Mortar Analysis On The Great Hall Ceiling

Imagine looking at a drop of pond water under a microscope and seeing the tiny universe of things that suddenly come into focus. You probably expect that a biologist could gather a considerable amount of information about a pond from a tiny sample of its water. But what if we replaced that drop of water with a similarly sized sample of ceiling plaster? What do you think you’d find when peering through the eyepiece or sifting through the tiny grains of powdery white plaster?

There are several ways of looking closely (very closely!) at plaster, but broadly we can call this process of close looking “mortar analysis.” Typically, mortar analysis involves taking a sample of mortar or plaster and breaking it down into its constituent parts to learn more about it. We can do this by crushing and then sifting a sample to isolate coarse sand from finer ingredients. Weighing a sample, introducing acid to “digest” the calcium-based components, and then weighing the sample again can help reverse engineer the original mortar “recipe.” We can also view samples under a microscope to characterize the types of aggregates used and to identify various species of animal hair commonly found in historic plaster.

Acid Digestion Sample

In October, Director of Preservation Trish Smith carefully removed a sample of ceiling plaster roughly the size of the average chocolate chip cookie and sent it to conservator Steven Stuckey at the Tower Heritage Center. There, Stuckey used several of the methods mentioned above to help us learn about the plaster in the Great Hall.

Ankerite

We discovered that the lime binder was made not from oyster shells or limestone, as is often the case, but from ankerite—a mineral that can be found in sedimentary rock and iron formations. Making binders from sedimentary rock was more common in the 19^th century, which fits with the timeframe of this plaster ceiling’s installation. Stuckey was also able to identify two species of animal hair used in the plaster sample’s “scratch coat.” Hairs from cows and deer were used in this rough coat of plaster to enhance its adhesion before the application of the final “skim coat.”

Deer Hair

Cow Hair

Conducting mortar analysis is one of several ways that we are studying and documenting the historic plaster ceiling in the Great Hall in preparation for a much larger effort to address the ongoing structural concerns that threaten the ceiling’s preservation. Your support makes it possible for us to take on projects as large as the Great Hall ceiling with an eye to the microscopic details, and for that we say thank you!