Plaster lath board © Daniel Friedman Plaster Ingredients / Components

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Components or ingredients used in plaster walls & ceilings:

What are the ingredients in historic & modern building plaster? Do they include asbestos?

This article describes the common ingredients used in making both trowel-applied and sprayed-on plaster used in building ceilings and walls as coatings, fire-resistant sprays, ornaments, and for similar indoor as well as exterior plaster coating applications.

In this article series we describe and discuss the identification and history of older interior building surface materials such plaster, plaster board, split wood lath, sawn lath, and expanded metal lath, Beaverboard, and Drywall - materials that were used to form the (usually) non-structural surface of building interior ceilings and walls.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Ingredients in Plaster Used in Buildings

Plaster containing horsehair from an 1870 New York home (C) PHPReader Question: asbestos in horsehair-plaster?

2015/12/08 PHP asked [by private email] [Click to enlarge any image]

I appreciate your website since I have recently purchased a home built in 1870 that has many additions and upgrades. There are a few areas of concern. One is some white ceiling tiles in the closet. Not sure when addition was done but it has paneling for walls.

They are roughly 21x16. White with brown cardboard looking substance in the cracks. The other thing that scared me was the basement. I have attached a picture of the wall containing long fibers.

Thanks again. - Anon [by private email 2015/12/07]

Update: The homeowner had a sample of this plaster tested for asbestos and reported:

Just to update you. The hairy plaster came back negative for asbestos.

The insulation around the pipe [referring to a white sealant paste around the connection of a flue vent connector to a masonry chimney] was 40% Chrysotile. - Anon [by private email] 2015/12/22


Yes some plaster contains asbestos, but certainly not all plaster used asbestos in its mix. Details are at PLASTER ASBESTOS CONTENT

Two coat and three-coat plaster on lath systems are detailed at PLASTER TYPES & METHODS in BUILDINGS

Let me see some photos of the ceiling you are discussing;

Your basement photo looks like old plaster that contained horse hair. Some added sharp photos might help.

Watch out: asbestos was used in some (not all) plaster formulations in both fiber form (as reinforcement) and fine particulate form (as a filler or to improve workability) (Croce 1954) in both hand-troweled plaster and later (perhaps in the U.S. beginning in the 1940's and confirmed by patents in the 1970's) in fire-retardant plaster based spray coatings.

Lange (2002) identified chrysotile asbestos as a significant ingredient in a trowel-applied plaster coating on ceilings and walls in a school rotunda.

My research found U.S. patents describing the use of asbestos in plaster preparations from as early as 1896 (Knight, 1876 - not as a surface coating but in plaster-asbestos mixed for use in a gas regulator) which suggests asbestos addition to plaster was in common use even before that time.

Watch out also not to panic but at the same time to avoid creating a dusty mess. Some uses of asbestos may not produce measurable levels of airborne asbestos in buildings. (Burdett 1986).

Layers of gypsum lath and plaster (C) Daniel Friedman

My photo above illustrates layers of plaster atop fiberboard wall sheathing and a top-most layer of modern drywall at the left side of my wall cut.

Historically the common ingredients in plaster have included the materials in the following list. Keep in mind that plasterers "back in the day" may have made use of locally-available cheap materials such as mud and straw or waste animal hair, also in coastal areas coral or seashells.

Before naming the specific ingredients in plaster it's useful to understand the concept: the plasterer prepares a plastic material that can be troweled onto a ceiling or wall surface, ultimately to provide a finished surface or for fireproofing.

The mixture will require water, lime, an aggregate that provides body (sand, stone, shells), and tensile strength (hair, straw, hay, other fibers). Here are common ingredients in plaster (not all used in any given mix).

The specific plaster mix prepared also varied depending on the plaster coat and whether a two coat or three-coat system was to be applied. The brown coat or scratch coat was more coarse, put on first, and provided the base, often comprised of sand and lime and after about 1910, portland cement (adamant plaster).

The plaster base coat was very coarse and was referred to as a scratch coat or brown coat. Sometimes in attics and basements just the brown coat or scratch plaster coat was applied, which is why I was able to take the photo shown just below.

The gouges or scratches in the brown coat were made by the plasterer using a notched tool or saw-tooth-edged tool to improve the adhesion of the next coat. The first two plaster coats would each be about 3/8" thick while the final coat was quite thin, just 1/8" in thickness.

The final top coat was intended to be troweled smooth and typically would have included the highest lime or gypsum content of the two or three layers.

Before about 1900 in North America the principal plaster mix included lime, sand, horsehair and of course water. After about 1900 gypsum became the main ingredient - it's safer to work-with for one thing - in a version of plaster most-widely referred to as either "gypsum plaster" or plaster of Paris. Your plaster from the 1800's in the U.S. will be the older lime and sand and horsehair mix.

Your plaster was probably simply hand-troweled onto the stones of your basement foundation wall without any lath backing.

Brown coat or scratch coat plaster (C) Daniel Friedman

Spray-on plaster coatings, widely used as a fireproofing in ships and buildings were more likely to include asbestos as well as rock wool, vermiculite, and fiberglass.

Asbestos was been used in other plaster preparations too. Spray-on plaster ceiling coatings beginning much later (perhaps in the 1960's [citation needed] included styrofoam or similar beads or fragments.

Reader Follow-up:

I thought about horsehair and hay is what it felt like. New old house has me paranoid. Thank you so much. I will get some better pictures as well.


Horsehair is quite coarse - thicker than human hair - at least mine.

The ceiling in your photos looks like a cellulose fiber product - not asbestos, though there are some cross-contamination exceptions.


Reader Follow-up:

And yes I looked through the tile section but couldn't find anything quite like this. And I used my brain and went down with a lighter and light the fiver, definitely hair so that is a relief! Since it is hair does that normally rule out asbestos due to the timeframe?


Yes some plaster formulations included asbestos. We discuss this here, at PLASTER ASBESTOS CONTENT and in PLASTER TYPES & METHODS in BUILDINGS [where I've repeated this email conversation]

Reader Follow-up:

After reading that, seeing how this was applied, I guess I should treat it like it contains asbestos and send off for a test? I spent 2 hours dry there vacuuming the broken bits up with just a dusk mask on and a fan going out the window, probably not the best thing to do. I live in up state NY, can you recommend a good affordable place to send samples to?


It is safest to treat the material as presumed to contain asbestos; a lab test costs about $50. and is worth doing IF there is a large area to be demolished or cleaned-up such that you'd have to hire a professional.

Watch out: STOP VACUUMING unless you have a HEPA-rated vacuum as IF there is asbestos in the material you're cleaning-up you may be increasing the airborne particle level. Ultra-fine particles pass right through conventional vacuums.

Damp wipe and HEPA vacuum is best. More details about typical asbestos-containing plaster or asbestos-suspect plaster removal are given in the citations later in this article. Eye and respiratory protection is also important during demolition of plaster. OSHA's PEL [Permissible Exposure Limit] to plaster of paris dust in the workplace is 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday.

The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. [Wikipedia 2015/12/08,]

My OPINION is that asbestos was more likely to be found in plaster formulations that were designed specifically for fire-proofing, particularly spray applications such as described in Bragg (1973). Plaster in private home use on walls and ceilings is less likely to contain asbestos but we won't know for certain in individual cases without a lab test.

Visual inspection of the plaster won't answer the asbestos question. The use of plaster as a fire-retardant covering dates (in the U.S.) from the time of the revolution and (if my memory serves) was recommended by Ben Franklin as a fire resistance improvement in homes.

Expanded mesh metal lath for plaster walls and ceilings © Daniel Friedman

Research Citing Use of Asbestos in Plaster Coatings & Products

Reader Questions: Ingredients in plaster: asbestos, horsehair, other fibers. Plaster chemicals used in the UK?

(June 23, 2011) Shoe said:

First of all, thank you for this site.

My questions concern horse hair plaster and/or asbestos plaster. My walls and ceilings are wood lathe and plaster that look very much like the photo above from the Wappinger Falls house and the photo next to it. In a few places, the plaster is damaged, very loose, or missing. I've been operating under the assumption that its horsehair plaster; there are visible threads that look like hair.

Until today, I wasn't aware of the possibility that there could be asbestos, and from my reading, I'm still unsure of whether that's actually likely. As background, the date of record for the house is 1896 (although I believe that a section of the house may be older, and I guess its possible that some or all of the plaster is newer). The house is located in upstate New York.

So here are my questions:

1) Do asbestos fibers look distinctly different than hair?

2) Is it reasonably safe to assume that the presence of horse hair indicates that there probably isn't asbestos? (I've read this claim in forums, but the authors were speculating, not speaking from experience.) What about the powdery asbestos?

3) I read that OSHA guidelines requires that all troweled plasters must be treated as presumed asbestos containing material (PACM). Does that apply to my plaster? What does that mean if I want to hire a contractor to do any work in the house?
Thanks for your help,

12/06/2014 aw said:

would lathes in 1930 UK houses that were used for plastering ceiling been treated with any type of chemical?

Reply: asbestos may be mixed into plaster


About the risk of asbestos use in horsehair based plaster, Gman has made an important point: that material was used into the 20th century so some plaster mixes may include asbestos even though asbestos was not in common use in plaster in the earlier 1800's.

I'd respect OSHA's guidelines, and in particular, do not allow a contractor to make a dusty mess in any building if the mess might contain asbestos - not only are there health risks but you're risking a more costly cleanup project than if the material were handled properly in the first place.

What this means in a practical sense is that ANY restoration or renovation work in your home could be much more costly once you notify the contractor that the walls and ceilings may contain asbestos-containing plaster.

Because of that risk of added cost, it would be worth having a few representative plaster samples from your home tested by a certified asbestos testing lab. Individual tests are not expensive, typically costing less than $50. each. If the tests indicate that your particular plaster is asbestos free, you will significantly reduce the cost of renovations and repair work on those surfaces.

- Moderator

(June 27, 2011) Gman said:

Horse hair plaster has be known to contain Asbestos as a bonding matrix, although I'm not entirely sure how common Asbestos use was in the late 1800's. Horse Hair plaster was used right through the 1800's until even the mid - late 1950's.

It was mainly during the period between 1920 and World War 2 that asbestos was the material of choice way up until the late 80's and it was contained in a lot of things - even talcum powder!!! If your house was built from between 1920 - 1950 I would pressume there would be some asbestos within the hair hair plaster as a bonding agent, maybe ranging from about 1% - 10%.

Anything containing asbestos at 1% isn't considered an asbestos material as it is fairly harmless. The fact you live in America might add to the risk as asbestos was more widely used and manafactured in the US more than anywhere else!!

Asbestos will softly crumble in your hands, it will be powdery and you will see tiny needle like hairs (distinctively different from long coarse horse hair)emerging from the substance.

It is definetly correct and safer to treat all troweled plasters with cautuion because 95% of the time they do contain asbestos!! Better being safe than sorry as they say!

You sound very sensible and well done for taking this seriously.

I highly advise you to send a sample off to a contractor or asbestos removal company, they examine and run tests on each sample for about £20 - £30 or in your case $30 - $45 per sample. Its always best to call out a contractor to remove the sample piece for you - in the UK this is usually free of charge, I dont know about the US.

Asbestos removal is very pricey which is why people try to avoid it...bodge it themselves...and then end up dying about 30 years later!

Reply to AW: common constituent ingredients in plaster


Not that I've come across in research - but speaking more generally, water is a chemical as are normal ingredients in plaster. Hydradted lime - gypsum - water. Adapting from Wikipedia and other sources on the ingredients in plaster,

Lime-based plaster such as that found in UK homes is a mixture of calcium hydroxide (lime) and sand (or other inert fillers).

Carbon dioxide in the atmosphere (in other words on exposure to air as the plaster sets and dries) causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone or CaCO3). Incidentally, whitewash is based on the same chemistry but was applied as a thick white paint on building interiors or exteriors.

If you could be more specific about your concern and the reasons for it we may be able to offer some suggestions.

Question: hard gray substrate below plaster

(June 1, 2014) Chris said:

While clearing away the damaged ceiling covering, I discovered a grey hard substrate "below" the white thin plaster-like veneer. The home was built in the 1950s. Inspecting the back of these ceiling boards from the attic, I found brownpaper-backing with numbers printed on (includes 5PH). My guess is that I have a plaster over cement board ceiling, correct?

Reply: scratch coat or rock lath base for finish plaster surfaced walls & ceilings

The gray substrate you see is probably the "scratch coat" on which the finish coat was applied. Or plaster may have been applied over cement board as you infer.

Certainly I have read of plaster application over cement board or "asbesto cement board" or "millboard" in several applications, particularly where fireproofing was important such as in boiler rooms.

But plaster was more widely applied over rock lath or gypsum board.


(June 1, 2014) Chris said:

Thanks, Moderator; I'm pretty sure that a 1/16" plaster application was made to a cement board with brownpaper-backing--the white plaster-like material came away cleanly in shards using a putty knife.

The consistent thickness of the plaster application makes me wonder if the boards came with it factory-applied back in those days (presuming the ceilings are original, which I don't know).

I plan to apply a bleach-water mixture to the discolored cement area where the water came through, use a mortar caulk/sealant in the cement board crack, and use joint compound to replace the scraped away plaster.


Chris, Yes it sounds as if you're looking at a version of rock lath - a plaster board that was used as a base for a finish coat of plaster applied by hand.


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