Evaluating oils for earth floors

oilsMany of the wonderful properties of earth floors, including their strength and water resistance, derive from the oil used to harden and bind them – I find it useful to think of earth floors as linoleum made with earth, instead of cork and sawdust. Linoleum was originally hardened with boiled linseed oil, but linseed isn’t the only oil that works for earth floors. The main types of oil that have been tried for earth floors are boiled linseed, raw linseed, hemp, and tung (note that boiled linseed oil actually contains drying agents, and you should source oil that is free of heavy metal dryers). Sometimes resins are also included in oil blends used for earth floors. While each type of oil has its adherents for earth floors, it seems that there is little data that may be used to compare them – there isn’t even that much anecdotal evidence, other than that they can all work to some extent.

IMG_6209Because it haven’t seen it done elsewhere, I tested hardness of several oil and solvent combinations using an improvised Brinell hardness test. Brinell hardness is a simple test performed by pressing a 10mm ball bearing into a sample at a set pressure (usually 220 lbs) and time, and measuring the width of the resulting indentation. I used an improvised setup (inspired by this discussion), and the sample size is small – but despite a lack of scientific rigor some very striking trends can be seen. I’ll summarize the results first, but the actual measurements are presented in the table below.

  • Tung oil has very high initial and later strength, but has terrible penetration, and must be cut with thinner 25-50% in order to be usable, or it can blended with other oils.
  • Boiled linseed oil had relatively low 10-day strength, but gained a lot of strength with curing and was second only to tung oil in terms of hardness after 3 months. Mixing it with thinner reduced initial and probably late strength (the sample cracked and failed during the 3 month test). However it was clear that more oil-thinner mix could be applied to the sample, so the lesson is that when thinner is used, make sure to use extra coats of oil to ensure good saturation. Boiled linseed oil had moderate to poor penetration, and needs to be either heated (safely?!) or usually cut with thinner.
  • Raw linseed oil had slightly better initial strength than boiled linseed, but gained virtually no strength over the next 90 days, making it one of the weakest samples after 90 days. Raw linseed oil has moderate penetration.
  • Hemp oil has excellent penetration, and reasonable 10-day strength, but does not appear to gain strength very much over time, certainly not when compared to boiled linseed oil.
  • Blending oils can capture the benefits of each. A blend of 1 hemp: 1 tung: 1 boiled linseed, diluted 10% with citrus solvent had very good strength (though not quite as good as tung oil and thinner alone), as well as intermediate penetration. The resulting hardness of 1.81 is better than that reported for Swedish pine flooring (1.6) but much lower than most hardwoods (e.g. black cherry is 3.6, whereas Swedish birch flooring is 2.6).
  • Note that the boiled linseed oil I used was old, and may have suffered from some oxidation. Even so it appears to be one of the best oils for earth floors, though blending even a little tung oil into it will improve the resulting hardness. The raw linseed oil wasn’t fresh either.
Oil Comments Penetration (1=best, 8=worst) Hardness 10 days Hardness 90 days
100% Boiled Linseed strong odour, intermediate penetration 5 0.94 1.38
100% Hemp mild odour, excellent penetration 3 1.06 Sample broke (weak)
75% Tung / 25% Thinner Poor penetration. Smell is nutty and smoky, reminiscent of chipotle 8 1.71 2.05
100% Raw Linseed intermediate odour, excellent penetration 2 1.03 1.04
75% Boiled Linseed / 25% Thinner best penetration 1 0.85 Sample broke (weak)
75% Boiled Linseed / 25% Tung Poor-Intermediate penetration 7 1.01 1.61
75% Hemp / 25% Tung intermediate penetration 6 1.24 1.49
30% Boiled Linseed / 30% Tung/ 30% Hemp / 10% thinner intermediate penetration 4 1.37 1.81
White pine Trim grade 0.97
Pine Flooring Value from internet 1.6
Spruce-Pine-Fir Construction lumber 1.24
Cedar Home depot 0.77
Black Cherry Flooring Value from internet 3.6


Drying oils are very reactive, and oily rags are prone to catching fire when left bunched up (sponges are even worse, and should be avoided). Immediately after oiling a floor all rags, rollers etc should be spread out outdoors – otherwise the photo you take of your freshly oiled floor might be the last photo of your house before it burned down.

A few tips

  • Get as many coat on as you can over about two days (usually 4-6 coats depending on how heavily you apply each one).
  • Wait until one coat is completely penetrated before applying the next one (initially the coats can be applied almost immediately, later coats may take hours to finish penetrating).
  • Wipe off any excess oil after a few hours, don’t leave it overnight or it will gel.
  • Your freshly oiled floor is incredibly fragile – use bare feet or socks and roll up pant legs. Use soft rollers or brushes. Try to wait until oil has penetrated before crossing it.
  • Read Sukita’s book.

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.

A review of the Natural Building Companion

Two years ago I saw a presentation by Ace McArleton, co-author of the Natural Building Companion, and based on that I knew I needed to order the book. For a while it sat on my shelf, alongside a collection of other books about building and natural building. However it has distinguished itself from its companions in the true test – when I have a question, or encounter a problem in my work as a straw bale builder, it is the first book I reach for. I now consider this book to be an essential reference for those planning to build with straw bales, clay-straw or other natural building materials – and while it is useful for the pros, it is still accessible for beginners.

Jacob Deva Racusin and Ace McArleton have spent the last decade working on high performance natural building – marrying the building science of the superinsulation and passive house movements with natural materials. The natural building movement in general has been moving to adopt better practices around managing heat loss, particularly making buildings more air tight, but the Natural Building Companion is the first book to address it comprehensively, and also to offer practical solutions to common problems.

For example, it’s common to couple straw bale construction with timber frame, however the junction of each post or beam with the plaster is likely to become an air leakage problem – movement of the building can even break caulking along these joints when it is used. The best solution to this problem is air fins that run behind the post and several inches under the plaster. Racusin and McArleton discuss proper installation, and share their experiences using different materials as air fins – and the choice of material is critical since the wrong choice can simply displace the crack from the post to the edge of the air fin, or cause other problems.
Bales squared before installation
A controversial topic in the bale building world is the use of cement-lime plasters. A track record in Ontario has shown that cement-lime plastered buildings can work in our climate, if designed properly. However I would agree with Racusin and McArleton that cement-lime plaster is not the best choice in this relatively wet, cool climate – and the authors offer a very good alternative plastering system that is more appropriate for our climate. A lime-stabilized earth base coat with a lime finish coat is a system we learned about from European builders, but we had just plastered our first house with this system when we discovered that there was a wealth of knowledge just across the border in Vermont. The authors have mastered this system and share the details of it in their book.

The treatment of moisture management is also thorough; like everything in this book it starts with design considerations and follows through to building techniques and detailing. The authors have also done extensive testing of their buildings post-construction to see how well they performed over time, while they were being lived in.

Vampire stakes

Vampire stakes

I even learned tricks to improve our bale work, such as squaring bales before installing them, or the use of “vampire stakes” to anchor straw bales to post and beam structures – the most functional and elegant system I have yet seen for doing this.

The book comes with an instructional DVD filmed at Yestermorrow, which shows exactly what they explain in the text, filmed at each step along the way. I’ve only consulted this a few times so far, but each time it clarified something that wasn’t quite translating from the page for me. Beginners to straw bale will likely want to watch it right through.

I have only picked out a handful of examples, but the Natural Building Companion covers every step from design, through construction, to finishing details, and does it all well. They take it from philosophical underpinnings to technical details of the systems they have evolved with their company New Frameworks.

In short, the authors nailed it, they have written a standard for the natural building world. Having used the book, and spent some time with the authors, I would also agree with Andrew C. Gottlieb that these are definitely builders I’d want to work side by side with, for their professionalism, expertise, and knack for having fun.

I would recommend their website and blog as a reference, in particular I’m keeping an eye on the development of their new system that aims to reach passive house standard while using straw bales. Here is a video of Racusin and McArleton discussing high-performance natural building.

The Dumont House

The first time I saw Rob Dumont’s house I was unimpressed. I was visiting an ex-girlfriend in Saskatoon, I mentioned that I was doing some research into sustainable homes, and she said “there’s one near here, we should walk by it.”

It just looked like any other house. The Dumont house is in the colonial revival style, it’s simply built and doesn’t stand out in the neighbourhood, which has a suburban feel to it (though it’s not far from the downtown). I’m used to seeing half million dollar ecohomes, so when you take away the architect and expensive finishes, solariums, thermal mass walls, radiant floors, etc., it’s hardly recognizable as an ecohouse. I arranged to go back later and visit Rob Dumont, who gave me a tour of his home and some other projects he was working on. What initially turned me off about the Dumont House (because it challenged my preconceptions) now makes it one of my favourite sustainable homes.

16 inch double wallWhen Dumont built this house in 1992 it was one of the world’s most highly insulated homes – but the house, like Dumont himself, is understated. Dumont took the double wall system from the Saskatchewan Conservation House, stretched it out to a full 16 inch wall cavity, and filled this space with blown-in cellulose insulation (which is just recycled newspaper with borax added for fire proofing and pest control). There are about 16,000 lbs of cellulose in the house – but what makes this insulation system really special is that the two walls have very little framing between them, so there are far fewer pathways to lose heat through the wall, either through leakage where the insulation doesn’t meet the wood perfectly, or by ‘thermal bridging’ through the wood itself. It may seem obvious, but it needs to be said: wood is way worse insulation than insulation is. A 2×6 stud wall with R20 insulation batts has an overall insulation value of about R13. Rob Dumont’s walls are R60, the attic is R80, and the windows are R5. The whole house is carefully air sealed. It takes less than 1/4 of the energy to heat Dumont’s house that it would for a conventional house. In 2000 Dumont wrote an article describing the energy efficiency features of his home.

Hot water heat-exchangerAs we continued the house tour, Rob showed me some things that really could be added to any existing house. The first was a drain water heat exchanger, just a copper tube wrapped around the shower drain – as hot shower water comes down the drain pipe the cold incoming water in the coil is warmed. “On a shower it will recover about half of the heat that’s otherwise going down the drain,” said Dumont. He quipped that “I’m worried that with the price of copper, in a home invasion someone will steal it.” Still, barring home invasions, the economic payback is pretty quick – if your hot water heater is electric the heat exchanger will pay itself off in 5 or 6 years, with gas maybe double that. Next we looked at the hot water heater itself, which is a standard tank but wrapped with batt insulation and a thermal blanket adding up to about R28.“Without the insulation it loses about 100 watts of heat continuously,” Dumont said, “with the insulation it’s down to about 25 watts.”

Painting of the house by Phil

Because of the simplicity of the Dumont house, it wasn’t expensive to build. The insulation, upgraded windows, and a solar thermal heating system, added about 7% to the building cost. “If I’d put brick on the outside of the house instead of hardboard siding,” said Dumont, “the brick would have cost more than all of the energy conservation features. I’d much rather have an energy-efficient house than a brick house.” In fact, the energy efficiency finished paying for itself in 2008, after 16 years – now it’s turning a profit. And he pointed out other non-monetary benefits – no draftiness, no cold feet, and the nice aesthetic of the deep window ledges.

Rob Dumont, like many of the builders and designers I’ve met, started his work in the 1970’s, and spent some time wandering metaphorically alone in the desert in the 80’s and 90’s. “Society has got a very short attention span,” he said, “there are waves of interest, but mother nature bats last. I started working in the 70’s on the Saskatchewan Conservation House, one had to really keep the faith through a part of the time since, because not many people were very interested. I must admit back in 1973, with the oil shock, I thought the reasonable thing to do would be to change the way we do our houses radically. That was my youthful naivety at the time.” He showed me a book of solar homes that was written in the late 1970’s, a sort of hippie version of what I’m trying to do, and I realized that I’m just the latest emissary of societal interest, something Dumont has seen come and go. I feel like this time it may be different, but I’m not sure. “It’s encouraging,” said Dumont, but “it’s not nearly at the level I’d like it to be. EF Schumacher put it nicely, he said the wind may not always blow but at least we should have our sails up. That’s the way I feel.”

Rob and his wife Phil took me to see a college basketball game, in which the home team, the Huskies, thoroughly trounced the competition (both women’s and men’s teams). I pictured Rob in his younger days playing basketball, fit and idealistic, believing he could change the world. And he did – it just changes very slowly. I wonder, when I look back in another twenty or thirty years, how I will remember this time. As the beginning of real change, or as lost opportunity? All I know is that my visit with Rob Dumont left me more optimistic than when I arrived.

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 naturalplaster.ca