Newbold-White House, Hertford, North Carolina, ca. 1730
Significant historic buildings that house museum collections present an environmental control challenge. The optimum environment for the protection of a collection may create stresses on the historic building, or may require alterations to the building that compromise significant architectural elements. Environmental systems are a balance between visitor and staff comfort, the protection of the collection, and the preservation of the historic building. Often, each has different ideal ranges that are sometimes incompatible. But, instead of choosing between comfort or preservation, it is possible to balance the requirements of building use and
collection protection by determining the environmental control range for all functions and the effects various designs will have on the historic structure and collection. Design considerations may include separate control zones within the building, providing micro-environments for sensitive collection items, reducing environmental cycles, and incorporating active monitoring of architectural elements.
Below is an example of environmental monitoring and material testing conducted at the ca. 1730 Newbold-White House in Hertford, North Carolina. Deterioration of masonry and wood frame elements of the historic house museum was determined to be related to below-surface condensation produced by the environmental control system. Tests included monitoring the relationship between the environmental control system, exterior environmental cycles, air infiltration, rates of absorption on architectural surfaces, and the effects of visitor use. A key component of the study was the evaluation of proposed alterations to the museum’s historic masonry walls and the installation of additional insulation elements.
The conservation and environmental studies resulted in alterations to the existing environmental control systems, isolation of unfinished crawl space areas from the interior museum spaces, changes in the operation of the museum, and a monitoring program for sensitive collection and architectural elements. The proposed alterations to the museum’s masonry and construction were shown to not address the causes of deterioration and were not implemented.
ca. 1730 Lime-Shell-Sand Mortar - 10X
1978 Lime-Cement-Shell Mortar - 5X
Surface Eruption - 1981 Gypsum Plaster
RILEM (Reunion Internationale des Laboratoires D'Essais et de Recherches sur les Materiaux et les Constructions), with headquarters in Paris, is the International Union of Testing and Research Laboratories for Materials and Structures. As with the American Society for Testing and Materials (ASTM), the purpose of the organization is to develop standard methods for measuring properties and evaluating the performance and durability of many different building materials. RILEM Test Method 11.4 provides a simple means for measuring the rate at which water moves through porous materials. (Torraca, Giorgio. Porous Building Materials, Rome: ICCROM, 1981).
The use of RILEM tubes - above, right - to determine the rate of moisture absorption through masonry has become a common practice in material conservation. The test is useful in comparing the response of different masonry materials to common environmental factors. The graph on the right shows the moisture response of the original ca. 1730 mortar, the 1978 restoration mortar and the 1981 restoration plaster at the Newbold-White House. Each sample was tested in situ by attaching a RILEM tube to the surface with clay. The response of the original lime mortar varied widely, due largely to variations in the initial compaction of the joint during construction. The smaller aggregate of the 1978 mortar produced a slightly denser mortar, but the absorption rates are characteristic of a very porous material. The initial absorption rate in the 1981 plaster varied according to its thickness and the condition of the bonding agent applied to the brick substrate. The absorption rates are an accurate predictor of the mortar’s ability to protect the adjacent brickwork from moisture retention and rising damp.
Condensation on Interior Plaster Surface (5X)
RILEM Tube Mounted on Exterior Restoration Mortar
Graph of the ca. 1730 and 1978/1981 Mortar/Plaster Responses to Moisture Using the RELIM Method of Testing