Low-tech adhesive testing for Egyptian polychrome limestone
BY CAROLINE ROBERTS, Conservator, Kelsey Museum of Archaeology
One of my favorite conservation activities is researching practical solutions to complex condition problems. Example problem: how to stabilize flaky, powdery paint on deteriorating Egyptian limestone artifacts. The solution? Some kind of adhesive. But which kind would work best in an outdoor environment on salt-laden painted stone?
To figure this out I took a look at published information on the treatment of painted Egyptian limestone sculpture and wall paintings. There’s a lot of information out there on this topic, and I wanted to see for myself how some of the adhesives tested by conservators and scientists performed on a painted, salty limestone surface. The Kelsey conservation lab has limited equipment for this type of research, although we often partner up with labs that do (check back with the Kelsey blog for an upcoming post on our collaboration with UM’s Aerospace Engineering department http://www.engin.umich.edu/aero/research). One thing I could do in-house was to create mockups of the problem surface to approximate how each adhesive might perform in situ.
Creating mockups that accurately represent the materials and conditions of an ancient paint surface required some creativity. I used travertine tiles as a base, and soaked half of them in a solution of sodium chloride, or halite. This type of salt is present in much of the soil in Egypt, and has been shown to have an impact on adhesive performance. Stone was often covered with a ‘preparation’ layer (or layers) of plaster before paint was applied, so I applied Plaster of Paris to each tile. I then applied a layer of red ochre in gum Arabic – a plant-derived binder used in ancient Egyptian wall painting – with a high pigment-to-gum ratio representing the often diminished state of ancient binders on polychrome limestone. A section of each material – stone, plaster, and paint – was left visible on each mockup.
I applied five different adhesives to the tiles, leaving a number of them untreated as a control. I recorded their working properties, absorption, and resulting color changes, and then placed them outside to see how the adhesives fare in an exposed environment on both salty and un-salty mockups. From this low-tech experiment I hope to determine, from a practical angle, which adhesive to use on artifacts both at the Kelsey Museum and in field settings.
Filed under: Archaeological research, Conservation