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An Introduction to Traditional and Modern German Clay Building

"Don't be afraid of being called unmodern. Changes in the old methods of construction are only allowed if they can claim to bring improvement, otherwise stick with the old ways. Because the truth, even if hundreds of years old, has more inner connection than falsehood which walks beside us." Adolf Loos, 1913

The aim of this short introduction is to give an overall view of the qualities and varieties of the use of clay as a building material in combination with straw and other elements.

Clay, pure or mixed with sand, is a universally existing material. Because of different geographic, climatic and cultural conditions, regional building techniques were developed throughout the world, which can be traced back for thousands of years. Now, after a period of decline, building with clay is making a comeback. However, the so-called "Third-World" nations are not the only ones constructing buildings with earthen materials; ecological and economical problems of the industrial nations are leading to a return to clay there as well. Clay as a local, non-toxic, affordable and recyclable material, is much more than just a substitute. Home-owners seeking money- and energy-saving building materials are also seeing the advantages of the durable, healthy and agreeable room climate of structures built with natural materials like wood, straw and clay.

Building with clay has a long tradition in Germany and other European countries. Framed structures (half-timbered houses) from the 12th century, filled with a mixture of clay and straw fibers, still exist. This traditional technique called "wattle and daub," is still in use in the preservation of historic buildings. Oak stakes are installed vertically into the frame, woven with willow and covered with a heavy mixture of clay and straw. When the infills are dry, they are plastered with a mix of lime, sand and animal hair. Finally the surface is painted with lime. Some other historical techniques still practiced are "Lehmwickelstaken" which consists of oak stakes wrapped with a mix of straw fibers and clay paste, and mainly used as a ceiling infill; and "Lehmwellerbau," a technique practiced since the middle ages which involves straw-clay loosely stacked with a pitch-fork and then compressed. After a couple of days drying time, the layers are shaved with a triangular spade and the next layer is stacked.

Due to a lack of building materials after World War II, an increased amount of interest was shown in clay building and a standard clay building code was introduced. The advent of the "Wirtchaftswunder" with its modern building materials and "modern thinking," ousted building with clay from everyday life. Not until the end of the '70s were clay products again tested, improved and used with renewed vigor. As a further refinement, "straw/clay" was developed. The term light-clay means the mixing of liquid clay with large quantities of light materials such as straw, wood chips, cork or minerals.

For the production of straw/clay, clay "slip" is mixed with straw. More straw added to the clay will result in a lighter mixture, while less straw creates one which is heavier. To get a very light mixture, the use of a slip with a high-percentage of clay is necessary. Medium-weight mixtures are most realistic for practical work on the building site.

The straw/clay mix is tossed together, then is covered with a tarp for a day or two before the mixture is placed into a "slip form" and tamped. To preserve the mixture's insulation qualities, it is necessary that the tamping not be too hard. After filling and tamping a section, the form boards can be removed immediately, and moved up for the next section. Although dependent on water content, outside temperature and wind conditions, final drying time for a 12 inch thick wall is approximately 12 weeks (about l" per week) during the warm season. Walls thicker than 12" are not recommended as the straw may begin to rot before it can dry.

The use of wood chips instead of straw as an aggregate in light-clay mixtures has increased in popularity in the 90s. The drying time, shrinkage behavior and most importantly, labor intensity are reduced when using wood chips. While temporary slip forms or permanent form work can be used, the use of reed mats stapled to a light lath framework is common. The ingredients wood chips, fibers, and shavings can be mixed easily with clay using a mortar mixer or even a cement mixer, then poured or shoveled into the formwork with no tamping required. For mineral-light-clay, the plants and straw in the mixture are replaced by an artificially-foamed clay product to reduce both the drying time and shrinking problem. This is more energy intensive and not commonly used.

An industrial prefabricated dry board has been developed in the last few years called "Lehmbauplatte," a fabric-coated, plant-fibre reinforced clay board in which a clay paste is applied to an unrolled burlap fabric jute net. Two to five or more layers of reed mats are laid crosswise with alternate layers of clay paste, over the burlap. Finally, it is covered with another layer of burlap and transported to a drying station. Material tests with this board have shown excellent fireproofing, sound proofing, deformation and diffusion values. The Lehmbauplatte can be used as a permanent form, combined with blown in cellulose, or as a ceiling and insulation board. It can be screwed, nailed and sawed.

Another possibility is the production of straw/clay blocks as a ceiling infill between the beams or as a light dividing wall. By installing two laths (bamboo or branches) as reinforcement in the center of the ceiling blocks, small amounts of weight can be supported temporarily during the time of construction. As required, these blocks can be made either "light" or "heavy." In addition, these light-clay products are easily sawn.

Frank Andresen is available for workshops and consulting on light-clay materials, plasters and dry clay products.

(winter) Kiefernstrasse 25, 4000 Dusseldorf Germany; 02 11/7333216

(summer) PO Box 247, Brownfield ME 04010


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