plan - JL

Getting a bit more specific, there were in my mind issues with some of the lectures themselves. The first of which was entitled, "What does an anti-capitalist building site look like?" I think one ought to be cautious about a negative title but fair enough, I personally hold the view that a capitalist economy is not going to be viable for much longer for humanity. I would've appreciated the speaker actually addressing the question and relating it back to earthen building. Instead we received an inundation of postmodernist philosophy, specifically a mash up of deconstructionism and feminism and poorly conveyed at that. A diatribe against an undefined patriarchy typified by white supremacy that painted two caricatures: the violent, aggressive, tyrannical masculine contrasted with the tender, generous and honest feminine. The solution? Again, the question was never addressed.

You tell me it's the institution 
Well, you know 
You better free you mind instead - JL

A second lecture, "Integrated technical training for women in northern Nicaragua", was delivered far more coherently and perhaps for that very reason, was more disturbing. Actually, the lecture had very little to do with technical training beyond its practical use as an instrument for feminist activism. I've no doubt that many women, have a hard life in Nicaragua and made even more so because they are women. Furthermore, I've no objection to support folks who are trying to develop their own capacities. It's the entire reason I teach, speak and write. However, it's one thing to support someone's personal development and another to indoctrinate and activate the young as avatars of your own ideology. It came across to me that the speaker was dripping in condescension of Nicaraguan culture. It's the same disastrous attitude the West has displayed internally and towards the rest of the world for almost a millennia, sending out our missionaries to convert and "civilise" the masses. That kind of "helping" has bred a maelstrom of resentment. Replacing the church with a secular ideology doesn't change the basic disposition.

But if you want money for people with minds that hate 
All I can tell is brother you have to wait - JL

In conclusion, I'll say this: the natural building movement grew into existence from the ashes of lost tradition, it came about organically. If we now attempt to graft to it postmodernism, feminism, socialism or any other rationally constructed ideology, it won't be racists, misogynists, industrialists or any "other" that will destroy the movement from without, but tragically it could very well be us that poison it from within. Please, let's not let it come to that.

Don't you know it's gonna be  
All right - JL

Incidentals

I noticed many folks at Clayfest had the pictured symbol on T-shirts or as badges. The extinction symbol, an hourglass within a globe, has been taken on by a newly formed social movement, Extinction Rebellion. I read their manifesto which actually came across as measured and reasonable. I'll be keeping an eye on how they interpret this manifesto and develop.


Contributed by Patrick Webb

Natural Building & Traditional Craft

Initially published on Traditional Building Magazine online February, 4th 2019

Last autumn I was passing through the Shenandoah Valley of Virginia and decided to drop in on a good friend of mine, traditional timber framer Jordan Finch. Jordan and I had taught together at the American College of the Building Arts in Charleston, SC. When I arrived he was busy hand chiseling and fitting the mortise and tenon timber joints for a project with fellow timber framer Robert Laporte. Robert and his wife Paula have a company EcoNest that specialises in design build of homes, traditionally timber framed with light clay straw infill, that are typically lime plastered inside and out. I gave them a hand for a day and ultimately Robert recommended me to the homeowners for the plasterwork. The whole process was fascinating and I'll endeavour to share from this experience the type of home that is today possible by the combination of highly skilled means and methods of traditional craft with the truly sustainable ethic of a contemporary natural building approach.

Timber Framing and Light Clay Straw

Robert Laporte has a young apprentice

You can't get more sustainable than responsibly cultivated and harvested timber. A properly constructed and maintained timber frame structure can endure for centuries. Timber framers are really architects in the original sense of the term, master (arkhi-) craftsman (tekton), who design and undertake the construction of the primary structure that holds the building up. The next generation of Finches were in on the action as well. Apprenticeship can't start too early on the long road to mastery!

Light Clay Straw Infill

A benefit of timber framing is that almost all of the construction of the members can be carried out in shop conditions and brought to the job site for a rapid assembly. Lincoln logs for grown ups! The next phase of the project was to shutter and infill the timber framing with light clay straw. Light clay straw is a contemporary adaptation of some very ancient traditional wall assemblies: cob and rammed earth. Similar to cob the mix is a combination of loam (sand and clay) and perhaps a bit more straw or other woody fibres. That cob-like mix is then stuffed and lightly tamped between form not unlike rammed earth but not quite as dense. The thermal mass of the loam and the thermal insulation of the straw make for a very interesting "dynamic insulation" that can contribute significantly to a very comfortable indoor environment in a variety of climate conditions with a minimum of energy consumption. A similar approach was used for most of the earthen floors which utilised a mix richer of clay, finer fibres and finished with linseed oil. Next up: loads of plastering with another good friend, Rob Wozniak and his team from Preservation Works!

Interior Lime Rendering and Clay Finish

For the interior perimeter walls we installed lath followed by removable wooden grounds that we used for scratched render followed by applying and leveling the brown coat with rods and compressing with wooden floats the following day. The plaster mix was a combination of two local sands, hemp fibres for tensile strength and Lafarge Natural Hydraulic Lime as the binder. The first week all of the preparation and scratch render went up and with the hydraulic properties of the lime setting in we were in good shape for the next float coat and clean up by week two. Lime plasters were the perfect choice as they are exceedingly durable and share the moisture permeability of the light clay straw substrate.

Whereas all of the interior perimeter walls received lime plaster, the inner walls had been framed and received drywall. For the finish we applied American Clay plaster as a nominal 1/8" veneer over both surfaces, the lime plaster and the drywall, for a unified finish. Lime and clay are both traditional materials, practically inexhaustible resources that are completely non-toxic. The means and methods of application go back to the dawn of civilisation: hawk, trowel, rods and floats. The work is physical but not brutal on the body. Essentially plastering makes for good exercise!

Exterior Lime Stucco

The exterior lime mix and application was basically the same as for the interior; sand, hemp fibre and hyraulic lime applied in a scratch and brown coat. However, instead of finishing with clay, the final coats were of lime stucco as well pigmented with mineral tints. The traditional design principle of "having a proper hat and a good pair of boots before going out in the weather" was followed here. There was an impermeable stone water table up to two feet to prevent rising damp or backsplashing as well as extended eaves to prevent the possibility of streaming water that might erode or discolour the stucco finish. Very smart application of embedded traditional wisdom as this part of the country can have some brutal winter and spring weather including blizzards, ice, sleet and heavy rains.

Hand Carved Islamic Dome

Included in our commission was the design, construction and installation of an enriched plaster dome of Islamic geometric design for the "mihrab" or private chapel of the owners who presented me with a rendering of an hexadecagram, a 16-sided star polygon as a point of departure for the design. The first challenge was to determine the surface to receive the design, to research what kind of curvature for the dome was possible given the architectural constraints of the room height, the potentially obstructing rafters above and even the thickness of the dome itself. We managed to squeeze out the most curvature possible with a profile generated by a three-centred arch, an approximate ellipse.

The hexadecagram pattern could now be adapted for the determined surface. This was all carried out traditionally, geometrically with compass and rule. The initially provided design was modified, the proportions of the 'safts' or petals of the star-like tessellation were adjusted to produce a more harmonious composition and the entire pattern was reduced in scale so as to be fully observable from a prostrate position on the ground. A calligraphic element, "ﷲ‬, Allah" was placed in the direction of Mecca to provide orientation for prayer.

Translating the interlaced pattern from a scaled two dimensional drawing to the three dimensional surface of the dome was another important step involving more geometry. The dome was to be cast; this meant that the "void" had to be first extruded in plaster. Onto that inverted surface the pattern was carefully and methodically transferred, physically inscribed utilising several methods to verify its precision. This accomplished, the casting of the dome in a relatively thin shell of plaster could commence.

Islamic geometry participates in a  universal tradition of sacred geometry. Principles of the sacred feminine in harmonious relation with the sacred masculine were intrinsic characteristics of the tessellated dome that I sought to augment. For example, the "void" of the dome can point to the vault of heaven but has a perhaps stronger correlation with the universal "womb", the unseen and unknown mystery out of which all emerges. Rather than being considered empty or being nothing, the void symbolises that which is as of yet undifferentiated...potential itself. The gentleness and ethereal nature of the void is contrasted with the rigourous order, the imposed and revealed pattern of the tessellation.

For the enrichment of the surface we chose to further accentuate these principles of softness/sharpness, darkness/light, the sacred feminine and masculine. The oculus at centre surrounds a lightwell, literally bringing the light of the sun to be the focal point of the design. The tessellation continues to emanate from there in successive waves. The sixteen pointed star received a soft dimpled pattern representative of the sacred feminine, next contrasted by the sharp "flamed" texture hand gouged with wood chisels. The safts of the tessellation were so proportioned as to provide balance between these soft/hard, feminine/masculine elements. The end result is an harmonious composition that takes disparate, contrasting elements and interlaces them together into its own symbolic "universe", a single turn of the divine compass, encompassing quite literally the sacred feminine and masculine, seen through the symbol of unity, the perimeter of the circle through which it is entered.

Contributed by Patrick Webb

From Dust We Come: A Look at Clay

 

Courtesy of American Clay

Originally posted February 2017 on Traditional Building Magazine Online

Clay was undoubtedly the first binder discovered and used to make plasters and earthen, clay-based
construction is our oldest continuous building tradition. Clay is superabundant and clay-based products such as roof tiles, pottery and bricks are benefits of clay technology that we almost take for granted. The topsoil in your backyard is likely a mix of organic matter, sand and clay. Having clayey soils is vitally important for most kinds of agriculture. Because of this there is hardly a habitable region of the planet that doesn’t have large, easily accessible clay deposits suitable for construction.

What is perhaps less widely known is where clays originate. Clays form from millions of years of mineral erosion. Mountains break down into boulders, boulders into rocks, rocks into pebbles, sand, silt and eventually, when the silt reaches a certain size of fineness, an amazing transformation occurs. Instead of just being a loose mix, the fine particles manifest an attraction for water and each other at a molecular level. Clay can be thought of less as a material and more of a behavior, the phenomenon of very finely eroded minerals to agglomerate.

Chemistry

Clays can and do form from a wide variety of minerals. The mineral sources most interesting as raw material for plaster are from the silicon dioxide family, examples include granite or feldspar. Fortunately for us these minerals make up more than 60% of the earth’s crust, explaining clay’s wide availability, practically an unlimited resource! When eroded, granites and feldspars form hydrous aluminum silicates having a simplified chemical notation: AlO•2SiO•2HO. This notation helps us to identify the alumina, silica and water components; however, this is not just a mixture. Instead, at a molecular level clay reorganizes itself into a platelet arrangement better described with a formula such as AlSiO(OH).

Manufacture

To convert naturally occurring clay into a binder for plaster requires minimal processing. In many areas clay can be found just a few feet under the topsoil. Manufacture often is simply a matter of harvesting. It is one of the few plasters you can easily and inexpensively make yourself! If your underlying soil has 20% or more clay content it is quite likely a very good candidate for use as a plaster binder. Even if the percentage of clay in the soil is low or has a relatively high silt content usually it can be easily amended with a suitable local soil rich in clay. There are simple, inexpensive tests that yield trustworthy results for determining if a given clay is suitable for plaster or construction more generally.

Clay has many practical uses in many industries including pottery, masonry supply as well as fields related to civil engineering. As a result, clays that have already been tested, dried, sifted, amended if necessary, are readily available for purchase as an inexpensive raw material. Industrial manufacturers of processed clays typically take advantage of the sun in the drying process, resulting in a significantly lower embodied energy and cost than most other construction materials.

Properties & Specifications

One of the most unique characteristics of clay that distinguishes it from other binders such as gypsum or lime is that it has a mechanical set, that is to say it undergoes no chemical change from a wet to a dry plaster. Rather, it simply dries out. The aforementioned platelet structure makes the clay very plastic and workable when wet, just what is needed for a good plaster. However, as the water evaporates from the plaster it becomes rigid. A major and unique benefit of having a mechanical set is that if damaged, clay plasters can be rehydrated and reworked.

Because so much water evaporates out of clay plasters, precautions have to be taken to ensure that the plaster is not overly friable, in other words loose and weakened because of voids. Base coats are typically loaded with aggregates and fibers such as straw to prevent shrinkage. Finish coats will receive a hardening consolidation of the surface by rehydrating slightly and compressing with a trowel.

Clay plasters contribute to a very healthy indoor air quality. As clay has a high degree of permeability it helps to regulate humidity in the air. At as low as 50% relative humidity clay plasters will act as a reservoir, adsorbing excess humidity out of the air and releasing it later as humidity levels in the air diminish. A few years ago I plastered my bathroom with a clay plaster. In winter it was great to take a long, hot shower coming out immediately to shave, the walls adsorbing all of the excess humidity before it could condense on the mirror.

Rammed earth, Root Down Designs; image courtesy of David Quick

In exterior, earthen plasters have been the most traditional building material around the world and throughout history. Wattle & daub refers to earthen plaster applied over interwoven reed laths, a typical infill for traditional timber framing. Adobe bricks are sun baked clay-based plaster molded masonry units that in turn receive an earthen plaster finish. Cob is similar to adobe; however, damp lumps are unmolded and hand applied. Rammed earth, as the name implies, compresses earthen plaster between forms. Clay plaster’s greatest vulnerability in exterior is erosion which can be accounted for in a building design that includes extended eaves or other means of preventing water from streaming on the façade.

Traditional Adobe Santa Fe, NM

In arid climates having a diurnal cycle of warm sunny days and cool nights, the thickness of earthen construction can be managed to take advantage of its thermal mass. Earthen buildings can slowly absorb the radiant heat of the day, releasing it in the interior of the building during the night. In more humid climates the thickness of the walls can be increased even further creating a highly insulative wall assembly. Earthen wall assemblies combined with smart building placement, natural shading and ventilation can create comfortable living conditions that negate or diminish reliance on mechanical systems, a traditional construction solution that is both economical and ecological.

In our next essay, we’ll delve into gypsum, a plaster binder whose properties are nothing short of magical.


Contributed by Patrick Webb

Plaster Word of the Day; 31 - 40

Image courtesy of Palladio Mouldings

FIBROUS PLASTER, STAFF

Pre-cast plaster mouldings really did not become widespread until the late 19th century. The innovations in gelatins and incorporation of jute cloth and wood reinforcement became known as ‘fibrous plaster’ in England and ‘staff’ in France.

Staff sounds particularly un-French. Urban legend has it that they originally used sticks, ‘staffs’ to hold up the mouldings until they set. I tend not to believe that the French adopted an English word even if that ever was their practice. More likely it derives from old French ‘estofer’ meaning to enrich or decorate.

STRAW BALE

Image courtesy of Plâtres Vieujot

Today associated with the natural building movement, the 19th century American innovation of utilizing straw bales as a building member provided affordable, highly insulated dwellings that comfortably withstood the harsh winters of the Mid-West prairies.

Straw is an ideal substrate for traditional plasters made from mineral binders such as clay, gypsum and lime.

GFRG

An acronym for Glass Fiber Reinforced Gypsum. Sometimes described as GRG or GFRC if cement is used instead of gypsum. Layered fiberglass between brushed or sprayed on coats can provide a significant increase in tensile strength. Pieces can be cast much lighter, as thin as 3/16”. Often this is a good specification for large ceiling mouldings such as vaults, domes, coves and arches.

Image courtesy of Palladio Mouldings

STRAND

Fiberglass is available as a spool of linear thread that can be chopped up with a gun and mixed directly into the plaster. Also available in bulk pre-trimmed to specified lengths.

VEIL

A finely woven fiberglass cloth that is very thin and flexible, allowing for multiple coats

SCRIMM

A coarsely woven fiberglass fabric that is more rigid, usually applied in one or two coats only. An alkali resistant version is commonly used with GFRC work.

Image courtesy of Plâtres Vieujot

ROUGHCAST, HARLING

‘Harling’ is an external render technique common to the UK and Northern France to finish a masonry substrate. After an initial coat of stucco is applied and whilst still wet, a variety of materials such as pebbles, gravel, sand or shells are added to a slurry of the same material and thrown against the surface using a shovel like tool called a ‘harling trowel’.

PEBBLEDASH

A similar technique where just the aggregates are ‘dashed’ or thrown against the wet stucco surface.  





Contributed by Patrick Webb

Plaster as an Architectural Specification

From its origin as a British colony the United States inherited a fully developed plaster tradition that would

East Room, White House

circa 1951

expand considerably from the mid-19^th century until the 1940s. The fine craftsmanship can still be enjoyed in public settings such as period railway stations, banks, courthouses and capitals across the nation. Many fine plaster ceilings enduringly grace private residences in historic neighborhoods such as Brooklyn Heights and Peninsular Charleston.

However, in the decades following World War II plaster ceased being specified and plastering rapidly diminished as a trade. What happened? Two distinct movements figured prominently: the ascendency of architectural Modernism and cheap, industrialized residential construction.

Pre-fabricated home. Levittown, PA

circa 1951

Modernist architectural programs were no longer teaching the traditional language of ornament. By contrast, students were learning that ornament was born of a superstitious and deceitful past, craft was a criminal enterprise injurious to the human spirit and that industry and technology were to be embraced as the basis for a new, purer aesthetic. At the same time, prefabricated temporary housing developed for the military during the war was being modified by developers for residential use. Factory produced construction systems were designed to be assembled by unskilled, replaceable laborers. Part of the package was to replace traditional plastering with nascent “dry” wall systems. By the 1980s there were practically no traditional plaster apprenticeship programs, little opportunity for training, by all appearances the trade had died.

Grand Central Station, NYC

With the destruction of New York's Penn Station in 1963 and the proposed destruction of Grand Central Station in the 1970s, an architectural consciousness began to arise among the public at large, a widespread awareness of how much of value from our traditions was being lost. This coalesced both into an historical preservation movement and a renewed interest in traditional architectural design. By the 1990s plaster was being once again, if cautiously, specified in new construction. For several generations of architects plaster has become somewhat of a mystery not understood or taught by architectural programs. Many architects would like to specify plaster, yet being unfamiliar with the medium are concerned with exposing themselves to risk or appearing irresponsible with their client's budget. Let's see if we can't ease that trepidation by considering some of plaster's strengths and its most practical, effective uses in contemporary architectural specification.

Traditional Plastering

Gypsum drywall has largely displaced traditional plastering for interior walls and ceilings. Many homeowners are surprised to discover that the vast majority of the gypsum used for drywall is the waste byproduct of coal-fired power plants pollution control systems. This is in contrast to gypsum, lime and other plaster binder materials that are mined from naturally occurring deposits. I would like to highlight a few specifications where traditional plasters should be considered as a practical alternative to drywall:

1. Monolithic substrates
2. Curvilinear surfaces
3. High durability

Plaster applied directly to

straw bale

Monolithic substrates (as opposed to cavity wall systems) are solid substrates, common examples being brick masonry, cast concrete or CMU blocks. The maturing natural building market also typically use monolithic substrates such as straw bale, adobe and compressed earth blocks, cob, rammed earth, hemp lime, etc. Exterior plaster or “stucco” will invariably be a practical solution in the exterior. However, interior plaster applied directly to the substrate is likewise a practical alternative to furring strips and drywall. With a carefully selected binder (clay, gypsum, lime, hydraulic lime or cement) a plaster can be formulated that closely matches the thermal, expansive, permeability and other characteristics of the substrate resulting in a far superior surface that is completely integrated into the substrate.

Running a barrel vault in place

courtesy of Sloan Houser

Curvilinear surfaces which may include walls but are often horizontal ceiling surface such as domes, vaults

and the underside of staircases are a logical consideration for plaster specification. I've been on many projects where vaults are painstakingly framed out with what is commonly called “ship hull framing” to receive multiple layers of ¼” drywall that has to be cut into small strips, soaked and scored in the back to adjust to the curvature. This is completely unnecessary and an inferior construction to traditional plaster over lath, requiring only nominal framing.

Courtesy of Louvre Museum
and Plâtres Vieujot

High impact and abrasion resistance may not be expectations of walls surfaces for a residential home; however, for commercial and institutional specifications of hotels, museums, shopping centers, universities, airports and similar settings, long term durability and low maintenance are concerns to balance against initial cost. Gypsum, lime and cement plasters are often prudent investments that can also provide a pleasing aesthetic.

Cast Mouldings and Ornament

As the millwork industry became increasingly sophisticated, soft “paint grade” woods such as pine and poplar began to displace plaster as the economic plain moulding specification. The integration of ornament enrichment into mouldings slowed this transition until ornament itself was largely stripped from architectural design in the mid-20^th century. Nevertheless, there are many strong arguments for specifying plaster mouldings with the following specifications often being competitive or less expensive.

1. Medium to large curvilinear profiles
2. Non-radial curvilinear profiles
3. Large, complex crown mouldings
4. Curvilinear oriented mouldings
5. Ornament
6. Low maintenance

Image courtesy of
Palladio Mouldings

Straight mouldings can be produced just fine in plaster but it excels like no other medium in being able to contour to curvilinear shapes. I say curvilinear as opposed to radial because plaster is not constrained physically or economically to arcs of circles but can readily accommodate ellipses, hyperbolae, or free formed curves. The process helps to explain this property. The first step in creating a plaster moulding is hand-cutting a reverse metal profile from a template. The profile is mounted on a jig and the plaster is built up in successive layers on a table or ramp. For plaster it matters very little if the profile is large or small, very complex or composed of non-radial curvilinear elements. In fact, the moulding itself can be curvilinear such as vertically for architraves surrounding arches, or horizontally as for moulding applied against a curvilinear surface or even complex helix shapes as sometimes encountered in the stringers of descending staircases.

 

Students learn to sculpt, cast and apply
plaster ornament at the
American College of the Building Arts

With a return to interest in traditional architecture there has been a corresponding interest in ornamentation. As with plaster moulding profiles, the work of plaster ornamentation is by hand so there are no mechanical constraints. Often much time is invested in a single model of high quality from which a mould is produced. Plaster is an inexpensive material and multiple casts can be easily and economically produced. The more detailed the enrichment and the more units produced the more value can be attained with ornamentation.

Particularly when large or ornate mouldings are specified I have found clients concerned about maintenance becoming an issue. The coefficient of expansion of soft woods is relatively high with changes of temperature and especially humidity. This is exacerbated by the reality that wood mouldings are typically affixed mechanically against drywall or plaster materials that have a very low coefficient of expansion. The wood moulding moves, the wall does not and cracks develop quickly between the disparate materials that are either addressed with caulk or lived with. Alternatively, plaster mouldings are affixed with plaster to a plaster (or drywall) substrate. The result is a monolithic system, the bond is so strong that the mouldings literally become a part of the wall. Most plasterers will guarantee that aside from structural movement their work will not crack, ever.

This article is a brief summary of a subject that can become very specific for a given project. As a technical consultant for plaster materials and application, I provide services to architects helping them properly specify plaster and plaster systems. I also work with plaster contractors providing training and onsite consultation services as needed.

Contributed by Patrick Webb

Plaster Word of the Day; 11 - 20

ADOBE

The work ‘adobe’ comes directly from Spanish who in turn inherited it from the Arabic ‘al-tob’ (الطوب). The term simply means ‘brick’, having its origins in ancient Egypt as verified by surviving hieroglyphs.

Today ‘adobe’ has a more specific meaning of a sun dried brick formed from clay, sand, water, straw and sometimes having additives such as manure, soil, lime, etc. A similar mixture is used for mortar and as an earthen plaster to bond and protect the adobe bricks.

SCRATCH, BROWN, FINISH

This description of a 3 coat plaster system (usually over lath) is more common in the US and applies to all types of plasters: lime, gypsum, cement etc.

The ‘scratch’ coat is the base layer and as you might guess receives scratches to receive the next coat. It is important that the scratches run horizontal, essentially forming little shelves for the next coat to sit inside and lock into

The ‘brown’ coat is the middle coat. Traditionally either brown sand was used or some mineral tint was added so that the plasterer could easily gauge if he had sufficiently covered the scratch and to make sure he had good coverage when applying the finish.

The ‘finish’ is applied last. Unlike the first two coats where thickness was achieved and the geometry of the wall was established, the finish is typically a thin veneer to create a smooth surface.

Image courtesy of Franco Saladino

MARMORINO

‘Marmorino’ describes an entire system of lime plastering inherited directly from the Romans as recorded by Vitruvius. It enjoyed a vibrant revival during the Renaissance, spreading from the Veneto region, where it had continued as a craft tradition, to the rest of Italy.

The word ‘marmorino’ is the diminutive form of the Italian ‘marmo’, meaning ‘marble’. So ‘marmorino’ has a direct translation something like ‘little marble’. Outside of Venice it has taken on the more specific meaning of the final coats which are rich in lime and taken up to a high polish

Image courtesy of Simple Construct

EARTHEN PLASTER

In discussing Adobe construction a few days ago we mentioned that the bricks traditionally would receive an earthen plaster. Earthen plasters are undoubtedly the oldest form of plastering because no cooking is required. Rather than having a chemical ‘set’ it simply dries out. They are still used in Adobe construction as well as over other natural building substrates such as Straw Bales and Rammed Earth.

The binding component of an earthen plaster is clay, meaning clay is the material that holds the other ingredients (silt, straw, sand etc.) together. A certain percentage of clay is required to make a suitable plaster. Too little and the plaster is weak and friable. Too much and the shrinkage of the clay will lead to cracking.

The image provides a good initial test to see if a site soil has a good percentage of clay or will need to be modified.

FRENCH PLANE

A common tool in the French plastering tradition. Wider versions are used to level wall surfaces whereas the narrow version is the tool of choice for creating faux masonry joints.

The narrow tool is called ‘Chemin de Ferre’ or ‘iron horse’. An appropriate metaphor as the plane has the shape of a train engine and runs along a straight edge or ‘track’.

Image courtesy of Plâtres Vieujot

RUN-IN-SITU

Previously we discussed the running of plaster moulds on a bench to later be affixed. However, a more traditional method is to realize the work on site or ‘in situ’. This type of work is usually referred to as ‘run-in-place’ in the US.

The same mechanical process of running a profile along a track is used for run-in-situ as for bench running; however, the level of skill required is much higher. Instead of using Plaster of Paris which has a rapid set, a common mix is to use lime gauged with gypsum plaster and a small amount of retarder to provide more time to complete the moulding.

Image courtesy of Palladio Mouldings

MODEL

Modeling describes the art of placing enrichments on a moulding in preparation to create a mould. Hence you’ll hear the expression ‘model and mould’ although they are two distinct actions not always performed by the same person.

A good modeler must understand layout well and is responsible for geometric enrichments. However, an experienced modeler will develop at least limited sculptural ability for repeated motifs such as egg-and-darts and acanthus leaves

Image courtesy of Palladio Mouldings

ARCHITECTURAL SCULPTING

There is a distinction made between the responsibilities of the modeler and that of the architectural sculptor. Although still possessing a solid understanding of geometry and precedent in ornamentation, architectural sculpting goes beyond planting repeated motifs on a moulding to embrace free formed, often asymmetrical or unique designs such as cartouches, bas-relief or ornate column capitals such as the Corinthian capital being developed here.

Having been trained classically in figurative study and having worked in mediums as diverse as clay, plaster, wood, stone, the architectural sculptor often assumes the responsibility of art director working with his team of modelers in larger ateliers.      


Contributed by Patrick Webb  

An Ecological Material for Restoration and Construction

The innovative, original American system of straw bale construction has been warmly and increasingly embraced as a beautiful and ecologically sustainable construction alternative for France and other countries of the European Union.

Traditional French plasters have been used extensively as coatings for EU straw bale construction. The article below is based on the presentation “Plaster - An Ecological Material for Restoration and Construction” delivered by Frédéric Charpentier, consulting architect for Maisons Paysannes de France at the 2010 Bâtir Ecologique (Ecological Building) conference in Paris.

Currently, a number of coatings are utilized for straw bale construction: cob and other clay or earth based coatings, lime stuccos of varying degrees of hydraulicity as well as traditional or Portland cement. The primary need of an exterior coating is to protect the insulating straw substrate against damage by water and fire. All mineral based coatings properly applied can achieve this fundamental purpose. Additionally, an ideal coating would possess the following properties:

• high breathability 
• porosity  
• fire resistance in addition to incombustibility 
• low embodied energy 
• ease of installation 
• a beautiful aesthetic

Traditional French plasters such as Stuc Pierre and Terre de Séléné exhibit the above characteristics. The importance of breathability, the capacity of a coating to transmit water vapor, is widely known for monolithic substrate construction. Often less understood are the benefits of a porous coating. To be sure, guidelines are to be followed to prevent water migration due to capillary action; however, the porosity of gypsum allows for the evacuation of condensation or water penetration caused by structural cracks or extensive inundation.

As with all mineral based coatings, gypsum is a non-combustible material. However gypsum plaster, technically known as calcium sulfate hemi-hydrate when combined with water, returns to gypsum or calcium sulfate dihydrate (CaSO₄·2H₂O) chemically combining calcium sulfate with two water molecules. When exposed to fire, a wall coated with gypsum plaster will release water vapor which retards the spread of fire and maintains the temperature in the adjoining room less than 350°F thus inhibiting the spread of fire via spontaneous combustion and often starving the fire through lack of oxygen. In this way gypsum plaster is not just incombustible but actively functions as a fire retardant.

This same chemical property of raw gypsum allows the manufacture of gypsum plaster to be achieved at comparatively low temperatures. Typically gypsum is baked in a horizontal kiln at the relatively low temperature of under 500°F for a couple of hours under autoclave conditions to create plaster. This is considerably less than the embodied energy expended in lime (1500°F) or Portland cement (2700°F) stuccos. Unlike hydraulic limes and cements, gypsum coatings can be removed and recycled as a plaster.

Another property of gypsum plaster is that it is lightweight and self binding. Average weights of coatings are between 5 to 6 lb per ft² at 1 inch thickness. As added water is bound chemically in the set there is no shrinkage of the coating and no need for sanded aggregates to bind the material. For this reason gypsum plaster can be applied as thick as 1 ½” in a single coat. The set is reached in a few hours. As a result, plaster can be applied so long as there is not a frost the same day. Plaster is first applied as a scratch coat directly over straw bale or lathe to a thickness necessary to fill in the inconsistencies of the substrate. The rapidity of set also permits a subsequent coat to be applied the following day. Generally this can be applied at less than 1” thickness to achieve a planar surface. In Europe, wool wood sheathing such as Fibralith is often utilized to supplement insulating performance and eliminate the need for a scratch coat. This relative ease of application is an attractive quality for both professionals and DIY’s.

 

Not to be forgotten is a most important aspect of sustainability; beauty. The surface of the plaster can be finished with scraped, sanded, washed or even ashlar cut techniques so as to appear like limestone block. Running and casting versions can be utilized for running cornices, adding quoins and decorative ornamentation. Often shells, chaulk, limestone, shale and straw are added in the finish coat for strictly aesthetic effect.

French plaster is a proven coating technology that has been utilized for centuries in hundreds of thousands of buildings in Paris, Normandy, Toulouse and throughout Europe.

The Vieujot family has maintained continual manufacturer of gypsum plaster of Paris since 1880. In 1996 Plâtres Vieujot established Plâtre.com to better serve the market for decorative, ecological and restoration coatings of gypsum, clay, hydraulic and aérienne lime.

Contributed by Patrick Webb

US Technical Consultant

Plâtres Vieujot

patrick.webb@platre.com