Overview: Build-up of Earth-Based Plasters
To look into earth-based plasters across cultures, countries, and climates, they can be seen (and overlooked!) in different disguise. For instance, plastering in India is usually found with additive of cow dung, German is known to enhance its plaster strength mixed with oxblood, China has its plaster mixed with sticky rice solution, US with potato solution, and UK with wheat solution. Ancient earth-based construction can be found in all continents except the Antartica. It is due to the nature of earth paster being such a local crafts, that they develop their own recipe adapting to their local abundance.
It would be hard to generalize the build-up of earth-based plasters into categories. To study earth-based plaster that maybe applicable to the city context though, 3 variants are selected for their distinct appearance and applicability for closer inspection. They are:
Plain Earth Plaster
Earth plaster with oil
Earth plaster with lime
Plain Earth Plaster
Plain earth plaster, an earth-based plaster in its most primitive form, of course. But it is also these rough, crude and highly textured qualities that is being revitalized and celebrated in modern design.
- coarse texture
- porous/ permeable, able to regulate humidity by absorbing and releasing moisture
- easily susceptible to abrasion, unless with additive
Places of Application:
Unless positioned in well sheltered area or in arid regions, like Africa, Middle East, and parts of India where rain is minimal, plain earth plaster is usually applied on the interior. Otherwise maintenance is expected to be executed frequently. As mud structure was observed by anthropologist, Edward L. Ochsenschlager in his book, Iraq’s marsh arabs in the Garden of Eden,
‘He chooses a fairly thick reed stick, dips it in water, and rubs it back and forth across the surface to create a smooth finish with the plaster. This plastering is crucial for the preservation of the structure and the plaster must be renewed every year. Usually this is done in the villages immediately after the wet season.’
And earth plaster is applied in building of all types as said in document of Earth Structures and Construction in Scotland,
‘Clay Plaster is common in pre-nineteenth century houses of all categories (in Scotland) and its use continues into the twentieth century in small farm, croft and cottar houses.’
Build-up and Mechanism:
As a rule of thumb for good practice, plastering would be applied at least three times. The coating would be composed of clay, aggregate and fibre. The three coating will each have decreasing thickness, and increasing fineness.
Japanese Architecture and Art Net Users System explained their practice of earth plaster, tsuchikabe,
‘The base coat, namely ara-nuri, 荒塗, is a course mud plaster usually mixed with straw. A middle coat, nakanuri, 中塗, is a finer mixture of daub. The top coat, uwanuri, 上塗 is either smooth whiter plaster or one that has a sandy finish.’
Like plastering of all kind, clay plaster is applied with tools like trowel. Traditionally though, It is applied with hand. In some indigenous societies, this work is practiced exclusively by women, which earns them the name Enjarradora, in New Mexico.
Plaster with Oil
Not being exclusive to plastering, oil used in oil painting gives us an initial idea of its ability to set, to protect and to withstand the test of time. Earth plaster, when applied with a coat of oil, can have its weathering resistance effectively enhanced. This oil coating not only provides a protective skin to the plaster, but also a distinctive appearance. Yet applying this oil coating too thick will lose the permeability of earth plaster.
- darken the coloured substance in the mix, and increase colour contrast
- relatively water resistant
- resistant against abrasion, due to the collective power of compressive strength and binding force of oil
Places of Application:
Journal of the Architecture of North Mexico (1966) writes that
“when the (mud) plastered area has dried, it is smoothed over once more with a piece of dampened sheepskin.”
In such instance it is the tallow from the sheepskin that brings effect to such practice. Apart from the traditional use of animal oil, refined plant-based oil can be seen being practiced in the US and parts of Europe. Loudon J. C. (1846)
Since the weakening in vapour diffusion of oil coat, it is less suitable in area that is too cold where frost and condensation easily happen between inside and outside.
Though the common use and consumption of oil in everyday life, plastered building being coated with oil is not as widely observed.
This may due to the fact that,
- oil, to be used as construction material, is expensive
- oil when exposed to prolonged light and UV will become brittle
Build up and Mechanism:
As a principle, all oils become thickened by age. And some oils becomes harden as they undergo oxidation, Field G. (1896). This property of oil contributes to the increase in compressive strength of earth plaster when oil is applied.
Tallow oil, and animal oil in general, has been phased out because of its lingering smell and low workability and instead plant-based oils enjoy a higher popularity. Of all plant-based oil, linseed oil is deemed to be the best. Field G. (1896) describes it to be ‘by far the strongest, and that which dried best, most tenaciously, and firmest under proper management.’ It has to be boiled or even twice-boiled prior to use. Earth structures and construction in Scotland (1996).
i) Brush of oil on earth plaster
Too much of oil will lose the breathability of plaster, hence oil should be applied as the last finishing coat.
Clay & Lime Renders Plasters & Paints (2008) recommends to use oil together with a thinner, such as citrus thinner(made of citrus peel oil), spirit of turpentine to assist the penetration of oil to the pores of earth plaster.
ii) Oil with limewash
Oil will react with lime to form a fatty acid synthesis to make lime soap. This emulsion oil within limewash makes it water shedding.
Holmes S.(1997) makes a remark on oil coat that
‘When tallowed limewash dries it can be impossible to apply further coats as they run off the wall just as water run off a duck’s back.’
It suggests that oil coat is applied as a finishing touch. And if several washes have to be applied, they should be applied in short interval before the previous one dries off.
Earth Plaster with Lime
Lime is a versatile building material. Due to its unique setting properties and exceptional smoothness, lime can be used in conjunction with others to become structural elements, like foundations, walls, floors as well as decorative finishes like plastering, moulding, fresco painting.
- Breathability (high porosity, high water vapour permeability)
- – breathable walls control internal humidity, reducing condensation and preventing mould growth. This improves the indoor air quality and the health of a building’s occupants.
- Low thermal conductivity
- ~1 W/(m K) – and a moderately high specific heat allow the plaster to act effectively as a thermally massive heat store, contributing to the thermal comfort of the internal environment.
- – With proper usage, lime can last for a long time. Pantheon in Rome has been standing for more than 900 years now.
- – lime acts as a very good ‘sacrificial’ external surface, protecting the essential structural elements of building from the weather, fire and vermin, although it must be maintained.
- Auto healing
- – as lime buildings move small cracks can appear. Water penetrates into the cracks and dissolves any free lime. This brings it to the surface where the lime carbonates and heals the crack automatically.
Sources: May (1998), Holmes and Wingate (2002), Building with lime forums (2005), BRE (2001).
Places of Application:
Building lime has been used as a binder for building work for thousands of years all over the world. Pantheon in Rome is built out of hydraulic lime and volcanic ashes(called Roman concrete) in A.D. 113 that still remains standing.
Before the popular use of cement, earth plasters in the UK were routinely used on internal wall surfaces alongside lime, up until the end of the nineteenth century. Often they were used as an undercoat, to even out the wall surface before receiving a top-coat of lime plater/render or limewash.
Lime plaster is preferred over cement-based plaster especially on restoration work of historical buildings. Holmes and Wingate (2002) explain that
‘cement mortars are typically stiffer and less permeable. When used in conjunctions with softer materials or on weaker backgrounds this can sometimes cause serious problems by trapping moisture and creating high local stress.’
Depending on the location of application and personal preference, lime can be applied as a thin coat, a limewash or a thicker coat, a lime plaster.
Limewashes are a family of paints which are particularly suitable for use on porous surfaces such as lime plaster, earth and stucco. Advantages of limewash over synthetic paint include:
- it allows surfaces to breathe, so that water will no be trapped behind impervious surface finishes.
- it is unaffected by UV rays in sunlight, which will destroy synthetic paint over time.
General properties include:
- would not facilitate the spread of fire
- dries without giving off any fumes
- helps to make rooms bright and hygienic
- can be mixed with pigments to give off soft colour and patterns
But good workmanship has to be exercuted, otherwise poor limewash wall will easily rub off on clothing due to the in incomplete conversion of lime to a film of calcium carbonate.
as lime plaster
Lime plaster is often chosen
- when a building is intended to have a pleasing and smooth appearance
- as a base to receive decorative finishes on top
- to cover poor backgrounds, such as differences in level due to the cost of raw materials of lime plaster is relatively low compared with other plastering media.
Similar to lime paint, lime plaster is porous. Synthetic or any non breathable paint should not be applied on top of lime plaster. Moisture can get trapped between the paint and the wall surface, creating flaking and blistering, and ultimately complete delamination.