Figure 1. Close-up view of the deteriorated serpentine stone façade showing substantial surface delamination. Photo: Anne Grimmer

Serpentine stone was frequently used as a building material in the late 19th century because of its distinctive greenish color. Numerous residential and moderate-scale institutional buildings were constructed using serpentine for entire facades or simply as a trim material in urban areas such as Philadelphia, New York, Washington, Baltimore, and Chicago, as well as in rural areas close to the quarries such as Chester County, Pennsylvania, and northern Maryland. However, serpentine’s popularity was short-lived because of its fragility.

Serpentine is a soft stone composed primarily of hydrous magnesium silicate which is basically the same mineral found in talc. It is not tightly consolidated, but a fibrous material that tends to absorb water. The presence of water accelerates the deterioration of the stone due to the effects of the expansion and contraction of the stone during freeze/thaw cycles. Since serpentine is an alkaline stone, it is also sensitive to attacks from sulphuric and sulphurous acids, and carbon dioxide dissolved in rainwater.

A stone consultant was retained to assess the condition of the serpentine stone and to recommend remedial work. The consultant conducted an in-depth visual inspection and evaluation and concluded that the serpentine stone was spalling extensively, due to the inherent softness and poor quality of the stone and the chemical forces interacting with the stone. The deterioration had been further exacerbated by the general lack of maintenance the building had received for the last forty years which had allowed large amounts of moisture to penetrate the stone. Moisture, laden with acids, had entered the stone through open mortar joints and the delaminated surface, accelerating the stone’s deterioration. In this case, the problem was compounded by additional moisture spilling onto the surface of the serpentine from clogged, leaky, and inoperable gutters, downspouts and drains. Given the inherent problems with serpentine stone as an exterior building material and its overall advanced deterioration on Six Logan Circle, the consultant recommended that the serpentine stone, which was used as a veneer covering structure brickwork, be replaced rather than attempting to repair using traditional patching techniques. This conclusion was supported by a survey undertaken to determine how many of the stones retained their integrity (i.e. how many remained substantially free of fractures and crumbling and retained their surface tooling, texture, and shape). The survey showed that 868 of the original 946 face stones which made up the façade no longer retained their integrity. In fact many were so deteriorated that they could literally be pulled apart by hand (see figure 1).

Solution

Because only 78, or less than nine percent, of the stones remained intact, total replacement of the historic stone was justified in this particular case. However, the replacement material needed to closely match the historic stone in size, shape, dimension, texture and color in order to preserve the historic character of the façade. To meet these requirements, three replacement alternatives were considered:

1. Replacement in kind with new serpentine stone
2. Replacement with another type of natural stone
3. Replacement with a substitute material such as pre-cast concrete

Although replacement in kind is generally the preferred preservation approach when dealing with a non-repairable material, serpentine stone is no longer quarried for use as an exterior building material due to its poor performance historically. It is thus not readily available for large-scale applications such as that proposed for Six Logan Circle. An alternative stone (such as limestone) could be cut to match some of the visual characteristics of serpentine, but it could not duplicate its distinctive green coloring. Therefore, the material that offered the most potential for use as a replacement material was pre-case concrete. This material had recently been used to replace extensively deteriorated brownstone on the Renwick Gallery of the Smithsonian Institution in Washington, D.C., and it was determined by the consultant (who had worked on the Renwick Gallery restoration), that the same technology could be successfully applied to replace the serpentine stone at Six Logan Circle.