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.
This post contains affiliate links. Upon purchase, I earn a small commission at no extra cost to you.
Gypsum Plaster
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.
Clay Plaster
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
Lime Plaster
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
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.
Concrete Plaster
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.
Buy me a kofi to support the research behind this blog.
Sources
- https://www.youtube.com/watch?v=-1CACkgUJcU
- https://www.bayareaplastering.com
- https://italianlimeplaster.com
- http://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plaster-systems-en-SA920.pdf
Mia Evans says
Thanks for helping me understand that clay plaster is usually used for a wood lath and it can also be applied to drywall with a sand coating. I have to know about these things because I probably need to get plaster supplies for the old house of my parents. I inherited it from them and now I plan to move into it after getting issues in it fixed for my own peace of mind and comfort, but I will probably hire a professional to help me out before I can start buying the supplies to ensure that there will be no wrong material while they give me the assurance that they can restore the walls of the property.
michael says
i used potassium silicate / liquid glass to stabilize interior lime plaster walls that were unstable / decomposing. the liquid glass worked great to stabilize the walls, but during the application process i became sensitized to the potassium silicate & still react to it months later. i’m considering either
1. sealing the lime plaster walls with shellac & painting over with latex paint. would BIN shellac primer work for this or should i mix my own shellac & spray apply?
2. skim coating the lime plaster with gypsum plaster.
thoughts? i’m inclined to skim coat with the gypsum plaster first, if that doesn’t do the trick shellac seal & paint that.
Corinne says
Hi Michael, sorry this is still going on. I would keep the walls breathable. I would not use shellac.
michael says
thanks, corinne. at this point i’m going to seal the walls with the BIN shellac & paint with ecos. how many coats of the BIN do you recommend? thanks again.
Corinne says
Tricky, I would not use shellac. But have you tested BIN, because that needs quite a bit of time to offgas for extremely sensitive folks.
michael says
is there a product you would recommend instead of the BIN that will seal as effectively? no i have not tested BIN. i’ve had difficulty testing products at the same scale as i end up using them (the potassium silicate for example). i’m aware that it varies, but what’s “quite a bit of time”? thanks.
Corinne says
I don’t feel comfortable recommending any shellac. But I’d be cautious with BIN.
michael says
thank you. is your concern with the shellac the duration of the off-gassing for sensitive folks, or is it the non-breathability? the wall structure is masonry thru & thru. my understanding is the alcohol-based BIN is pretty intense as it off-gasses but once dry is odorless. one of the reasons i asked is that you recommend BIN + paint as a sealant in a june 8 post below. thanks.
Corinne says
For someone as sensitive as you I would be cautious with BIN and only use it if you are sure it will be fine for you. You are much more sensitive than most folks. concern with Bin is offgassing, concern with all shellac is non breathability.
michael says
thanks. my latest bright idea is to tile the walls.
RY says
Which option do you recommend to minimize off gassing of paint &some material of the house?
1.-Apply 2-3 layers of AMF safe coat primer over it then paint new AMF safe coat and at last, seal it by Safecoat Acrylacq to encapsulate off-gas of the original paint.
2.apply American Clay over the original paint
3.apply Vasari lime plaster or vasari Lime paint over the original paint
Will the American clay or Vasari lime plaster or Vasari lime paint chip or crack when it is applied over the original paint? Will the touch up easy when it happens?
If I cannot afford to apply clay or plaster for all ceilings and walls of the house, will I just apply all walls to help off gassing?
Corinne says
AFM Transitional Primer + AFM Paint or BIN Shellac plus paint. Noting that these are semi-vapour retarder paints and noting that once you seal it in you slow down the offgassing of the original paint. It’s better to do a bake out first and try to let the original paint offgas. see post on offgassing new house odors.
RY says
Thank you Corinne. I do not need Safecoat Acrylacq sealer at the final finish, is it correct? Could you tell me why AFM paint is better than plaster? I have a conventional cabinets and other material the builder’s selected we could not change. I thought Vasari lime plaster or American Clay may be absorb those toxins but is AFM Transitional Primer + AFM Paint will help absorbing the toxin too?
Corinne says
No, just primer and paint. Clay might absorb a little bit and blunts the ability to smell through it but not much. The VOCs will likely come through. Though that is the breathable option.
Carolyn Schroeder says
Your article starts by saying you will tell which of the plasters are the healthiest or cleanest for the highly sensitive to chemical persons.
Tricia says
Will any of these plasters become hazardous or melt if heated? I’m thinking of using a plaster to over grout and cover the natural stone of our fireplace. Do you know which would be a good option for that? Thank you
PF says
Are you sure that clay plaster cannot be used in wet areas like bathrooms and kitchens?
Here’s a quote from the American Clay Plaster website that seems to suggest it is fine to do so
“American Clay plasters work beautifully in a bathroom with a shower. Our plasters naturally absorb moisture from the air and later releases it back as the air dries out (humidity buffering). This leads to less water vapor on the surface of the wall which can lead to mold or mildew growth (like on many painted surfaces in bathrooms). This process is also demonstrated in a short video (watch by clicking here) or in a handout (read more by downloading here).
The plaster will be fine everywhere except inside the shower unit where it will be exposed to water contact continuously — this will erode the plaster away very quickly if done inside the unit. Areas above the shower, near the shower, or the backsplash behind the sink are fine when plastered with American Clay. If soap residue or water spots occur, they can be cleaned using a damp tile sponge with a light amount of pressure.
The choice of plaster will not matter as they all react with moisture/humidity in the same ways. They are basically the same products with minor differences in aggregate sizes only.”