Plaster usually means a material used to coat interior walls while “render” more often refers to external coatings even if it’s the same material. Stucco is another word for an exterior wall coating.
The most common types of plasters have a base of gypsum, lime, cement, or clay.
Clay plasters are not very common anymore since they lack tensile and compressive strength. They are still used in some buildings, most commonly in natural buildings, like in adobe or cob homes. Clay plaster in conventional wood-framed homes was replaced by lime and then by gypsum plaster.
This article will look at the chemical composition of plasters and where you can use each type. Plaster is generally a healthy material no matter the base mineral, but those who are chemically sensitive will want to know which additives each type contains to choose the non-toxic options.
Early versions of many plasters used to contain asbestos fibers, so we want to avoid another mishap like that.
I will also look at non-toxic backers for plaster. Wood or metal lathe used to be common in old homes, but now moisture-absorbing board known as “blue board” has mostly replaced lathe.
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What is Gypsum Plaster Made of
Gypsum plaster is made from 95% gypsum, plus fillers and functional additives.
Fillers can include: quartz sand, limestone, dolomite flour, chalk, ash, and perlite.
Additives can include: cellulose or starch ethers, cellulose fibers, citric acid and its salts (sodium citrates), tartaric acid and its salts (sodium tartrates) and surfactants (listed here).
Where You Can and Can’t Use Gypsum Plaster
Gypsum plaster can be applied directly on brick, solid or hollow concrete blocks, AAC blocks, and drywall.
Gypsum plaster cannot be used on outdoor walls or in bathrooms due to high moisture levels in those areas.
What You Need to Know About Gypsum Plaster
It is more finicky and less expensive than slaked lime plaster. It also requires fewer coats. It can be tinted.
Gypsum plaster is more expensive than cement mortar plaster (cement and sand mixes) for the same thickness. Though a final layer of gypsum plaster is often used with cement base layers.
Plaster of Paris is a gypsum plaster that is usually tolerable for the chemically sensitive as it’s a very simple non-toxic product. The USG brand comes recommended by sensitive folks and can be found at hardware stores like Home Depot.
Drywall compound like USG Easy Sand 20, are also gypsum-based and are similar to gypsum plaster. Structalite is a base coat of gypsum-based plaster that is commonly used.
Nowadays it’s usually applied over gypsum board, aka drywall. Usually, the type of backer board that is used is colloquially called “blue board”, which is a more textured version of regular drywall. I haven’t seen any difference between blue board and drywall in terms of toxicity.
Sometimes a bonding agent is used like Plasterweld. You don’t need to use that.
Veneer Plaster – This is blue board, with a very thin 1/8-inch layer of plaster applied on top. I have seen samples of this from USG and it seemed fairly benign to me. USG Veneer plaster is highly tolerable for the chemically sensitive, many other people report.
What is Clay Plaster Made of
Clay plaster is a mixture of clay, sand, natural pigments, with plant fibers for tensile strength. You mix it with water.
American Clay Plaster brand is made of clay, aggregates, non-toxic mineral pigments, and boric acid.
Where You Can and Can’t Use Clay Plaster
You usually use clay plaster over wood lath. It can be applied to drywall if it has a sand coating as the first coat (see instructions on American Clay Plaster website). It’s not used in wet areas, such as bathrooms and kitchens.
What you Need to Know about Clay Plaster
The earliest European settlers’ homes used clay plaster or a clay-lime plaster mixes. But now it’s gaining in popularity again. While it’s very popular in earthen homes it’s also gaining popularity in regular homes. I used clay plaster in my tiny home on wheels, over my MgO board walls, but you can use it over regular walls if prepped correctly.
I personally love the totally natural feel of clay plaster. Here is a detailed post on my experiences using clay plaster and lime wash in my tiny house.
It comes in powdered form in a range of earthy tones and has a very matte, rough-looking finish.
It’s quite labor-intensive to apply and you need to have some practice with it. If you are very handy taking a short course first might be enough for you to apply it yourself.
To learn more, get yourself a copy of The Natural Plaster Book.
Where to Buy:
In the US through Green Design Center
In Canada via Amazon.ca
What is Lime Plaster Made of
Lime plaster is a mixture of calcium hydroxide and usually sand, though other inert fillers can be used.
The plaster “sets” when carbon dioxide in the atmosphere transforms the calcium hydroxide into calcium carbonate (limestone).
To stabilize the lime plaster, some Plaster of Paris (gypsum plaster) often is incorporated into the mix
Where You Can and Can’t Use Lime Plaster
Left unpainted, lime plaster can take on moisture and release it, just like clay plaster. But unlike clay plaster, lime plaster can be used in kitchens and baths (but shouldn’t be exposed directly to water unless it’s the Tadelakt variety). A protective layer of beeswax or Marseilles soap can be added to the plaster for water resistance.
What you need to know about lime plaster
Lime plaster was a common building material for wall surfaces in the lathe and plaster system. The lath is the wooden slats that are the backing for plaster pictured above.
The plaster used in most lath and plaster construction in North America was lime-based.
A modern form of lath is expanded metal mesh over wood or metal structures.
Slaked Lime Plaster
This is what the plaster walls in your great grandparents’ house were probably made of.
Starting with wood lath, a coarse coat of plaster was added first. This first coat is called a “scratch coat”. It’s plaster mixed with shrinkage-preventing aggregates, back in the day that was horsehair. Next is the “brown coat”, which is plaster mixed with sand, and finally a fine skim coat that does not have coarse aggregates.
Hydrated, or slake, lime plaster is made of limestone that has been baked at extreme temperatures. It comes in a putty form, not a dry mix.
It’s pure white in its basic form (brighter whites than clay plaster can achieve), it now comes in a huge range of colors.
Venetian plaster is pigmented slaked lime plaster. What makes Venetian plaster unique compared to other lime plaster is that there are no aggregates mixed in.
Marmorino plaster is when marble dust is added.
Tadelakt is a type of lime plaster from Morocco. Tadelakt is composed of lime plaster, with calcium aggregate (crushed marble), optional pigments, and black soap made from olives.
When the ingredients are combined, a chemical reaction between the lime and the soap creates a waterproof surface. Tadelakt can be used right in the showers and baths as the walls and on vertical surfaces like countertops and for sinks.
What is Concrete Plaster Made of
The most basic cement plaster (usually called stucco when on the exterior ) is made from Portland Cement, sand, and water.
There are also mixes that are cement, lime aggregates, and water.
Admixtures may be added to it for various purposes including workability.
Where You Can and Can’t use Concrete Plaster
Portland cement plaster is applied either by hand or machine to the exterior and interior walls in two or three coats.
It is applied directly to a masonry or concrete wall (concrete bricks, solid concrete, red bricks) or it could be applied to a metal lath attached to frame construction.
When applied to metal lath, a vapor-permeable, water-resistant building paper is used over the framing.
Concrete plaster is used in much of the world in masonry buildings. It’s applied to all walls of the house, including the bathroom and kitchen walls, and then painted.
What to know about Concrete Plaster
Interior surfaces sometimes are finished with a final layer of gypsum plaster.
Concrete plaster can also be called adamant plaster.
Corinne Segura is a Building Biologist Practitioner with 8 years of experience helping others create healthy homes.
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