Compo Deterioration and Damage

To some degree, the longevity of historic composition ornament is related to the ratio of ingredients in the original mix and to the skill of the craftsman in applying it. But it is far more dependent upon interior climatic conditions and the long-term effects of heat and dampness on both the compo and the wood substrate.

A conservator replaces broken pieces of compo ornamentation on a mantel during a restoration project. Photo: Courtesy, Irving Haynes and Associates.

Variables in mixing and application. Dried compo is inherently hard and somewhat brittle; its increasing brittleness over time is primarily due to the oxidation and hardening of the linseed oil component. The drying oil, in turn, contributes to age cracking. Thus, during initial manufacture, if the oil content was low and the dry filler content (chalk) high, shrinking and cracking over time is less likely to occur. Originally, the compo was probably attached using small, headless brads (1/4") that penetrated the hardening compo as well as the wood substrate. They were used to keep the compo from shifting or warping after it was set in place. If an insufficient number of brads was used by the craftsman during the application process, the compo simply falls off as cracks develop.

Interior environmental conditions. Compo was conceived as a durable substitute for hand-carved wood or marble and decorative plaster; its potential for structural failure is generally due to substrate failure rather than to the compo mix itself. Theoretically, composition will move with atmospheric changes due to the moisture-sensitive glue component. Its breakdown typically occurs when the wood base expands and contracts at different rates than the compo during extreme temperature and humidity fluctuations. Especially when it is close to a source of heat, such as directly over a fireplace, compo develops fissures or shrinkage cracks. Contemporary heating systems in old buildings also contribute to the drying and cracking syndrome.