CLAY: Earth PLASTERS IN PRACTICE
By Jessica Barber Goldblatt
Earth or clay plaster is a blend of clay, fine aggregate, with an addition of fiber and pigment, depending on the look one wants to achieve.
Earth materials were used extensively in buildings of all sorts prior to the 19th century. The disappearance was not because the material was considered primitive or inadequate in any way, but because of the changing economic conditions of the time. This is an important fact to remember when looking at clay plasters in practice. They are not only primitive materials, even though they are not synthetic or a result of high-energy processes, but clay plasters still perform well after many centuries.
Earthen plasters are less toxic and energy-intensive than many other wall coverings, which makes them appealing to the environmentally-conscious. They provide protection, structural and insulating abilities as well as texture and color. Clay plasters have many complex qualities and require a proper understanding. Given that understanding, they can perform very effectively, and are durable as well as amazingly attractive.
Because the rate at which clay plasters absorb moisture is much higher than that of other materials (timber, for example, takes in and releases large quantities of moisture but over a much longer period), clay plasters can act to protect vulnerable organic materials (and in particular timber) from high levels of relative humidity, when microbial and insect attacks can be triggered. In addition to the building benefits of clay, the hygroscopic qualities mean that molds caused by condensation are minimized, and that a relative humidity of 50 to 60% is maintained. This is the ideal level for healthy mucous membranes of the human body, and also for the control of dust mites and other organisms which affect human health.
Preparation: Clay plasters can either be made from local soil with a careful process, or can be bought in a proprietary form, from companies such as American Clay Plasters.
Application: Clay plasters may be applied like any other plaster, troweled by hand or by spray application onto standard drywall, concrete block, brick and many other substrates. Where exposed to direct spray of water, for example in showers, just above sinks and in other vulnerable areas, it is necessary to use tiling or some other impermeable material. It is also suggested that timber is preferable to clay plaster for lining the reveals (perimeters at the wall-window intersections) around cold windows where condensation occurs frequently on the frames. Where clay plasters get wet only occasionally, then a coat of casein, silicate paint, or limewash is usually sufficient protection. Clay plasters are very forgiving, and easy to use even to a novice plasterer. However, very fine smooth finishes require practice or professional experienced help to perfect.
Warm to the touch in winter and cool to the touch in summer, clay plasters bring health, comfort and what can be described as organic, biological and living beauty to interior spaces.
Jessica Goldblatt Barber is the owner of Interiors Green, a home and living store, in Bethlehem, NH. Jessica supports sustainable principles in her everyday life and when creating designs for clients. www.interiorsgreen.com