Archive for Natural Plasters

Binders part 2: gypsum and lime

In Part one of this post we looked at clay-based plasters; now we’ll examine the other natural binders, all of which are different from clay in that they have a chemical set.


Gypsum is one of the oldest plasters, and because it can be cooked as low as 350 degrees Fahrenheit to create a binder, it is in fact among the most ecological. Gypsum is a soft, fairly common mineral that is formed when sulphuric acid (typically from volcanoes) reacts with limestone. It is carried in solution and deposited in layers on sea-beds, and over time it forms into a soft stone which we now mine. gypsum-cycleChemically gypsum is hydrous calcium sulphate (CaSO4·2H2O), but when cooked at temperatures you would use to cook a casserole, three-quarters of the chemically bound water is driven off, forming 2CaSO4.H2O, which is moulding plaster (plaster of Paris), or gauging plaster – the latter is chemically identical, but is ground coarser to slow the set time. This is the most common form of gypsum plaster, and really the only one that is readily available in North America – and it cannot be used where there is any exposure to weather or moisture.

However if gypsum is cooked at 850 degrees Fahrenheit the remaining water is driven off and anhydrous gypsum (CaSO4) is created, which makes a very strong plaster that sets relatively slowly. Anhydrous gypsum, also called dead-burnt gypsum, can be used as an exterior plaster that lasts over 100 years, and historically it was fairly commonly used in France and Belgium.

In North America gypsum is hardly used by natural builders, however it could certainly play a greater role as an interior plaster, either on its own or mixed into earth or lime plasters. The unique properties of gypsum are very fast set time (tens of minutes to hours), and that it swells rather than shrinking as it cures. It can therefore speed up the set time of plasters, and reduce cracking, and it can be used with or without the addition of sand or fiber. Gypsum plasters also have relatively high permeability, though not as good as earth plasters. While many drywall products contain gypsum, pure gypsum plasters may need to be ordered as a specialty item, and will be sold as moulding plaster (very fast set) or gauging plaster (slower set).

Many drywall muds are best avoided, as they commonly contain toxic compounds – drywall products with a set time are usually more natural than those without. Joint compound that hardens by drying (rather than setting) contains no gypsum, and is typically rather synthetic. Drywall itself is arguably much less ecological than most gypsum plasters, because the gypsum used to manufacture drywall is usually a by-product of pollution control on coal-fired power plants. A great deal of energy is used to process this manufactured gypsum, and furthermore it may contain heavy metals or other toxins. Recycled, as well as toxin-cleaning drywall panels are available.


Lime has been used in construction for at least 9,000 years. The earliest known uses of burnt lime is in floors and plasters in the middle east; subsequently it was widely used in Greek and Roman architecture.

Today it is the most important of the natural binders for its balance of weather resistance, permeability, strength and flexibility. Lime is arguably the most perfectly balanced of the natural plasters, and it also works well in combination with the other binders. lime-cycleLime is manufactured from limestone, which is sedimentary stone created from the skeletal remains of marine organisms – simply put, from seashells accumulating and compacting over geologic time. Limestone, which has the chemical formula CaCO3, is heated in lime kilns to at least 900 degrees Celsius (1500 degrees Fahrenheit), which drives off CO2 and leaves behind highly reactive calcium oxide (CaO), or quicklime. This in turn reacts rather violently with water to form hydrated lime (Ca(OH)2), which can be either dry hydrate or lime putty, and is what we use for plastering. It gets a little more complicated because impurities in limestone can dramatically change the properties of lime used in plastering. These changes can sometimes be quite useful because they create hydraulic limes and natural cements – but more on that later.

As we mentioned, lime is a well-balanced binder. Its permeability is less than clay, but still good enough to work well with natural buildings. It is relatively strong, but still flexible enough to move with natural buildings and have fairly low cracking. The biggest downside is that it can be finicky to work with – it likes weather that is “not too hot, not too cold, but just right.” It also needs to be protected from sun and wind, and regularly misted after application for a week or more.

Burnished lime plaster

Burnished lime plaster

Hydrated lime should dry slowly over a period of days, and then it will benefit from numerous wetting and drying cycles in the following weeks, because it requires a combination of both moisture and carbon dioxide from the air in order to cure. Although it’s rare, overzealous wetting of lime plaster such that it never dries out in the first few weeks can slow carbonation and be harmful. Much more common is allowing lime plasters to dry too quickly, which also interrupts carbonation and leaves the plaster weak and chalky when carbonation does occur.

When limestone that contains impurities is burned to create lime, natural hydraulic lime (NHL) may be created. The impurities react with the lime and give it a hydraulic set, meaning it will start to set as soon as water is added even in the absence of air (hydrated lime by contrast can be mixed into a putty that will store indefinitely if it is covered with water). The advantages of the set are that natural hydraulic lime (NHL) cures in days instead of weeks, and the resulting plaster is a little harder and less porous, and can withstand freezing much sooner.

NHL 2, 3.5 or 5 are available, with NHL 5 being the most hydraulic (reactive with water), NHL 2 the least. The main disadvantages of hydraulic set are somewhat lower vapour permeability, and also the plaster is harder to work with (it is not very sticky or creamy, it feels a bit like trowelling on wet sand). Also cost – NHL is often imported from Europe. That can be a deal breaker for natural builders who believe in using local materials, but if it means breaking away from Portland cement (for late season plastering for example) that’s a big plus.

When natural hydraulic lime is hard to get locally, it can be created by adding impurities to hydrated lime. These are called pozzolans, and commonly include brick dust or other fired clay, or certain types of ash (especially volcanic ash).

Lime plasters in general tend to be somewhat porous, letting moisture in but also impeding its release more than earth plasters would. Vapour permeable paints or other sealants can be important over lime plasters, especially on fairly exposed sites. Lime also needs to be applied in relatively thin layers (3/8 inch is safe) so it may take three or more layers to level some straw bale walls.


Finally there’s cement, which is a dirty word in natural building circles. Natural cement occurs as a result of very specific (aluminate) impurities in lime – on the other hand, Portland cement is an artificially created cement. Natural cement can be used almost interchangeably with Portland, except that it has a very quick set time, which can be partially managed using retardants. Portland cement has higher embodied energy, contains more toxins than natural cement, but is nevertheless more widely used in plastering natural buildings because of its low cost, availability, and controlled set time. When we talk about cement, we therefore are usually referring to Portland even though it is manufactured in a way that excludes it from being a “natural” plaster.

This plaster is 2 lime: 1 cement: 9 sand

This plaster is 2 lime: 1 cement: 9 sand

Cement plasters are very strong, but prone to cracking, and they have low permeability. Pure Portland cement has such low permeability that it virtually guarantees rot in natural materials that it is bonded to – but when mixed at least 1:1 with lime to make a cement-lime binder, it can have limited applications in natural building, though this is controversial. On natural buildings I would almost always recommend pure lime-sand plasters over cement-lime-sand plasters. For one thing, lime has much lower cracking, therefore less maintenance and less potential for water to enter your walls. If you’re considering using portland, the minimum amount of lime is 1 lime:1 portland: 6 sand, the maximum amount of lime is 2 lime: 1 portland: 9 sand (see Building with Lime: a Practical Introduction p.121)

And Finally

Now go read Patrick Webb’s essay about blending natural binders. Special thanks to Patrick for some of the information, especially on gypsum.

Binders for Natural Plasters (Part 1)

As their name implies, binders glue the other elements of a plaster together. More than anything else the binder defines the properties of a plaster including strength, permeability, and resistance to weathering. Over thousands of years of natural plastering there are three major binders that have been traditionally used: clay, gypsum, and lime. There’s a lot of variation within all three. Lime in particular is further categorized into hydrated lime, natural hydraulic lime, and natural cement; all of which have very distinct properties and are considered as unique binders. But for simplicity let’s look at the properties and origins of the three major binders first, and then break them down into further categories.The table below summarizes some of the properties of the trinity of natural binders, as well as portland cement and a portland-lime blend (recognizing that portland cement is not a natural plaster, but has been used in natural building).

Binder Vapour Permeability (US Perm/inch) Weathering Resistance Strength Embodied energy
Clay Excellent (18) Poor Weak Low
Gypsum Excellent (18) Very poor Weak Low-Medium
Lime Good (14) Good Strong Medium
Cement-Lime Poor (7-10) Very good Strong but brittle High
Cement Very Poor (1) Very good Strong but brittle Very High

Now let’s look at each binder more closely.


Home made earth plaster (yellow ochre)

Earth plaster pigmented with yellow ochre

Clay is usually considered to be the most ecological of all binders, because it can be dug from the ground and used as-is; or even when it is mined and sold industrially, the energy cost of processing it is typically lower than other binders, which are mined and then heated to relatively high temperatures to achieve their binding properties.  Clay plasters, or earth plasters, are the most vapour permeable of all the natural plasters, and the most flexible – they readily allow humidity to pass through, and adapt to movements of the substrate without cracking. These properties are important when plastering over natural wall systems. Clay also has the interesting property of readily taking in water, but when it is saturated it becomes rather waterproof and prevents further water penetration. However earth plasters trade these virtues for lower strength and erosion resistance – wet clay may be water proof, but under driving rain it will erode relatively quickly. Clay also has very high shrinkage as it dries, so earth plasters are either applied very thin, or contain large amounts of fiber and/or aggregate.

Clay is the product of many thousands of years of erosion of rocks (particularly feldspar), and the deposition of very fine particles, often on ancient lakebeds. Sometimes clay deposits are formed at or near the source rocks, and are relatively pure – these are called primary clays. More often they are transported by water and deposited far away on the beds of ancient lakes, where they are typically a blend of many minerals – these are called secondary clays. Clay is primarily composed of the mineral kaolinite (Al2O3•2SiO2•2H2O) but with widely varying quantities of aluminates and silicates, as well as oxides of iron, calcium, magnesium and many other compounds / impurities. But this doesn’t tell us much about what clay actually is – which is incredibly fine particles that are typically flattened into miniature platelets. It is the interaction of these platelets that gives clay its properties.

The platelets are easily lubricated with a layer of water, so that they stick together very strongly, and yet readily slide over each other – making clay extremely plastic and malleable when wet, yet quite hard when it dries out and this lubricating layer is gone. Clays vary in the size of their particles, and generally clays with very fine particle sizes have high plasticity, and high strength because the clay has a lot of binding power. Unfortunately this goes hand in hand with higher rates of shrinkage. The most extreme example of this is bentonite clay, which has such fine particle size that it behaves quite differently than other clay. Bentonite is typically 10 times finer than any other clay; it can have a surface area of almost 1000 square meters per gram. The very high rates of shrinkage, high plasticity, and extremely low permeability generally make bentonite totally unusable in natural plasters.

On the other hand if you’re using pottery clay, you’ll probably want to use some of the most plastic pottery clays, the ball clays, which are fantastic for mixing with lots of aggregate and fiber to make a strong base coat that can be laid quite thick on the wall. Kaolin clays, which are much less plastic, are often used in thin finish plasters for their desirable white colour and very low silica content – it’s very important to realize that most clay contains significant amounts of silica, so the dust is little safer to breathe than portland cement dust. Clays that lack plasticity (called short clays) will tend to break rather than bending when wet, and generally speaking are less desirable for plaster (but this rule is often broken). When you’re using clays dug up on site, however, they are rarely so pure – your biggest concern will be ratio of clay to non-clay elements in the soil

Evaluating soil types

The most important property of your soil, when assessing it for plaster, is the clay content. Ideally soils used in earth plasters should contain 20-30% clay or more. It may be possible to make a good plaster with as little as 10-20% clay content in your soil, but proceed with caution, particularly at the lower end, because your plaster will be weaker. Particles that are slightly large than clay are called silt, which is often mistaken for clay because it has the same slippery feel when wet – however it lacks the binding properties. Silt can be either benign or very harmful in plaster, depending on how much there is. As a rule It should equal less than 1/4 of the clay content of your soil, so if your clay content is 20% the maximum amount of silt should be 5%; any more than this and there’s a chance of plaster failure. Plasterers will certainly break this rule, but be careful and make a lot of samples  You should aim to use the best soil possible for your plaster, even if it means trucking it in or using bagged clay- plastering a house costs thousands of dollars, so if you’re going to experiment, do it on a shed.

There are several ways to test your soil, you should use all of them and compare the results. You may want to compare soil from a few different places on your site, as it can sometimes vary over short distances.

The ribbon test

Make a ball of clay that is malleable and not-too-wet, not-too-dry; squeeze it between thumb and forefinger to produce a ribbon about 1/8 inch (2-3 mm) thick and less than ½ inch (about 1 cm) wide. Keep pushing the ribbon out to see how long you can extend it before it breaks. A minimum of 1 ½ inches (40 cm) usually indicates at least 20% clay, but to be safe it would be good to have a ribbon double that length. Evaluate the feel as you squeeze it: does it feel smooth and plastic, can you feel sand grains in it? Is the sand fine or coarse, sharp or rounded? Next do the jar test to get even more detailed information about your soil.

The jar test for soil

The jar test for soil texture

The jar test

The jar test is a fairly accurate and easy way to determine your soil type, but it takes time because clay can take up to a week to settle out of suspension. Any jar will work for this test, but a 1 litre mason jar is a nice size. Fill the jar no more than 1/3 full with soil, then top it up to at least ¾ full with water (leave enough air space to get a good shake). Optionally you can add some detergent or salt to help disperse the clay particles. Shake the jar well, until you feel that any soil clumps have broken up. If not using a dispersant, you’ll likely want to let it soak for a day or more, and come back to shake it again. Note that it should be shaken, not stirred; swirling will tend to throw off the accuracy slightly. Have a timer (with an alarm) and a permanent marker handy. Shake for several minutes; start the timer when you stop shaking. After 40 seconds all of the sand component of the soil will have settled out – mark this level on the side of the jar. After 2 hours all of the silt will have settled out, mark another line at this level. Wait until the water starts to look clear (up to a week), at which point all, or nearly all of the clay will have settled out. Measure the height of each layer and divide it by the total height to obtain a percentage by volume of the soil. The measurement of clay will be a little high, because clay continues to compact over time, and even more so when it dries. For more accuracy you may carefully scoop off most of the remaining water and leave the jar to evaporate in the sun for a few days.

Clay plasters are fun, but because of the varying properties of soil, fiber and aggregate it can feel more like art than science.  Different regions can also have very different clay soils, which makes knowledge sharing a little more complicated.

In part 2 we’ll look at gypsum and lime, and talk a little about cement. I’ll also share some links.

Sand for natural plasters

Sand is underrated. It provides the structure of plaster, and the quality of your sand can make the difference between success and failure. So what makes sand good or bad? In general, good plaster sand should be sharp, with a diversity of particle sizes, and clean.

marble sand

  • Sand should be sharp and angular, not worn and rounded. Imagine trying to build any kind of structure out of balls vs blocks, and the reason for this becomes obvious. Unfortunately this means that many natural sands are poor plaster sand. Beach sand in particular should be avoided, because waves have often been rounding the sand grains for thousands of years.
  • Particle size diversity is important to create good structure, and to reduce the amount of binder needed. Imagine a bucket filled with softballs, how many golf balls could you add to the bucket without changing the total volume? Then how many marbles could you add to that? Ideally you’d have a mix of nearly every grain size so that there are few large voids left – this creates a structure that resists movement, and also requires less binder to fill all those voids. Less binder equates to less cracking, and shrinkage cracks are one of the plasterer’s number one enemies.
  • Sand should not, however, contain silt – which is the particle size below sand, slightly coarser than clay. Silt fills the voids in place of the binder, resulting in weak plasters. Clay can also cause serious problems in lime-based plasters. Salt also can lead to plaster failure, as well as causing rusting of metal lath or any other metal used in plaster preparation. So when we talk about sand being clean, we mean free of fine particles, and unwanted salt, chemicals or organic matter.

As a conservative rule the largest particles in your sand should be no more than half the thickness of your plaster, but preferably would be at least one quarter the thickness of your plaster (larger aggregate can provide better structure, resulting in a stronger plaster with less cracking). So if your plaster coat is a half inch, your largest aggregate would ideally be between 1/8 and 1/4 inch. There are several types of sand that are widely available, so when you call a sand yard, or any construction materials supplier, you need only tell them what you want and it will promptly appear at your jobsite… maybe. Unfortunately the definition of sand types allows huge variability (even assuming it is followed correctly), and what you receive on the jobsite will depend on what that supplier carries, or what is locally available. Nevertheless, as a rough guideline the main sand types everyone carries are masonry sand, concrete sand, and (if you’re lucky) stucco sand.

Masonry sand

Masonry sand has a maximum particle size of 3/16 inch (4.75 mm), which could mean that it is a nearly ideal sand for many base coats, and particularly for lime and cement-lime body coats which are typically applied at a depth of about 3/8 inch. Unfortunately, using the ASTM standards for particle size distribution (see Table below), anywhere from 70 to 100% of the sand can be less than 3/64 inch (1.18 mm). Which explains why when we order masonry sand on a jobsite sometimes it’s a nice mix of coarse and fine sand, perfect for a base coat, other times it’s almost 100% fine sand. Worst of all, masonry sand can be 99% rather fine, but with just a few pebbles, making it useless for fine finish coats. With masonry sand you should look before you buy; however it is still often the best choice available for a given plaster. Brick sand can be a synonym for masonry sand, often a relatively fine version of masonry sand.

Masonry sand gradation (ASTM standard)

Sieve Size

Percent Sand Passing Through Sieve

Inch mm Mesh # Natural Sand Manufactured Sand
3/16 4.75 4 100 100
3/32 2.36 8 95 to 100 95 to 100
3/64 1.18 16 70 to 100 70 to 100
0.024 0.6 30 40 to 75 40 to 75
0.012 0.3 50 10 to 35 20 to 40
0.006 0.15 100 2 to 15 10 to 25
0.003 0.075 200 0 to 5 0 to 10

Concrete sand

Concrete sand (technically “fine aggregate”) has a maximum particle size of 3/8 inch (9.5 mm), and five percent should fall between 3/16 and 3/8 inch (4.75 – 9.5 mm). It also must have a variety of mid-size particles, and up to 10% can be finer than 100 mesh. Even though the 3/8 inch maximum is a little more than we’d like for most plasters, concrete sand may be a good choice for a plaster coat that will be applied at ½ inch or more. Large aggregate doesn’t necessarily interfere with a nice finish, as it tends to push to the back. However it makes finishing extremely difficult and frustrating if the largest aggregate is about the same size as the depth of the finish coat.

Stucco sand

Stucco sand is what you probably want for your plaster, the trouble is actually getting it. It’s similar to Masonry sand, with a maximum particle size of 3/16 inch, but is required to have a greater proportion of large particles, and less very fine sand. A large amount of variability is also allowed in stucco sand, but because it is formulated for stucco it’s more likely to have a desirable gradation. Stucco sand is ideal for anything but thin finish coats, however it is far less widely available than masonry or concrete sand.

Stucco sand gradation

Sieve Size

Percent Sand Retained in Sieve

Inch mm Mesh #
3/16 4.75 4 0
3/32 2.36 8 0-10
3/64 1.18 16 10-40
0.024 0.6 30 30-65
0.012 0.3 50 70-90
0.006 0.15 100 95-100
0.003 0.075 200 97-100

Silica sand

Most sand is primarily made of silica, however fine white silica sand is available in 80-100 lb. bags and can be very useful for some finish plasters. Silica sand is most commonly used in earth plasters that are applied in a very thin coat and pigmented (vs painted). Silica sand is available in various mesh sizes – typically mesh sizes between 40 and 100 are used in finish plasters. Forty mesh is almost coarse (but still much finer than window screening, and might be mixed with other mesh sizes to make plasters that apply slightly thicker (up to 1/8 inch, possibly with some fine fiber). One hundred mesh or finer would make an exceptionally fine and smooth finish plaster that would be prone to cracking unless applied in very thin coats. Silica sand can be used for sandblasting, so it can sometimes be found in rental centers, as well as being fairly commonly available in masonry supply and other stores. Silica is very hazardous to breathe, so wear a respirator during mixing to avoid breathing dust from silica sands.

Limestone sands

Marble and calcite are common forms of limestone, and these sands can be used in some of the most exciting finish plasters. Limestone based sands are traditional in many lime finish plasters, especially ones that are highly polished. They are also used in some unpainted earth plasters, where they impart a subtle sparkle. Marble and other fine white limestone-based sands can be hard to find in bags, and since they are usually being used in finish plasters a truck load is probably too much. Marble sand is sometimes used in swimming pools, and other types of stucco, so it may be available at masonry or stucco supply stores. If you have trouble finding it you can try contacting swimming pool plasterers, who might refer you to a supplier, or sell you some sand directly. In the USA marble sand is available online from Limeworks, at a reasonable price for small jobs or learning. You might also try contacting manufacturers to find out who distributes their products. Manufacturers of bagged sands include OMYA (SW350 white stucco sand); Superior Marble in Arizona (Plaster Sand); Universal White Cement (Universal Ultra White Marble Sand); Imersys (40-200 Dry-Ground Marble; Pool Mix). Imersys is one of the largest suppliers of marble sand, they are distributed in Canada by Debro (who sell by the palette = 50 bags). Be aware that the quantities you are looking for are tiny compared to what most companies usually deal with. You can also go directly to marble quarries who may sell (or even give) you some. Typically it needs to be sifted, however, which means it needs to be dry. Even bagged sands often need to be sifted for use in finish plasters.

Sieving you own sand

If you can’t buy the sand you need, you can make it. It’s not unusual to modify sand for use in fine finish plasters, by sieving out the larger particles from a commercially available sand. To sieve your own, you need dry sand (many sands are sold damp), and you need an appropriate screen. First you need to know what mesh size you want. Window screen is typically about 16 mesh. A smaller mesh size can be purchased in the form of splatter guards from the kitchen section of many stores – these might be found in the 20-30 mesh size range. Otherwise, you will likely be shopping online at a specialty supplier.

It’s worth taking the time to make a good setup for sieving sand – this may be as simple as cutting the bottom out of a bucket or plastic bowl and gluing the screen in (construction adhesive is good for this), or it might mean making an angled wood frame. For anything but small, occasional sieving jobs it’s worth investing time in a good setup. Build the frame to sit above a Rubbermaid or whatever bin you find most convenient. Leave the bottom end open for coarse aggregate to exit (hook a bag over the end so that it bags itself –then use it on icy spots in winter, if you have winter where you live). Try out some different angles (you can start with 45) until you find the one that works best for you.

Sand is typically 60-100% silica (and usually closer to 100), so avoid breathing fine dust. This is especially important if you’re sieving it – always wear a respirator to sieve any sand.

Maintenance and repair of natural plasters

When spaghetti sauce meets unsealed earth plaster, it’s a bad scene. But it’s fixable. Most bad things that happen to natural plasters are repairable. There tends to be a trade off between durability and repairability – an unsealed earth plaster is the easiest plaster to damage, and also the easiest to repair without a trace. Lime plasters can be a little harder, but there are definitely tricks for repairing them. Also the more polished and perfect a plaster is the harder it is to blend in a repair; if there is very little variation in your surface, any blemish is going to draw the eye to it. One of the most polished plasters, tadelakt, is still fairly repairable because there’s so much variation across a wall that your repair is like Waldo, or goldbug, camouflaged by the diversity around it.

Earth plaster repairs

1) Surface marks

  • If you just need to clean a smudge or a pencil mark etc. from unsealed earth plaster, you can remove it with a good quality pencil eraser. If that fails, if it’s a sponge finish try a slightly damp sponge (wet it, then squeeze all of the water out of it) or rag. In this case you don’t want to re-wet the wall much, as it will show a change in texture.
  • If your wall has a trowel finish, you are better off skipping the sponge, and go straight to re-wetting and very light surface scraping, then a light quick pass with a trowel if needed. Spongeing may change the reflectivity of a trowel coat, usually making the area appear lighter, so keep that sponge away from a trowel finish.
  • If this doesn’t work, scrape the stain off with a tool and repair it with the technique described below, for scrapes and dings. With something that penetrates, like marker, you’ll probably need to scrape the plaster off, whereas pencil, and usually crayon, can be erased with an eraser, sponge, or surface scraping alone.

2) Scrapes and dings

  • The key to blending the repair into the existing plaster is to properly rehydrate the wall around the damage before you start. Use a spray bottle on fine mist setting, spray an area a couple of inches around the repair. Mist lightly, try to avoid drips running down the wall. Wait a minute or so, give another light misting,
  • Go away and do something else for five or ten minutes. When you come back mist it again once more, then wait until all the sheen has left the surface of the plaster (maybe 30 seconds).
  • A flexible plastic trowel is ideal at this point, or you could use a small pointing trowel and plastic cut from a yogurt lid. Use a small amount of the earth plaster mix, just enough to fill the damage – if you put on too much, carefully scrape the extra off. Try not to get any plaster on the surrounding wall if you can help it.
  • Now using the plastic trowel or yogurt lid, compress the repaired area once, maybe twice if you need to – don’t overdo this or you will burnish the wall around the repair.
  • If it is a sponge finish you can touch it up with a sponge, very delicately when it is partly set, or wait until is is entirely dry and sponge over the area.
  • If it is a trowel finish, you can improve blending by scraping the surface of both the old and new mud, then retrowelling them. Be gentle. If the plaster is too dry it will burnish when you retrowel it, if too wet it may tear or pull off the wall – in this case finish the repair after a brief drying period.

The attached video of American clay repair will help make all this clear, it’s a little slow to watch because it’s filmed in real time (a bit like watching paint dry).

For large repairs, mist the existing plaster well, then trowel over the damaged area, trying to level carefully to the old mud. The junction will show, but may be blended somewhat with scraping the joint and retrowelling. I find it is then worth letting it dry significantly or entirely, then rewetting old and new mud and either retrowelling if it’s a trowel finish or spongeing a sponge finish. Large repairs require waiting time.

Repairing earth plaster that has been painted is even easier, just fill and compress it as above, then when it’s dry use a damp sponge to wipe any spillover off the surrounding paint. Then touch up the paint, of course.

American Clay have also produced a repair manual that you may find useful.

Lime plaster repairs

Lime plasters are so variable in their composition, and how they are finished, that any advice is going to be a generalization. So consider the following as a fairly basic starting point; and unfortunately you may have to learn from your mistakes, so practice repairs on sample boards first. It’s hard, but not impossible, to make repairs in lime plaster disappear as they do in earth.

1) Porous lime plaster (not painted or waterproofed)

  • Rehydrate the area to be repaired well with a misting bottle (or wet sponge).
  • V-open the edges of the damage area if needed- especially if edges are crumbly.
  • Using some of the lime-sand mix originally used in plastering, fill the the repair, compress, and let it dry.
  • Once it is dry sand it with around 180 grit sand paper to smooth the repair.
  • The same technique can be used in crack filling – generally you would V open the crack using a grout removal tool, backerboard scoring knife, or even a sharpened can-opener, before filling it.

2) Lime plaster with a waterproof surface (e.g. tadelakt) or painted lime plaster

  • Rehydrate and fill with the original mix (as described above for porous plasters), but wipe the excess off the surrounding plaster since it will not stick.
  • Compress the repaired area using a plastic trowel, or a stone in the case of tadelakt. As always, try to be as neat and careful as possible at every stage.
  • This technique can be used to fill large cracks in tadelakt, again it would probably be wise to V large cracks open. Do not open fine cracks, use the techniques described below.


3) Crack filling fine cracks in tadelakt

  • First, it’s important to realize that micro-fissures in tadelakt are normal – if water isn’t penetrating there’s no problem!
  • If you determine the crack is a problem, the best option is to re-compress the crack with a stone. Re-wet the area first with a dilution of black soap, I find this reduces the risk of scratching or damaging it. Even so, you may leave some undesirable marks which could, in the worst case scenario, draw attention to the crack. If the tadelakt was done recently, compression is the best way to deal with cracks and some other kinds of damage.
  • For very fine or hairline cracks in tadelakt that isn’t very fresh, I would still usually try stoning, but only after hydrating it very well with dilute black soap. Another option is to make a slurry from which you’ve removed all the sand, and rub it into the (hydrated) cracks.
  • To do this, use two straight, clean trowels, take a scoop of mix, scrape it down the length of one trowel with the other, pushing most of the mix off the trowel and just leaving a smear.
  • Scrape it again once or twice, discarding whatever is scraped off, until all that’s left is a thin film of plaster with no sand at all. Gather this with the other trowel – you may be surprised at how much there is.
  • Wet the cracks to be filled, then rub this slurry rapidly across the crack using a gloved hand, then wipe off any excess using a clean rag. The cracks will stand out at first, but will quickly fade as the mix dries, and may not be visible at all. Note that this technique only works after soaping, and works even better after waxing.
  • You will need to reapply diluted black soap over the area after crack filling – preferably daily for a few days. 

4) Crack filling cement-lime plasters

  • V out the cracks using a grout removal tool, backerboard scoring knife, or  a sharpened can-opener
  • Wet the cracks using a misting bottle, or paint on a bonding agent.
  • Fill cracks with grout. For most cracks you’ll want sanded grout, hairline cracks may call for unsanded grout.
  • Once the grout is dry, sand the area with a foam sanding block to remove excess – do this within a few hours at most, or it will harden and make your life difficult.

Planning ahead

  • When you’re at the planning stage of a project you should consider the strengths and weaknesses of different plasters. For example, we avoid unsealed earth plasters in kitchens. Or avoid putting any natural plaster on a corner where it’s likely to get bumped a lot – next to a door threshold, for example. Trim it out with wood if that’s an option, or there may be places in your home where paint or tile is more appropriate than plaster.
  • Plan for a finish you will be able to maintain – most people can learn to repair a troweled plaster, with some dedication, but if a homeowner wants to maintain their own plaster and wants it to be easy, a sponge finish may be the way to go.
  • When you are installing a natural plaster, always save some for repairs later. Most lime plasters can be stored wet, in a mason jar etc., with a little water over the top to prevent air from reaching it. Hydraulic lime must be stored as dry mix. Earth plasters should usually be stored dry, either the original powdered mix, or dehydrate some of the leftover mix.

Ongoing maintenance

  • Avoid contact of oil with any natural plaster. Except, perhaps, oiled earth plasters.
  • Use natural oil-based soaps to clean waterproof plasters or oiled earth plasters. Black soap is the best choice for tadelakt (available here in the US and here in Canada). Use a dilute solution.
  • Wax tadelakt every year or so as needed, if it’s in a wet area. Ryan Chivers, our tadelakt mentor, reccomends Howard’s wax, and I have found it to be good, easy to use, and cheap.
  • Earth plasters need little or no regular maintenance, repair as needed.

Slideshow of natural finish plasters

More earth and lime finish plasters at

Will straw bale buildings last?

After seeing problems in a few straw bale buildings, I’ve been thinking about this lately: is it a truly durable building system? By which I mean, will  a straw bale house measure its lifespan in centuries rather than decades? I’ve concluded that most will, some won’t. The ones that won’t are predictable, however, and for the most part they break the rules.

This wall is ready to be replastered after wet straw was removed. It had no overhang at all.

Architects occasionally design straw bale homes with no roof overhang, for instance. I’ve seen this twice, and in both cases an overhang was added before construction was completed. In one of them there were already some moisture issues a year or so after the wall was closed in. Water was sheeting down the wall in spring rain storms and working in through cracks. These were a few horizontal cracks which had reopened after crack filling. Straw at the base of the wall was saturated and had to be replaced – which was not as hard as I thought, and in a weird way I found that encouraging for the question of longevity. With the overhang in place I think this will be one that does last.

Other houses that I worry about don’t break the rules so blatantly, rather they push them a little, but they are on exposed sites. Driving rain is the enemy of straw bale houses, and gable ends are particularly susceptible. If you’re thinking of building a straw bale house on an exposed site – a hill or a lakeshore, or any site where you might consider using a wind turbine – your design must be impeccable. You might want to consider a bungalow with good overhangs all the way around, you should certainly avoid a large gable end on a windward side of the house. Gable ends in general should have some kind of skirt roof, and you may want to consider siding the upper part if it’s large or particularly exposed.

Cement-lime plaster tends to make things worse. There’s an unfortunate tendency to gravitate towards cement-lime on very exposed sites because it is the most durable plaster. Cement-lime won’t erode away under driving rain, but it will trap in moisture more effectively than any other plaster. High lime content helps a lot, but pure lime is better, or an earth-lime hybrid system; in rare cases exterior earth plasters may even work on their own (note that the right paint is important for earth and lime plasters). In any case, if you’re very worried about your plaster eroding under driving rain, you probably have a design problem and cement-lime plaster is likely to make it worse. You need to redesign, or possibly you just shouldn’t be building a straw bale house there. An oft-overlooked alternative that can eliminate most external moisture issues, even on exposed sites, is to use siding or rainscreen over bale walls. And keep in mind that whatever you build on an exposed site, bale or otherwise, you’ll need good design and attention to detail.

Cracks must be filled. I’ve seen a house that went maybe 8 years without crack filling and painting, and it was fine! But I’ve also seen disastrous results from unfilled cracks. Again, the site seems to make all the difference, but there’s no sense pushing your luck. Fill your cracks within a few months, or if you plaster in the fall, wait until the following spring or early summer – but not years.

This sounds like a whole lot of bad news, so why build straw bale at all? Is it worth the hassle, and is it really a sustainable wall system? To put this in perspective, when a 100-year-old hay-bale house was dismantled in Nebraska the hay was in such good shape that cows ate it. Or consider that straw bale building is not alone in having had its share of mistakes – modern building practices have created a “perfect storm” of stucco failures on conventionally built homes. In some ways, bale walls are better, they can be more resilient than some conventional wall systems. As soon as you add  insulation to a wall you’re inviting moisture problems – the more insulation you use, the harder it is for the wall to dry out if any moisture gets in, because the middle of the wall tends to stay cool. Superinsulated homes are built to have very low air leakage for energy efficiency, but also because air leakage can cause moisture problems if water condenses in the wall.

Straw bale walls can likely handle small to moderate moisture loads better than conventional wall systems because of the vapour permeable plaster skins on either side, and because the straw itself can act as a large reservoir for moisture without ill effects, so long as it does not exceed an upper limit, and the conditions occur for drying. It’s still very important to air seal a straw bale home properly, and many natural builders have been slow to realize how important air sealing is. In my experience those days are over and air sealing is a priority for most natural builders, which means some kind of air fin behind all plaster joints, and of course good detailing around electrical boxes etc.. This is not just a question of energy efficiency, but also is likely to extend the life of the home.

There are other benefits to straw bale, of course, that I should mention briefly: A relatively high R value (at least double that of a 2×6 stud wall with batt insulation, but still less than most superinsulated homes); low embodied energy and local sourcing of the building materials; and aesthetics. Straw bale is not for everyone, and is certainly not the only ecological way to build, but it has a role to play when done correctly.

A literature is beginning to develop around moisture control in straw bale walls. Here’s a short list of important resources

Design of Straw Bale Buildings

Moisture Movement and Mould Management in Straw Bale Walls for a Cold Climate

Moisture Properties of Plaster and Stucco for Straw bale Buildings

Building Science for Strawbale Buildings

Many of the best practices of design, air detailing, flashing, and other details of conventional homes also apply to straw bale homes, and for this one of the best resources is the Builder’s Guide to Cold Climates.

Video of leveling earth plaster with a darby

Trever Miller from Evolve Builders came out and helped us with some plastering. I asked him to bring his darby (a three foot long leveling trowel) to help us level the kitchen wall behind the cabinets. Trever has years of experience leveling walls with a darby, I filmed this video of him at work.

Notes on toxicity

People who are new to natural plasters sometimes think they are non-toxic: earth plasters are made out of materials dug from the ground, right, how could they be dangerous? In fact, while the end result is non-toxic, these products can still be hazardous to work with. Take clay for instance, which often contains large amounts of crystalline, or ‘free’, silica (fine quartz), which when inhaled causes silicosis (a debilitating lung disease) and lung cancer. Silica is also found in cement and fine sand, but not in pure lime (which nevertheless isn’t great to breathe in). The long and the short of it is that plasterers work with materials in fine powder form and need to be very careful about what they breathe in.

  • Always wear a respirator when mixing, or anytime there is dust  – including cleanup!
  • Use a mop, or a vacuum with HEPA filter, instead of sweeping when fine plaster dust is present.
  •  Read the Material Safety Data Sheet (MSDS) for materials you are working with, including bagged clays and pigments

MSDS sheets can be found for most materials using an internet search. For example an internet search for “msds epk” finds that EPK bagged clay contains 0.1 – 4% crystalline silica, whereas the MSDS for Bell Dark Ball Clay shows it contains 10 – 30% silica. Kaolin clays often contain less than 1% silica, and are good for earth plaster finish coats. Ball clays and fire clays are more common in earth plaster base coats, and typically have large amounts of free silica. Site clays are typically processed wet, so they are hazardous only during cleanup.

Pigments vary greatly in their toxicity. Here’s a website that gives a summary of composition and toxicity of a few common pigments. The colour index can be helpful in trying to figure out what pigments are made up of.