Another natural building material—clay plaster

Anna Wolfson demonstrates the use of a Japanese trowel on plaster. Photo by Dr. Sonia Vogl

By Drs. Robert & Sonia Vogl
President and Vice President
Illinois Renewable Energy Association

Another natural material has been used in Jim Hutchison’s straw-clay building: clay plaster. Anna Wolfson of Wolfson Earth Finishes, professional plasterer and art major from Oberlin College, introduced us to the material. Best of all, “it’s all natural!” she exclaims.

The first ingredient is clay—about 1-1/2 yards. Wolfson thought a local clay would be the best, but the colors weren’t what she or Hutchison wanted. They finally found the right color, which had been removed from a construction project in Fond du Lac, Wis. The only cost was hauling.

Fine, angular sand is mixed with the clay. Round particles will not cling. Straw, cut up in a leaf chopper, is used in the mix. Differing lengths assure a mixture that holds together by “weaving.” Silt is avoided since it can hinder cohesion. The plaster is processed in a cement mixer.

Clay, sand and straw are used in equal proportions. Either horse or cow manure, or both, can be added for helpful enzymes and the desired final color. In this case, horse manure was used. About two wheelbarrows full of the mixture can cover the inside of the building.

Jim Hutchison sprays slip on walls (left) before plastering and applies plaster with a friend (right). Photo by Dr. Sonia Vogl

Other colors can be achieved by using ochre, which produces a yellow; iron oxide for antique rose; or kaolin for white. Wolfson’s eyes sparkled as she discussed the qualities, colors and other features of her product. Linseed oil wiped on the exterior plaster will create a glistening effect. Several pleasing natural colors were featured on a permanent display wall.

Before actual plastering begins, a thin coat of slip (clay and water) is sprayed on the walls to assure the plaster will stick. It can be prepared in advance. What is not used will not spoil. One coat of plaster is about 3/8-inch thick. A second can be added if desired.

The actual plastering was done by Wolfson, and several of Hutchison’s friends, new to the process, who were doing a fine job.

The plaster may be smoothed when it is wet, may be given a texture or made to look like concrete.

Wheat paste is used to fill in irregularities in the wall before plastering. Burlap is used to protect edges such as around windows.

Framing for benches and hanging tools are built in, not added later.

Wolfson also does fine plastering in Chicago over either drywall or lath. She feels the advantages of clay plaster are that it absorbs humidity, breathes, is negatively charged and does not collect dust. Straw clay allows water to pass through.

Photo by Dr. Sonia Vogl

How did an art major become a plasterer? She took an internship where she learned the skill, and “never looked back.” Both Wolfson and Hutchison work with architects Lou Host-Jablonski and Sue Thering of Design Coalition, who work with natural materials.

Natural buildings are labor intensive, but she believes they are well worth the work. The finished product is healthy; there are no dangerous fumes, and the building can stand many years. Additionally, building materials are often locally available and inexpensive.

Previous columns about this building appeared in The Rock River Times in 2009 and 2010.

Drs. Robert and Sonia Vogl are founders and officers of the Illinois Renewable Energy Association (IREA) and coordinate the annual Renewable Energy and Sustainable Lifestyle Fair. The Vogls and the IREA are members of the Environmental Hall of Fame. Dr. Robert Vogl is vice president of Freedom Field, and Dr. Sonia Vogl is a member of Freedom Field’s Executive Committee. The Vogls consult on energy efficiency, renewable energy and green building. They have 3.2 kW of PV and a 1 kW wind generator at their home. Forty acres of their 180-acre home farm are in ecological restorations. They are active in preserving natural areas and are retired professors from Northern Illinois University. E-mail

From the Oct. 27-Nov. 2, 2010 issue

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