Recently, we received a few questions on the blog regarding about plaster. Fortunately, Matthew Webster was again nice enough to share his expertise on the topic. Matt is Conservator of Architecture here at Colonial Williamsburg, and is deeply involved in the Coffeehouse project.
The plastering project here at the Coffeehouse is largely accurate to the 18th century. Many people expect to see rough surfaces, poor materials, and distressed finishes; however, eighteenth century craftsmen were not only masters of their trade, but also of their materials. Plastering in the following manner has a number of advantages over modern techniques, especially when dealing with period framing.
All of our framing was cut by hand and arrived green, which means there can be variations in the surfaces, as well as movement long after the frame has been erected. Wallboard and gypsum plaster are very rigid, and these variations and movement would show as buckles in the wall or cracks. Lime plaster is much more flexible and can take minor shifts. It is also self healing, as the moisture will allow available lime to migrate to cracks.
The plastering process begins at the framing where lath is affixed as a skeleton, which the plaster is attached to. Instead of wooden lath, we have used metal lath, which is really the only aspect of plastering where we have used non-18th century material. While both types are functionally equivalent, and do not affect the finished look of the wall, the metal lath saves us an enormous amount of time and materials.
On the other hand, the Coffeehouses porch ceiling is being built using wooden lath. Its proximity to the street means that guests can view the whole process, done in a historically accurate style by a costumed member of the Historic Trades department. The wooden lath is split by hand, and is then attached with nails. Nearly 1,600 of these lath nails were produced at the Colonial Williamsburg blacksmith shop for this ceiling alone. It will take several days to split the lath and nail it to the ceiling.
We do have pieces of original plaster from the Coffeehouse, and these samples tell us the depth, number of layers, and makeup of the plaster. Analysis tells us the plaster is made up of four primary components. Hydrated lime is the first major component, and is made from burnt oyster shell that has been slaked and placed into a pit for at least six months. The oyster shells for the Coffeehouse plaster was burnt and slaked in the Colonial Williamsburg brickyard. For more information on slaking lime, see this blog entry from March.
The second component is sand: Sand is a major component in the coloring and texture of plaster, and is also a bulking agent which doubles the overall volume of the plaster. The sand used for the original Coffeehouse plaster had a very fine and worn grain, indicating that it most likely came from a creek or river. Sand with similar color and grain was obtained for the new plaster, giving it a similar appearance.
Our final two components are brick dust and clay; though less well-known than lime and sand, they were almost always present in plaster and mortar. Clay acted like a plasticizer, which allowed the plaster to stick as it was applied, as well as spread easily on the surface. On the other hand, brick dust was added as a strengthening agent, as was a small amount of horse hair. The mixture ratios for the plaster are 45% lime putty, 45% sand, 7% clay, and 3% brick dust.
Once the lath is installed, a scratch coat is placed. This is a thick layer of plaster, heavy with aggregate (sand), which serves as a leveling layer. The depth of this layer varied depending on changes in the wall, but averaged about 3/4-inch. Once applied, the surface is scratched to allow the next layer to adhere. At the Coffeehouse there are only two layers, the second being the finish coat. The finish coat is approximately 1/8 to 1/4-inch thick. This layer is troweled to a smooth finish, just as it would have been in the 18th century. The walls are then kept wet and troweled for 4 days to allow the plaster to slowly dry and avoid cracking.
The plaster needs to cure for no less than 6 months before oil based paints or wallpaper can be applied. This is due to a process called saponification, where the alkalinity of the plaster degrades the oil and glue causing failure. To bridge the time between the completion of plastering and the finish treatments, whitewash is applied, which is also a common 18th century practice. The whitewash applied to plaster is simply thinned lime putty, and has roughly the consistency of milk. Approximately six coats are applied; the whitewash provides a temporary finish treatment, and also helps to heal micro cracks in the plaster.
Photos by Peter Inker and Joshua Muse