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LIME PLASTER

Lime plaster is a mixture of lime and sand with fiber occasionally added. The primary component of lime plasters is lime putty, a hydrated-lime product (calcium hydroxide) with the consistency of sour cream. It can only be purchased from a few sources like the Building for Health Materials Center in Carbondale, Colorado [(800) 2924838; buildingforhealth.com ]. Lime putty also can be made from Type N or Type S hydrated lime (not agricultural lime). Available in many building-supply outlets, these dry powders are fairly inexpensive.

To make lime putty from hydrated lime, the powder is first mixed with water. Protective clothing and eyewear are essential, as the reaction between lime and water produces an intense heat that can boil and splatter this caustic material. Before you attempt mixing lime putty on your own, read more about it. (See " Resources ," Page 69.) Consider working with a skilled lime plasterer to gain hands-on experience.

After the water and hydrated lime are mixed, the material is left to sit, usually a day or two, although veteran lime plasterers like their lime to hydrate, or slake, for months or years to achieve full hydration. When fully slaked, the lime putty is separated from the water layer on top, mixed with sand and applied to the wall, either by gloved hand or trowel. Some lime plasterers like to harle — throw — lime plaster onto walls from a distance of about 3 feet. Generally, two or more harled-on coats provide a durable exterior finish, although harling is an art most often reserved for experts.

As lime plaster cures, the calcium hydroxide in the mix slowly reacts with carbon dioxide in the air. In the process, calcium hydroxide is slowly converted back to the parent material — calcium carbonate or limestone — producing an extremely durable and highly protective wall surface. After application in drier climates, walls need to be misted one to three times a day for one to two weeks to prevent the lime plaster from drying out too quickly, which weakens the material considerably and can cause it to crumble off the wall.

Lime plaster is the strongest and most durable of all plasters and stuccos on the market today. It provides superior protection over stone, cement, cement-block and brick walls, and is ideal for exterior applications, especially in rainy climates where driving rains are common. Lime plaster is quite suitable for interior walls and for application in straw bale, straw-clay, cob and adobe homes. Like earthen plasters, lime plasters permit the escape of water vapor within walls.

Because of lime plaster's long set time, working with this plaster requires more experience than earthen plaster. Its caustic nature makes it potentially dangerous to all who work with it. Intense energy is required to manufacture lime from crushed limestone, so lime plaster also has a high embodied energy. For these reasons, lime plaster may best be used as a thin top coat over an earthen plaster base coat. Approval from building-code officials should pose few, if any, problems.

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