Gypsum: A Naturally Occurring Stone

The Gypsum Cycle

Originally posted July 2016 on Traditional Building Magazine Online

Gypsum is a naturally occurring stone, a metallic salt of calcium. It commonly forms as an evaporite from the dissolution of limestone by exposure to sulphuric acid from volcanic activity. Under certain conditions, continual cycles of dissolution and evaporation will agglomerate into a “primary” deposit of gypsum.

Mineral gypsum so formed is interspersed among other minerals. Primary deposit gypsums are characterized by a loose crystalline structure and high solubility in water. Over geologic time gypsum from primary deposits is often carried away in solution, forming a “secondary” deposit of a much purer gypsum. These secondary deposits or “massifs” can be tens of feet thick, forming extended beds. Massifs are the primary source exploited as raw material for gypsum plaster.

Chemistry & Manufacture

Selenite: crystallized form of gypsum

The most common form of naturally occurring gypsum has the chemical formula: calcium sulphate dihydrate or CaSO₄·2H₂O. This “hydrous” or watery gypsum binds water to calcium sulphate molecules in a dry, crystalline state. As we'll see this imbues gypsum plasters with some amazing properties. If water held “frozen” at ambient room temperature doesn’t already sound incredible, the alchemy of burning stone to convert it into a plaster or mortar, to be subsequently reconstituted into stone in a place and shape of our choosing is downright magical!

Unlike clay, mineral gypsum must be baked in preparation for its use as a plaster. Fortunately, this occurs at a relatively low temperature so is not an energy intensive process. Gypsum rock can be efficiently baked at temperatures as low as 300° F. At this temperature gypsum quickly loses 75% of its water content, off-gassing steam. The resulting material has the chemical formula calcium sulphate hemi-hydrate or CaSO₄·½H₂O. Commonly known as Plaster of Paris, this is the most prevalent form of gypsum used for plasters.

In the 19th century it was discovered that gypsum baked under increased atmospheric pressure in a barometric chamber would result in dense plasters, having less water demand. These “gypsum cements” require less water to mix and manifest a distinct crystallization pattern that produces dense, hard sets very useful in casting work. Anhydrous gypsum is another form of gypsum stone that occurs naturally or can be manufactured by continuing to bake the hydrous form over a temperature of 800° F, producing calcium sulphate or CaSO₄. This anhydrous or “dead burnt” gypsum, sometimes with a small addition of alum, is characterized by a slower set and dense crystallization useful for floor, exterior and other specialty applications such as scagliola.

Properties & Specifications

 Modeling and casting ornament with gypsum

There are several characteristics that are inherent to all gypsum plasters. Notable among them is that gypsum plaster is self-binding. Aggregates may be added as an inexpensive filler or for decorative effect; however, unlike clay or lime they are not necessary for the plaster to hold together. A closely related quality is that gypsum plasters do not shrink as they set. As gypsum plaster incorporates most of the added water into its crystalline matrix it actually expands slightly as is sets. Plaster of Paris and the gypsum cements in particular are fast setting materials that permit work to be conducted expeditiously. Gypsum plasters have excellent adhesion to most any solid, fibrous or lath substrate and provide a permeable, breathable coating. Furthermore, the combination of these unique characteristics of self-binding and rapidity of set result in gypsum being the perfect binder for molding and ornamental applications. Both Plaster of Paris and gypsum “cements” can be mixed to a light cream consistency, capturing the finest of details.

Historically gypsum plasters have been used primarily for interiors. Although all natural plasters are incombustible, gypsum is practically miraculous in its inherent capacity to actively retard fire. This is due to its hydrous chemistry. Should a fire occur in one room, gypsum will continue to off-gas steam, thus suppressing the temperature on the other side of the wall well below the temperature needed for spontaneous combustion. This arrests the ability of the fire to spread, starving it of needed oxygen.

Although Plaster of Paris produces a plaster far too porous and soluble for exteriors and gypsum cements are simply not practical to use as a wall plaster, there is a long history of exterior stuccoes in Europe based on anhydrous gypsum. Similar to earthen renders, reasonable precautions need to be taken with overhangs and other flashing details to ensure protection from streaming water as well as establishing water tables to prevent capillary water rise.

  Running in situ, courtesy of Plâtres Vieujot

Nevertheless, the self-binding nature of the material itself allows a great range of technical and aesthetic freedom. Gypsum stuccoes are very manageable to work as a wall plaster and can be applied up to an inch or more in a single coat. They have a rapid set that permits working in almost any season so long as there is a brief window of good weather. Furthermore, molding profiles can be run in situ, ornamentation can be cast and affixed and a practically unlimited variety of aggregates can be added for simply decorative effect.

In our next essay we’ll begin taking a closer examination of the family of lime binders, materials intimately associated with civilization itself.


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

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

What is Plaster Anyway?

Plasterer, by John Cranch, 1807

Originally posted February 2016 on Traditional Building Magazine Online

Plaster is as old as civilization. I'll go out on a limb and proclaim that without plaster civilization was impossible! Mankind’s ability to leave the metaphorical 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 English word plaster has a rather direct lineage from the Classical Greek ‘emplassein’ (εμπλασσειν) ‎meaning to ‘mould or form’ as well as the related term ‘emplastron’ (εμπλαστρον) conveying the sense of ‘daubing, to salve.’ So it is that our contemporary speech has effectively retained these ancient meanings more or less unaltered, the word plaster still being used to describe a range of materials for casting and for coating. Physically plaster begins as a wet, mineral slurry characterized by either a chemical set or a mechanical one, meaning that it simply dries out. Having now a general idea, let's take a closer look at what makes up a plaster.

Plaster Ingredients  

The most important component of a plaster is its binder. As the name implies, it’s the component that binds or holds the plaster together. Think of it as a kind of mineral glue. Lime and gypsum are very common heritage binders and plasters that exclusively use one or the other are commonly referred to as lime plasters or gypsum plasters respectively.  Aggregates are typically the ingredient that physically constitute the bulk of a plaster. Materials such as silica sand act mostly as filler, something relatively inexpensive for the binder to cement together. Aside from sand many other aggregates have been used that impart very distinct properties to a given plaster. We’ll address several of these in a subsequent essay.  Plaster has to be wet to be ductile, spreadable enough to use. The proper level of moisture also makes a plaster sticky so that it can form a good bond or adhesion. Potable water is almost exclusively the material of choice to provide a fluidizing agent, being both inexpensive, readily available and safe. Sometimes the water will have additions that either ‘accelerate,’ speed up the set of the plaster or ‘retard’ it, slowing it down. On occasion loose fibers such as hair, cow dung or even woven fabrics such as burlap might be added to a plaster to increase its tensile strength if applied in a system that subjects it to unusual shear forces.

Types of Plaster  

There is a surprisingly varied range of vocabulary used to describe and subdivide types of plaster. Many of these are regionally specific and vigorously defended. You might hear terms such as render, coating, grout, mud, dash, harling, parging, daub, to name a few. I’ll describe the three most common divisions for describing plaster that have come into widespread use in the United States and Canada.

Plaster – plaster used as a coating in interiors, for moldings or for ornamental casting

Stucco – plaster used as a coating in exteriors

Mortar – plaster used to bond masonry units or rubble

I have definitely heard exceptions to these generalizations here in North America. Some masons would vehemently deny that mortars are plasters whereas others are more accommodating. Furthermore, I would say that all of the aforementioned descriptions are practically meaningless throughout the United Kingdom and Europe where local tradition and terminology predominates. Perhaps that is only to be expected in places where folks have had their own way of doing things for centuries. For thousands of years until as recent as the 19th century only a small handful of binders, that is to say unique minerals, were used to make plasters, stuccoes and mortars: clay, gypsum and an entire family of limes. In my next essay in the series, I'll introduce these amazing minerals including an overview of the physical properties that make them so special and useful for plaster.


Contributed by Patrick Webb

Setting up a Plaster Shop

There are many plasterers out there who do a variety of what is called 'flatwork' or 'solid plasterworking' such as stucco, traditional lime or gypsum plaster, veneer work or various decorative plasters. Many are intrigued by plaster mouldings but might be intimidated by the work itself, visually looking so different or by the thought of setting up a plaster shop, not knowing what it entails or how much it might cost. My experience has been that plasterers take to fibrous moulding work, both shop production and field installation, very quickly. Regarding setting up a plaster shop and getting tooled up for the field, plaster is really one of the most inexpensive trades to get started in. For about $10K and a little sweat equity any plasterer can be up and running. Compared to other trades, such as smithing or millwork, that's a bargain, a fraction of the cost easily covered by one or two projects. The really good news is that plaster has been steadily making a comeback in contemporary architectural specification. The market demand is there, the work is profitable and enjoyable. At this point we just need more individual plaster shops servicing local communities. For this post I'll pull back the veil of secrecy and show at least how I've set up a plaster shop for myself and others.

Tables


This is where the sweat equity comes in. Build your own tables from off the shelf lumber from the local stockist. Your main table will be for straight runs. I like a thin long table 2' x 10' if you have the space. It should be between 37" and 39" high depending on your height. High production shops might invest in a granite top. I've found that with a bit of maintenance a high quality, smooth surface 3/4" plywood doubled up and sealed with several coats of shellac performs admirably. This running table is your bread and butter, the important thing is that it is dead flat. Don't shy away from using 2" x 4"'s to create a solid frame underneath. I always like to incorporate a shelf into all my tables down below for quick access to supplies used for repeated work on that table. For my running rail I use extruded storefront aluminum, 1" x 4", it's dead straight and won't wear out or warp, exactly what you'll depend on to keep your run mouldings straight.

The next important table is your circular for radial work. I've found an 18" radius top works well for most things. Build it to the same exact height as your running table (actually build all your tables to the same height). This will allow you to attach an arm to it when you have large radial runs. The circular table acts as the pivot point, the running table the surface. You don't want the table moving when your running a large radius so build it sturdy with a heavy duty shelf below that can hold some sand if you need the extra weight to keep it steady. I like to use a 5/8" or 3/4" threaded rod right down the middle secured to the table with nuts and washers in three places. Again, you don't want either your table to shift or your pivot point to deflect when your in the middle of running your radial piece. Depending on the size of the piece, I'll use either a 2"x 4" or 3/4" plywood for my radial arm securing it between combinations of nuts and washers above and below.

The last table you'll need is really just a workbench that I use for quick sketches and to mount equipment such as a scroll saw and vise. It doesn't need to be nearly as sturdy. I'll suggest a couple of additional items to construct. First a fence as you'll need a flat vertical surface to run against for certain pieces such as cornices. Again, you want this surface dead flat and straight. I attach 3/4" ply to two pieces of 2" x 4" extruded storefront aluminum the length of my running table. Height can change based on your needs, I find 18" will service most needs. Finally, a large mitre box for trimming your mouldings with one 90° slot and two 45° slots going in both directions. 

Shop Tools, Equipment and Supplies

Yes, you'll need eye, ear, hand safety equipment and basic tools like tape measures, screwdrivers, wrenches etc. However, rather than exhaustively go through every possible tool or supply you'll need, I'll focus on ones more particular to plaster shop work. 

There is a bit of carpentry work involved in plaster mouldings. I have the following cordless power tools from Makita: circular saw, drill and impact driver set, multi-tool and jigsaw. The impact driver is very important for working with plaster, especially for affixing in the field. Same goes for the variable speed multi-tool, it makes delicate cuts in plaster, slicing through like butter without risk of fracture. Add to these enough batteries to go around and a variety of clamps in various sizes.

For constructing my running moulds I use smooth quality 3/4" plywood. For the actual knife a traditional method is to use zinc, layout the pattern with a scribe and punch, bending till breaking with pliers and cutting with aviation ships respectively. I quickly switched to thin sheet aluminum. I find it still cuts and files easily but is more damage resistant and won't bend under pressure when running. I get the .032 thickness online from Metals Depot. To cut I use the inexpensive Ryobi scroll saw available at Home Depot. I tried a couple of other more expensive scroll saws and went back. It uses 5" pin blades and can rotate up to 45° for kerfing the plywood stock backing up your profile. A good supply of coarse blades for kerfing the plywood come with the saw and are available from Home Depot and other stockists. For the finer blades for delicate cutting of the aluminum I buy online from Olson. Their 25 TPI regular is recommended and I'll admit it will turn on a dime. The 18.5 TPI skip blade is a bit sturdier and is my workhorse unless the profile has a lot of detail. A word of advice of cutting sheet aluminum, to reduce vibration back the aluminum up with some 1/4" MDF. Finally, you want a set of files and a vise to hold the aluminum for cleaning up your knife.

For tables, plaster cores and other utilitarian uses I use shellac as a sealer and an oil soap for a release agent. For plaster models I use Superseal and Universal Mould Release available from Reynolds. I'll use urethane rubbers, particularly if I have a mould that needs to be very rigid, having a shore hardness of 50 or higher also available from Reynolds, distributors of SmoothOn. However, I prefer the slightly more expensive silicone rubbers working in Charleston, South Carolina as urethanes have some issues in humid environments. They can go up to a shore hardness of about 40, which is firm enough for most application before they start to lose tear strength. I get mine from Silicones Inc. Added expense but nice options to have eventually for removing bubbles from rubber are an adjustable speed, 120 volt vibrating unit available from Vibco that can be mounted to the underside of your running table (this is also great for plaster castings) and a vacuum setup for silicone rubbers (SmoothOn has good specifications). You'll also want to have clean translucent measuring buckets and paddles set aside exclusively for mixing rubbers. Some additional miscellaneous materials for the shop:

burlap
1/2" mixing drill
drywall screws ; assorted sizes 1" to 3"
1/2" foam for crating

Last but not least I should mention plaster. I prefer USG® No. 1 Moulding Plaster for running and will combine it with or exclusively use USG® Hydrocal for casting. Georgia Pacific and National Gypsum are the other moulding plaster producers.


Field Tools, Equipment and Supplies

Some of the tools of course carry over from the shop, particularly the drill and impact driver and your mitre box. For layout you'll want a good quality 6' mason's level, a chaulk line and eventually a quality self-leveling rotary laser. Let the contractor provide you with a centreline for the room and a vertical benchmark and plan on installing your mouldings straight and level unless otherwise directed in writing. Ceilings may have dips and walls bows that will force your mouldings to depart from the specified face of finish. It is the architect's and ultimately the contractor's responsibility to give you direction in writing as to how they want the mouldings installed. Just because you're a plasterer, flanking is not your automatic responsibility anymore than it would be for a millwork installer. 

Usually precast mouldings are first dry fit. A minimum nominal 1/8" off the wall and 1/4" separation between mouldings if sufficient for a good bond. Cut plywood scrap can be used for temporary support underneath and a variety of shims to get the piece situated just right. I use both the typical wedge shaped cedar and pine shims as well as the flat drywall paper shims. Keep a number of countersinks on hand, using the drill to pre-drill for attaching the moulding with drywall screws of appropriate length using the impact driver. For permanent affixing of the mouldings with plaster you'll want to have a trough or large tub onsite so you can thoroughly hydrate the mouldings. I use a blend of moulding plaster and USG® Durabond 90 to affix the mouldings. Durabond (please note USG® Easy Sand is not an appropriate substitute) is already formulated to adhere to drywall without bonding agent and contains retarder giving you time to work. 

For pointing, filling the joints, edges and any dings in plaster, you'll want to have a spray bottle for repeatedly hydrating the surface and a set of ornamental tools, available from The Compleat Sculptor. I usually will use my moulding plaster and Durabond mix for the initial pointing and finish up with just moulding plaster. The addition of plaster retarder will give your pure moulding plaster mix more time. The Compleat Sculptor also carries USG® Plaster Retarder. 

Finally, I'll mention a word about painting. The plaster installer is primarily responsible for providing good geometry and a finish ready for flat paint. Plaster is no different than millwork in this regard. For higher sheens the painter has the same responsibility and essentially the same process for surface preparation. 


Contributed by Patrick Webb

Decorative Plastering

Students at the
American College of the Building Arts

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

Color and Texture

Clay plaster with osyter shells

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

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

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


Ornamentation

Moroccan "gebs" or Gypseries

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

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

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

Enrico Trolese, contemporary Venetian Stuccotoro

Scagliola and Buon Fresco

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

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

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

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



Contributed by Patrick Webb