The Challenges of Treating Delaminating Slate Headstones: New Treatments with New Materials

The Challenges of Treating Delaminating Slate Headstones: New Treatments with New Materials by Irving Slavid

Colonial settlers found slate to be abundant, and the quarries in the “slate valley” from Vermont to New York (and as far south as Virginia) supplied gravestone carvers with this stone. Slate was the predominant material used by the early New England carvers for markers. Many of those created by Boston-area craftsmen are found along the Eastern US coast as far as the Carolinas.

Some of the earliest markers survive in intact and pristine condition. The “crispness” of fine detail was not affected by the increasingly acidic environment associated with 19th century industrialization. Depending on the quality of the slate, many look new, despite more than 200 years of exposure.

However, the geological makeup and orientation of the slate can, over time, be responsible for delamination. Slate is a compact metamorphic rock, geologically formed from fine-grained sedimentary rocks of poor quality. When it is used as a headstone, zones of residual internal stress can result in the partial detachment of vertical planes. These are seen as gaps at the top of the marker, which are directly exposed to the weather. Over time, moisture can enter these spaces, and repeated freeze-thaw cycles can make the delamination more extreme, ending in total loss.

Slates are dark in color. In the sun, surface temperatures can be remarkably high, and will vary widely from one face to another during the course of a day. The result of this changing temperature is cyclical movement of the separated layers of the stone, further driving the delamination. (A similar condition can occur in sandstones, but as they are more porous and lighter in color, they are not as affected by temperature change.)

Successful conservation treatments to these markers are extremely challenging. Repeated movement at the top of the marker causes rigid cementitious fills or epoxies to detach and eventually fail. Historically, lead or copper “capping” was done to cover the exposed upper edge. While these applied caps were physically successful in preventing water intrusion and therefore limiting the effects of weathering, many were visually distracting. They were also easily removed, in part due to vandalism or to their salvage value.

However the markers were capped, filling the associated voids is paramount to limit water pooling and ice formation. Cementitious grouts, related to such fills along the top, were often used to fill these cavities. These would eventually become detached from one or both inner surfaces, sometimes only after a single season, allowing water to enter.

This presentation outlines our laboratory evaluation of flexible fills, begun 12 years ago, and a program of field tests which commenced in 2008. Since that initial fieldwork, our procedures have continued to be refined each year, with some additional materials. Our most recent field applications, done in 2012, were carried out on over 50 slate markers.

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