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Clay

Strawtec answer all of your questions about using clay in the straw building process.

Making sense of the enormous amount of information scattered throughout the abundant literature on building and rendering with earth may seem impossible. This contribution portrays information based on selected relevant research and many years of hands-on experience. We have attempted to use a straightforward approach for the presentation of the data in order to make it accessible to a wide range of interests, skills and applications and help to make informed decisions.

Advocating clay as the first choice for rendering strawbale walls as well as any other conventional wall system is predominantly linked to its great benefits to our health and the environment. Earth has a vast technical and architectural potential in the construction industry and the fact that it has been used in the simplest and most sophisticated structures all over the world supports its importance in this industry today. Traditional clay architecture is more than 10,000 years old and about a third of the world’s population live in homes constructed from clay. European monasteries, castles and mansions where rendered with clay and clay was used in the infill panels of traditional timber frame houses. Many of these buildings are hundreds of years old and still standing up proudly today.

Natural, non-toxic, healing, easily available, recyclable, low embodied energy, compostable, a pleasure to work with, soft & soothing, limitless creative possibilities – are some of our ways to describe clay -; others may call it primitive, inferior & dirty but the latter are views we hope to transform with the information provided here.

Huts

Yemen

Why Use Clay?

“We are our environment (and vice versa) at a very fundamental level”.

(S. & J. Baggs, 1996, p.14)

Depending on where we live and what kind of work we do most of us spend between 60 and 90 percent of our time within the buildings where we eat, breathe, sleep, work, learn, play and heal. A healthy, pollution free and comfortable indoor environment is therefore vital for our physical, emotional and spiritual wellbeing.

Today we know that many “wonder products” of yesterday, for example asbestos, pose a very serious health hazard, even killing the people who endured excessive contact with the material at their workplaces; not to mention the disposal problems of such products. We also know that health problems such as asthma, allergies, migraines, depression, malignant cancers and so called “modern life or mystery” syndromes such as Chronic Fatigue Syndrome, Attention Deficit Disorder, Serotonin Irritation Syndrome and General Adaptation Syndrome have increased throughout the population, including our very young. Pesticides, long-chain molecule plastics, chemical glues, fibreglass insulations, aerosols, radioactive and mineral pollutants as well as electrical and magnetic fields are all factors in our physical environment which can cause or contribute to these health problems. Using natural, non toxic building materials such as clay and straw reduces exposure to out-gassing toxic chemicals and provide us with safe and comfortable buildings, while easing the environmental impact of the construction industry at the same time.

We have probably all heard about the healing properties of clay. It played a large role in European medicine during the Middle Age, retaining importance until today, and has always been used in traditional healing all over the world. Usually the healing earth was named after its origins; for example healing earth from the Greek island Samos was called Terra Samia, Terra Agyptica was mud from the Nile in Alexandria and Terra Japonica came from Japan (G. Minke, 1994, p.305). This tradition seems to continue today seeing that we find for example Dead Sea Mud in most health food shops and beauty treatment facilities.

The healing properties of earth are due to its pain-relieving, anti-inflammatory and astringent qualities. It can be used internally or externally and is best known as mud bath, mud wrap, healing earth and essigsaure Tonerde (aluminium acetate). Taken internally healing earth helps alleviate diarrhoea, replaces lost minerals, binds acidity and toxins in the stomach and intestines and for these purposes is also used supplementing stock feed for weak and sick animals, here this supplement is known as Bentonite. Applied externally, mud helps to ease rheumatic and muscle pain, stimulates circulation, protects from the sun, provides relief from itchiness of insect bites, helps to heal burns, treats skin conditions such as acne and eczema and can be used to cleanse the skin. Healing with earth is also known as Pelotherapy and this website http://www.eytonsearth.org/eytonsearth.html is an excellent recourse to find out more about it.

Another interesting property of clay is its ability to absorb radiation. Apparently “Russian scientists use Bentonite to protect their bodies from radiation when working with nuclear material, by coating their hands and bodies with a hydrated bentonite "magma" before donning radiation suits. Bentonite absorbs radiation so well, in fact, that it was the choice material used to dump into Chernobyl after the nuclear meltdown in the former Soviet Union." (Note: there are many different kinds of Bentonite for over a thousand uses).

Natural healing centres all over the world offer therapies where clay or mud is used. Knowing just this, it comes at no surprise that using clay to build or cover the walls of our homes is valuable to our health and comfort.

Ghandi - "Though I have had two serious illnesses in my life, I believe that man has little need to drug himself. 999 cases out of a thousand can be brought round by means of a well-regulated diet, water and earth treatment and similar household remedies."

This reminds us of a little anecdote experienced with our two children. To our amazement we have surprised both of the boys from time to time licking and chewing on small dry balls of clay (which float around our home as they were kept from sample batches of soil we have tested over the last few years). Do they instinctively know that clay contains many essential minerals and has healing properties? And do they just have a bite when their body needs these?

Clay also has the ability to beneficially influence internal air humidity by maintaining a pleasant and constant humidity level, optimal for indoor air quality and health. Air humidity is as vital to human comfort and wellbeing as temperature, and can have more serious consequences than the physical discomfort associated with extremes of temperature inside the dwelling. Many people have the misconception that drier air is better, probably due to an association of moist air with the unfortunately typical problems of raising damp and mould in their homes. However, a relative humidity level of less than 40 percent can cause the mucous membranes of the nose and throat to dry out, allowing foreign bodies such as dust particles and bacteria to reach the sinuses and lungs rather than being carried back out of the body via the fine epithelial hair lining the mucous membranes. A high air humidity of up to 70 percent has many positive influences. It reduces the fine dust content of the air, helps to protect the skin against microbes, increases resistance against colds and flues, reduces airborne pathogens, absorbs odours and decreases the unpleasant electrostatic charge on surfaces of objects. However, if the air humidity in closed rooms rises over 70 percent an ideal environment for the rapid growth of fungi is created which in turn can cause or aggravate a number of allergic ailments. From this we can conclude that the optimum and most pleasant indoor air humidity should be at least 40 percent and not exceed 70 percent (G. Minke, 2000, p.15).

Porous materials have the ability to absorb humidity from the air and release or desorb it back into the air; this humidity balancing potential is especially effective with clay. If indoor air humidity rises above 50 percent clay absorbs the excess moisture and will release moisture back into the air if humidity levels fall under 50 percent. In a house built in 1985 in Germany, with all exterior and interior walls built from clay, air humidity measurements were taken over a period of 5 years. The measurements showed that indoor air humidity was nearly constant over the 5 year period, varying only between a healthy 45 and 55 percent. Humidity levels are particularly affected in cold and moderate climates where outside temperatures are much lower than those inside. For example, if outside temperature of 0°C enters a room and is heated up to 20°C, its relative humidity decreases to less than 20 % (G. Minke, 2000, p.15). For this reason, we often see bowls of water placed in heated rooms, so that the water evaporating into the air will raise air humidity. Obviously a wall built from, - or rendered with clay would achieve this much more effectively by maximising the mass and surface area available to absorb and desorb atmospheric moisture.

A solid wall of earth bricks can absorb up to thirty times the moisture of conventional burnt bricks. Nevertheless, a thin layer of clay render is still able to absorb an exceptional amount of water. Experiments undertaken at the Forschungslabor für Experimentelles Bauen, an experimental building research facility at the University of Kassel in Germany showed that a one sided sample of 15mm of clay plaster was able to absorb 5 times the moisture of a 15 mm sample of gypsum plaster. In setting the conditions for this experiment, humidity was suddenly increased from 50% (a comfortable and healthy level for humans) to 80% (at which level we would certainly feel uncomfortable in either cool or warm conditions) at a room temperature of 21°. After 48 hours the 15mm thick layer of gypsum plaster just about reached its full moisture absorption capacity at 40 grams of water per square metre of wall surface. After the same period the 15mm layer of clay render on the other hand reached a level of about 80 grams of water per m² but had not reached its absorption capacity for a long time. Further similar experiments showed that with the addition of natural fibres such as coconut fibres the absorption capacity of the clay plaster could even be raised (G. Minke & F. Mahlke, 2004, pp. 33-34).

To further support the use of clay renders in regards to healthy room climates we would like to refer to another experiment conducted at the University of Kassel. Here it is shown that a 20mm layer of clay plaster can absorb more than 250 grams of water per square metre, a 40mm thick coat more than 600grams and an 80mm coat more than 750 grams of water per square metre of wall surface. In this experiment humidity levels were also quickly raised from 50 to 80 percent at a room temperature of 21° and measurements where taken over a period of 16 days (G. Minke, 2000, pp. 16-17).

These remarkable humidity balancing properties may cause people to believe that clay building products, by absorbing moisture, would be setting up the perfect conditions for the growth of mould or the onset of fungal rot. The opposite, however is actually true: ideal conditions for mould or rot are caused when a building material absorbs moisture and is unable to later release it when air humidity drops, as in the case of absorbed water becoming trapped behind oil based painted surfaces, or when moisture is not absorbed but remains stagnant on a surface. This makes even bathroom and kitchen walls built from clay or rendered with clay more hygienic and far less vulnerable to the growth of mould than for example tiled walls. Where water splashes directly against clay, surfaces can be protected with casein, lime or silicate paints or small affected areas could be tiled or cladded with for example corrugated iron.

Before moving on to the more practical part of this paper we briefly like to mention some other benefits clay can offer to your home.

  • Thermal mass
  • Sound insulation
  • Reduces Electromagnetic Radiation (EMR)
  • Fire safety
  • Preserves

The insulating properties of clay are rather modest. However, clay render or internal walls from clay provide thermal mass to a building. Thermal mass depicts the heat storage capacity of a material. In climatic zones where the diurnal temperature differences are high, or when we wish to store solar heat gain by passive means, clay can balance the indoor temperature; storing the heat from the sun during the day and releasing it slowly during the night. Rudolf Steiner, founder of anthroposophy, once formed the sentence “only through mass can a good and comfortable indoor climate be achieved”. This has been confirmed by physical and technical data in regards to building materials since. We are left to suggest that the material providing mass for our home might as well have other valuable properties such as being natural, non toxic, vapour permeable, biodegradable, healing etc.

Being heavy or a high mass material, clay has good sound absorption qualities. It provides better sound insulation than lighter building materials such as timber, burnt bricks, fibre cement sheets or plaster boards. (E. Thoma, 2003, p.102)

Electrosmog or Electromagnetic Radiation (EMR) can be divided into three categories.

1. Electric fields and static charges which are found around TV screens by friction or in clothing etc. These are measured in volts per metre.

2. Magnetic fields are found where alternating current is present, they are measured in Tesla.

3. High frequency radiation which is present near radios, TV, mobile phone towers, mobile phones etc. This is measured in megahertz and gigahertz.

Tests conducted at the University in Munich, Germany in 1999 showed that solid timber and clay had by far better radiation shielding properties than for example concrete, bricks, concrete blocks or stud & plasterboard walls. From these tests they concluded that the superior performance of natural materials such as timber and clay is due to their unique cell structures made up of cavities, capillary tubes, cell walls, encased resins and various other materials and that man made building materials can just not compete with nature (E. Thoma, 2003, pp. 62-63).

German fire safety norms confirm that clay, even if mixed with straw, is not combustible, as long as the density is not less than 1700kg/m³ (Deutsche Normen, 1956, DIN 18951, Blatt 2). Fire tests of rendered (clay, lime & cement rendered samples were tested) strawbales conducted by the CSRIO in 2002 also validate the excellent performance of clay render for strawbale homes in high-category bushfire prone areas. Engineered clay provided by us was used to render the test bales. These tests concluded that it is clearly the render layer, which mainly contributes to the fire safety of the strawbale by providing a barrier for the combustible straw inside (Fire Safety Testing and Assessment of Rendered Straw Bales for Use in High Category Bush-Fire Prone Areas, 2002, CSRIO Fire Science and Technology Laboratory).

Clay preserves timber elements that are covered or in contact with the material. It keeps the wood dry and fungus or insects are very unlikely to cause any damage to the timber, because they need about 15-20% of humidity to survive.

From all the collected information we like to conclude that any wall system rendered with clay, even with a fairly thin layer of 15 to 20mm, will help balance air humidity and provide many other advantages conducive to our health and comfort. With today’s technology, advanced rendering tools & techniques, many years of experience, the right clay, reinforcement materials and the help of natural glues to provide a bridge between clay render and conventional building materials such as brick, brick veneer, fibre cement sheets, plasterboard, timber cladding, cement rendered and painted walls, etc. it is possible to provide a healthy & non toxic wall surface for any kind of building. Of course, not everyone is inclined or has the opportunity to built or move into a dwelling made from earth and other natural materials. But for those interested in the health-enhancing and beneficial properties of earth construction, it is still possible to reap the benefits of clay building products within a conventional urban or suburban house with the application of clay render over surfaces made from standard materials. You may wish to trial clay rendered walls in one room of the home first. The bedroom – a room where we spend a good part of our time to gain rest and rejuvenate a tired body - would probably be very suitable for the assessment of clay rendered walls in regards to their benefits to health and comfort. Such a conversion would be quick and affordable to do and the night spend in a healthy indoor environment may endow you with a fresh and relaxed start of the day ahead.

We hope that we have succeeded to change some of the negative attitudes towards clay and that the remainder of this paper will take away whatever doubt or insecurity is left.

What actually is clay?

Note: the scientific name for earth used as a building material is loam which basically means soil with a significant clay content, however, to avoid confusion we will continue to use the term clay throughout this paper.

There are many different types of clay ranging from Kaolinites - the least expansive and binding, to Bentonites – the very expansive and sticky materials; most soils contain a mixture of clays. All clays are composed of secondary or water-containing minerals of extreme fine and flat particles, sized smaller than 2µ (micro millimetres). They have been produced by the action of weathering agents (water, air and sunlight) on the feldspars and micas of igneous rocks (rocks of volcanic origin). Clay materials are composed of alumina, silica and water with minor amounts of lime, magnesia, soda or potash. Iron compounds, hydroxides or carbonates are nearly always present as impurities in clays and account for most of the wide range of colours (Technical Notes on Clay Bricks, Brick Development Research Institute, Melbourne, 1977). More important than the chemical composition however is the fact that when mixed with water, the clay minerals give a plastic mass which can be shaped and has binding power. This binding power is based on the electric force of attraction between the flat sides of individual clay crystals. The German Norms from 1956, DIN 18954, specify bonding strengths of clay in grams per cm² (g/cm²).

The following figures are taken from these norms indicating the binding powers of different clays.

o Lean clay: 50-110g/cm²

o Slightly lean clay: 111-200g/cm²

o Fatty clay: 201-280g/cm²

o Very fatty clay: 281-360g/cm²

They further state that clay with a binding power of less than 50g/cm² is not suitable for building purposes.

In contrast to fired clay, water can be absorbed and move freely between the flat particles of raw clay. The wet particles slide over each other, giving natural clay its extraordinary soapy or plastic consistence; the clay will harden again if dried out. This rather simple occurrence of softening and hardening shows that clay building parts must be protected from water on the one hand but on the other hand it demonstrates that clay is a very flexible material that can be recycled, easily repaired and shaped into limitless forms and applications.

Before using a particular soil for construction one has to be aware of its properties; these fall into four main categories.

Texture: Describes grain or particle size distribution

Plasticity: Describes the ease of shaping the soil

Compressibility: Describes a soil’s potential to decrease its porosity to a minimum

Cohesion: Describes the ability of soil particles to remain in association

The following examples show different soils according to texture:

Gravel: grain size distribution 2 – 20mm, gravel and pebbles predominate

Sand: grain size distribution 0.06 – 2mm, sand predominates, has the appearance of mortar

Silt: grain size distribution 0.002 (2µ) – 0.06mm, silt predominates, it is a fine silky soil with low cohesion

Clay: grain size distribution smaller than 0.002mm or 2µ, clay predominates, cohesive and pliable

Depending on the quantities of a specific grain size within the soil, characteristics of the soil can be defined. For example, if the soil contains 10 percent of clay it will have some degree of the cohesive and plastic properties characteristic of clay, whereas 20 to 50 percent of clay particles give a soil the properties suitable for rendering (soil for mudbricks or earthbuilding usually has a clay content of 5-15% only), it will be extremely cohesive and malleable when wet.

There are some simple visual and manual tests which can be performed quickly and easily without specialised equipment. These tests will give some indication of the composition of the soil and its suitability for a specific purpose. However, they are not very precise and more involved tests may be necessary, especially if experience with soil identification is limited. It is also highly recommended to always work with sample patches (blocks, walls) before rendering or building with clay in order to determine the suitability and performance of the soil and the needed ratio of other ingredients such as sand and water.

  • Sedimentation test:

One of the basic tests is a sedimentation test which is frequently described in earth building literature but has proofed to be very imprecise with error rates up to 1750 percent (G. Minke, 2000, p.22). However, in this test the soil sample is mixed in a jar with a lot of water, the coarse particles settle first on the bottom of the jar whereas the finest particles form the top layer. The different layers can be visually distinguished and proportions of gravel, sand, silt and clay can be roughly estimated. If the contents of the jar are frozen, the visually distinguishable layers separated, thawed, dried and then weighed, the percentage of different composites can be expressed in mass. This refined test is much more accurate than the simple sedimentation test; however, experiments at the University in Kassel still confirm an error quotient of up to 850 percent (G. Minke, 1997, pp. 33-35).

  • Shrinkage test:

Clay swells when in contact with water (absorption of air humidity does not lead to swelling) and shrinks when drying out. This characteristic is rather detrimental when building with clay. However, testing clay for shrinkage cracks can give an indication of its quality in regards to being a fat or a lean clay. If, after drying out, a clay sample batch displays large cracks it can be concluded that is a strong or fat material. A silty clay will usually display fine cracks after drying out and clay with a high sand content may have no cracks at all. It may be best to start working with a fat clay which can be made leaner until it is perfect for its intended use by adding sand. The change of grain distribution achieved by adding sand to clay is aimed at minimising shrinkage cracks.

Other basic tests include:

  • Chewing a small sample of soil. If a grinding sensation is felt the soil has a high sand content while a soft, squashy and sticky sensation indicates a soil high in clay.
  • A sign of gravel or sandy soil is given if when rubbing a wet soil sample between the hands grains can be clearly felt. If the moist soil sample sticks on the hands but can be rubbed of when dry a silty soil can be identified. A clayey soil is very sticky making it necessary to use water to clean the hands.
  • Cut through a dense ball, formed from moist soil, with a knife. If the cut surface is shiny the soil has a high clay content while a dull surface indicates a high amount of silt in the soil.
  • Last, form a ball of about 4 centimetres in diameter from soil just wet enough to be malleable and drop it from a height of 1.5 metres onto a flat surface. If the ball crumbles and falls apart the soil is very sandy and probably unsuitable for building/rendering. If the ball flattens only slightly and shows no cracks the soil has a high clay content and may have to be made leaner by adding sand before using it in construction.

More precise tests performed in a laboratory or by an experienced person should be sought if uncertainty of the clay’s suitability for building or rendering persists.

Rendering with clay

Clay plasters are usually made up from fatty clay which can be made leaner with sand or fibres to a suitable product. It is important to get the clay content of the mix right because if it is too lean the binding power and compressibility are reduced and if it is too fat shrinkage cracks will create a problem.

Some literature suggests that it is not recommendable to enrich lean clay however, we regularly add Bentonite to lean clay and this practice has worked very well.

Because of the difficulty identifying and the skill required to distinguish a soil’s suitability for rendering (building) purposes we believe it is crucial that a ready made product is made available at building supply outlets and hardware shops. For clay to become a mainstream rendering product and to be recommended by architects and engineers as first choice, it has to be standardised and made safe, easy and enjoyable to obtain and use. Engineered clay is used every day to manufacture bricks & tiles as well as for making pottery, therefore, designing a clay render mix should be possible without too much difficulty. Standards for testing soil for engineering purposes exist in Australia and many other countries; these could probably be used or extended to test soil for rendering purposes.

Bagged rendering mixes from clay and natural fibres are readily available in Germany and some other European countries, making clay a much more popular building material there. With standard clay building materials, product information in regards to ingredients, performance, application procedures and maintenance requirements would also be given. Knowing how to use the product, how it will perform and ensuring uniformity in appearance with a standard product would eliminate the need for time consuming experiments with sample batches of soil. We believe that readily available, standard clay building products would make the use of clay more attractive and could provide a real choice for people, even without any experience working with clay.

Clay & Sieves Local clay is processed through a makeshift sieve.

We will now finally describe our rendering techniques and mixtures. Please remember that these are guidelines only and no guarantee can be given that these will work unless you are using suitable and tested materials and follow proper application techniques. With most of our projects we have used engineered clay and other well trialled products which our company can supply if required.

Considering you have a suitable clay, we recommend the following render mix ratios for the specific coats. Please note that the mix for the first coat, also referred to as scratch or slip coat, varies, depending if it is applied by hand tools or spray rendering equipment. Also, the following mixes and application procedures are only relevant when rendering strawbales, rendering techniques for other surfaces will follow.

1st coat applied by hand tools:

Clay and water only, mixed into a slippery sticky blend with a consistency of smooth chocolate milk or runny custard. If applied by hand tools such as wide paint brushes or hand brushes make sure that it is thrown onto the bales with some force in order to penetrate into the strawbale as much as possible.

1st coat applied by spray rendering pump:

Because the pump provides the force it is possible to apply the first coat much thicker than with hand tools. The ratio of clay to Brickies sand we use here is 1:1 and the consistency of the mix can be compared to thick, grainy custard.

2nd coat (also often referred to as base, main or brown coat):

The ratio here is the same for trowel or pump application.

Clay to Brickies sand in a ratio of 1:2-3 mixed into a consistency which will not run off the trowel. If in a wheelbarrow you should be able to scoop it up easily with the trowel but it should be solid enough to not run off the tools and not slide down the vertical wall surface. The consistency reminds of a sandy sourdough bread mix. The mix is right if it has a sandy feel but should also be sticky due to adequate clay content.

3rd coat, also called top coat or finish coat: (This coat is usually about 2-5mm thick, all coats together add up to an average thickness of 35-45mm)

The mix for the finish coat is clay and washed Beach sand in a ratio of 1:1.5. This coat is always applied with hand tools. It has a smooth and easy to work consistency. We use a finer and lighter engineered clay for this application. The soil used for the top coat contains a clay content of about 5-8% as compared to the scratch coat where clay content can be up to 50%. The clay gradually should become leaner with subsequent coats to avoid shrinkage cracks. This translates into the coats becoming softer from the inside to the outside. The top coat provides a smooth and light finish which can be easily painted. Only natural paint products (coloured with pigments if required) should be used to maintain the vapour permeability and natural beauty of the clay render. Pigments may also be added to the render mix however, it has to be kept in mind that when using natural products colour variations may result in subsequent mixes or when doing repair/maintenance work on the plasters at some later stage.

Externally we usually recommend to apply a finish coat of lime render for extra protection from driving/splashing rain and hail. This is, unless the walls are very well protected by roof overhangs, off ground foundations, wide verandas etc. The Vipassana Meditation Centre in the Blue Mountains, NSW, Australia (see under our projects page) presents an example of an exclusively clay rendered strawbale structure. The walls are well protected by verandas but please remember that it is extremely important to further protect the plaster with paint. We recommend to use a natural and vapour permeable product such as casein paint, pit lime paint or a lime wash; all of these should be applied in three coats.

The external lime render finish coat is made up from slaked quick lime and washed Beach sand at a ratio of 1:3-4. We usually mix a small amount of clay, approximately 10% of the lime quantity, into this lime render finish; this practice has proved to make the plaster slightly stronger. This finish coat is about 6-8mm thick and is applied with hand tools. We will not go into much detail in regards to rendering with lime in this paper, however, please keep in mind that slaking quick lime requires some skill and is dangerous as the material gets very hot during the chemical reaction with water. It is possible to use other lime products such as hydrated lime but we do not recommend these products as they tend to be more brittle , less flexible and weaker than quick lime.

Historic lime renders analysed in Germany showed small amounts of clay included in the plaster. It has been suggested that clay may have been added to provide some extra bonding strengths to the lime plasters.

Before applying lime render over the base coat of clay we apply a bridging coat made up from lime putty. This is a runny mix of 1 litre of lime putty with 14 litres of water; it can be brushed or sprayed on. Note: if sprayed on the lime putty should be strained through several layers of Hessian (or similar) to remove any lumpy bits which may have sunken to the bottom of the vessel containing the material. We have found that, possibly due to different tension stresses during the setting periods of the two materials, the lime coat has been rejected and fallen off the clay base in some patches. With the thin bridging coat this has not happened again.

We also do not usually mix lime into our clay renders. In traditional Japanese plasters this has been done but with these, only a maximum of 5 percent of lime has been added to clay renders in order to be able to polish the surface into a shiny finish; we do not recommend to add more lime to any clay render mix. These Japanese finishes are usually only applied internally unless the walls are very well protected.

Lime render reacts with carbon dioxide from the air to eventually form the compound CaCO3. Moisture is needed for this chemical reaction to happen and this setting or curing process of lime plasters is slow. To allow lime render to cure it is important to keep the render moist for at least one week. It is best not to apply lime render in hot and dry weather. If the climate turns hot and dry after you have rendered the walls should be moistened with a mist of clean water three to four times a day. Hessian can be hung from the gutters to provide shade and the material can also be kept wet. For the first week the wall should also be protected from driving rain and hail as these may damage the surface. The main curing time for lime render is about 4 to 6 weeks, however, the material takes up to three years to reach its hardest state. Before the lime top coat goes on the base coat of clay the clay base coat also needs to be protected from driving and splashing rain to avoid wash out, again lengths of Hessian hung down from the gutters can be used to do this job. No plaster should ever be applied if frost is expected.

Because the structural soundness of a plaster depends largely on the background and bonding between coats we take particular care with this issue, i.e. application of bridging coats between different materials, normally no lime to be mixed into clay etc. It is also important that a coat of plaster has thoroughly dried before applying a subsequent coat and only the surface of the dried out plaster coat is wetted with a fine mist of water before applying a new coat. The first thin coat or scratch coat of clay render usually displays cracks because a strong clay is used and no or little sand has been added to make the mix leaner. The cracks and only thinly coated straw provide a good key for the second coat. A key between second and third coat is supplied by grooves scratched into the surface in a criss-cross or diamond pattern of the second coat when it is semi dry. A hayfork or similar can be used to scratch the about 10mm deep grooves into the plaster.

Render materials should be mixed slowly. The use of more water added to the mix to speed up the procedure will only results in more cracks. Instead of adding water to the mix to make it workable quicker, take more time in mixing and a better product will result. Water will move slowly in-between the clay particles and will be absorbed over time, giving clay its plasticity and bonding power so desirable for good rendering results. Mixing the plaster materials for 10 minutes or more has proved to be a good guideline for integration time of the materials.

We start rendering from the top of the wall working our way down to the bottom. This is done because water contained in the render mix moves downwards and would cause the render at the bottom to stay very wet for an extended period. Also, if some render material falls down from the upper parts of the wall, damage or soiling of lower parts of render can be avoided.

Reinforcements:

To make clay renders strong & durable and control cracking we use several reinforcement agents. Slightly overlapping strips of reinforcement netting (synthetic-resin coated fibre glass) are worked into the second coat of render as soon as it has been applied (either by pump or trowel) and smoothed out. The netting strips are stuck onto the moist clay and plied into the render, gently pushing it in with a trowel. This netting is used internally and externally, covering all areas of wall. In areas affected by strong winds, on walls where heavy objects are to be hung, over posts and joints with other building materials, at corners, around window & door openings and occasionally over the more vulnerable first two bottom courses of bales, we may work the netting into the first and second coat. We also add short (about 3mm fibre glass) reinforcement fibres into the 2nd coat of render and also into the third external lime finish coat. Usually we use about 900 grams of fibre per m³ of mix. A small amount (2 cups per mix) of Bentonite (please remember that there are many different Bentonites, all have different functions) is usually mixed into our render to give the material some extra binding power which allows us to apply coats in thicker layers. Some basic experiments we have conducted over the last few months show that clay render can be made very strong as long as the appropriate clay is used, reinforcement is included and application has been performed with professional care.

Other reinforcement materials such as hemp fibres, chaff, cow manure, animal hair coconut fibres, shredded paper, sawdust etc. could be used. However, we have decided to make a compromise in regards to natural versus synthetic reinforcement materials. This decision is based on the fact that we have achieved excellent results in regards to strengths, durability and crack control with the synthetic materials. The netting is quick & easy to install and the short fibres are not causing blockages when used with spray rendering equipment. We have not encountered any problems with mould & fungi growth and have been able to achieve uniformly good results using standard materials.

The photo below on the left shows a clay rendered strawbale wall holding two 15 litre buckets filled with water without showing any damage to the render. An 8mm wall-plug and screw were fastened to the wall, the full buckets are hanging without support and the 30 + Kg weight has not put any strain onto the clay render. This basic experiment demonstrates that frames, mirrors, light shelfs and other wall hangings can be fastened to a clay rendered strawbale wall without positioning pegs, dowels or similar in the bales before rendering. Our aim is to show that approximately 35mm of clay render is comparable to a standard gyprock (plasterboard) wall in regards to strength and durability.

The photo on the right shows reinforcement netting being worked into the second coat.

Clay rendered strawbale wall supporting 30 + kg. of weight

Netting is installed

No chicken wire or other metal laths are required for reinforcement or as plaster carrier with clay and lime renders. Tests and experience have shown that these wire reinforcements provide no or very little additional structural strength, the render does not bond as well with the strawbales as when applied directly onto the bales and air pockets can develop between strawbales – wire - and plaster, trapping moisture within the walls. Wire meshes are also labour intensive to install, are unpleasant to work with and walls being caged in wire have been known to cause electro magnetic fields.

We do not use straw or chaff as reinforcement or for crack control because the straw-clay renders take a very long time to dry. The risk of mould & fungi growth in these wet plasters is high (unless you built in a very hot and dry climate with little annual rainfall) and we have observed clay-straw plasters to be sprouting seeds. However, small amounts of chopped straw or chaff make for lovely special surface effects when used in the finish coat. Aluminium dust, oxidised iron splinters/shavings, coarse sand, crushed semi-precious stones, granite dust, shredded paper, sawdust and similar can also be added to the finish coat to achieve different textures and a number of creative surface effects. These effects are brought about by using sponges, soft brushes, sheep skins, rags, wooden floats, stones or similar to polish the semi dry surface.

Spray rendering strawbale walls is particularly suitable to ensure optimum bonding. It allows the use of sharper sand and less water in the mix. The pressurised application of the spray rendering pump allows to built up a plaster thickness of up to 25mm in one coat, speeding up the procedures while reaching optimum bonding at the same time.

Other surfaces such as bricks, plasterboard, fibre cement sheets, timber cladding, concrete or Hebel blocks can also be rendered with clay. Some of these surfaces, mainly fibre cement sheets, concrete walls and timber, may have to be prepared in order to create a key for the subsequent render application or in order to protect the background material. We have successfully prepared fibre cement and cement surfaces by painting on a coat of wood glue diluted with water in a 1:1 ratio, (Weldbond, Aquadhere or similar) which is than coated with a layer of sand. The sand must be thrown onto the wet glue and the surface should be completely and evenly covered; a second application of glue and sand can be applied if an extreme dense surface is to be rendered. Diluted Bondcrete could also be used instead of wood glue. For all other mentioned surfaces it has proved to be sufficient to increase the amount of Bentonite to the scratch coat mix to make the material very sticky. This scratch or slip coat can be sprayed on or painted on with wide paint brushes. This first sticky coat acts as the plaster carrier for the more solid base coat. These techniques of applying wood glue and sand or a very sticky scratch coat, plus the use of reinforcement netting, have provided strong and durable clay plasters to all kinds of wall systems in an effort to create healthier indoor climates as well as for aesthetic reasons.

The facing short dividing wall is a rendered plasterboard wall. This was done to achieve a uniform look with the rendered strawbale walls - see curve over door.

Cracks and failing plasters

When following the above outlined techniques and working with suitable materials, cracks should not be a problem, however, it is not always possible to avoid them altogether. Weather conditions may not be right or change dramatically during the rendering process. If the plaster dries out too quickly cracks may result. Heavy rain can wash render of the wall and frost is one of the biggest problems during the rendering period. Rushing to put on subsequent coats may also cause failure of the render; underlying coats should be thoroughly dry before applying new coats. Other building materials within the wall system may pose a problem too. Timber swells and shrinks with water moving in or out and this can cause cracking of the plaster. Steel expands when warm; this can cause similar problems as experienced with timber components. The absence or unsuitability of additives and reinforcements can all be the cause for faulty plasters. Unsuitable or unprepared backgrounds can be a problem and plasters often show cracks or gaps at joints with window & door frames, ceilings and floors. Of course another important reason for render to become faulty is when the plaster mix is not right. In this case most cracks are probably due to the mix being too fat therefore, displaying severe shrinkage cracks. Plasters can also be overworked, i.e. if the surface of a render is rubbed for too long or too hard, water is drawn out of the underlying coat, resulting in significant reduction of the binding power. Movement of the building can also cause substantial damage to render, however, this is a structural problem rather than a rendering problem. Cracks occurring through shifting of the building cannot be repaired or avoided until the building has completely settled. Last, poorly designed details such as missing drip lines underneath window sills have frequently been a cause for damage to render and should be avoided from the beginning.

There are probably many more things that can go wrong with the render and professional skill in proper application plays a vital part for successfully rendered walls, however, there is some good news too. Fine cracks of up to 0.4mm will usually cause no problem with the vapour permeable clay and lime renders. Any moisture entering the wall through such cracks will be able to move out instead of being trapped as would be the case with cement renders. It is also fairly easy to repair such cracks using a runny top coat mix painted on with a wide paintbrush however, this should only be done once the render has completely dried out and settled in order to avoid more than one repair session. More severe and deep cracks need proper identification and repair work has to be adjusted accordingly; professional advise may need to be sought at this stage. Painting the walls with natural and vapour permeable products such as casein paint, pit lime paint, silicate paint or a lime wash will often fill in small cracks. It is recommended to paint the rendered surfaces anyway, externally for extra protection from rain and internally to reduce dusting and allow the surfaces to be wiped with a wet cloth. Aesthetic reasons may be your main incentive for painting internal rendered walls. It is recommended to paint the walls only after they have thoroughly dried, 4-6 weeks are probably the average time before this is advisable. Often people decide not to paint the rendered walls because at this stage the budget is commonly running low hence, painting does not seem a priority. However, painting at least the outside walls will protect the walls and may reduce future repair and maintenance efforts.

We have attempted to cover the issues of why clay, what is clay and how to use it to our best knowledge and with the aid of expert literature on the topic. However, it is impossible to cover every issue in a short paper like this and every single topic covered here probably deserves a paper for itself; so please accept our apologies for any shortcomings or overlooked subject. Concluding we like to remind you that good rendering results require good materials, good techniques and professional skill for the application. The latter is probably the hardest to come by and we highly recommend that some basic practical skills are learned before embarking on a rendering project. We offer regular workshops at current building sites, or you may wish to lend a helping hand at a friends or neighbours place, any time you spend getting your hands muddy will help to gain valuable experience for future use.

Clay, a material from Yesterday, of great value Today, to help our planet for Tomorrow