By OHJ staff

	Because adobe bricks are constantly swelling and shrinking, modern materials can cause problems. Traditional mud plasters remain the safest coatings.

Our adobe was built in 1929 and is not on the National Register. Nonetheless, we want to leave it in as good a shape as possible, but we’re getting older and find that a 50 lb. tub of adobe plaster mix weighs more every year. Any advice?
Sid Rimmington
Taos, New Mexico

Adobe, or sun-dried brick, is an ancient building material, and though some adobe structures are quite old, maintaining adobe in the modern world has become a tricky issue. Traditional adobe bricks are molded from sand and clay mixed with water; straw or grass is often added as a binder. Since the bricks are not fired in a kiln, but instead air-dried for weeks before use, they are particularly hygroscopic—that is, they absorb moisture and will swell and shrink constantly, depending upon their water content. This makes traditional adobe unstable by today’s construction standards, and a problematic match with modern materials, such as Portland cement.

Because adobe buildings are always swelling and shrinking, historically they have required constant maintenance, particularly patching and replastering the surfaces with mud mortar. Though lime or Portland cement were sometimes added to plaster mortars in the 20th century to increase their durability, these mixes do not have the same expansion rate as the brick and can cause them to crack and crumble. In fact, the National Park Service Preservation Brief on the subject notes that “Techniques acceptable only 10 years ago are no longer considered appropriate.” For traditional adobe buildings then, a traditional mud plaster made from the same ingredients used on the building historically remains the best approach. For more background, consult Preservation of Historic Adobe Structures, now available online at http://www.nps.gov/ history/hps/tps/briefs/brief05.htm.