In our previous posts we discussed the materials and methods used by Claes Oldenburg to create Floor Cake, the artist’s unique and popular piece of painted cake currently undergoing conservation treatment here at MoMA. This week, we investigate the properties of Floor Cake’s surface dirt, to help us prepare an optimal cleaning solution to remove dirt and grime from the sculpture’s painted surface.
The majority of Floor Cake’s canvas surface is covered with acrylic paint, a painting medium that first became available to artists after World War II. Compared to traditional mediums such as oil paint and tempera, acrylic emulsion paints are also very stable, are highly durable to color changes, and remain flexible over time.
Because the research and conservation of acrylic paints is still a relatively young field, we have fewer years of testing and treatments on which to base our research and treatment approach. One method for understanding the nature of soiling on an acrylic surface has been developed by Richard Wolbers of the University of Delaware. Last summer Wolbers, along with several other conservators and scientists, lectured on applied strategies for cleaning acrylic painted surfaces at the Getty Colloquium—Cleaning Acrylic Painted Surfaces: Research into Practice. It was suggested that measuring the pH and conductivity of the surface dirt on an acrylic surface would aid in tailoring a cleaning solution. These measurements could be done with handheld meters.
So what are pH and conductivity?
The pH of a solution measures the degree of acidity or alkalinity relative to the ionization of the water sample. The pH of pure water is neutral, i.e. 7. Most pH readings range from 0 to 14. Solutions with a pH less than 7 are acidic. Solutions with a pH greater then 7 are basic or alkaline.
Conductivity is the ability of a material to conduct electric current. Electrical current is conducted by ionic charge. A conductivity meter can be used to determine the concentration of ions in a solution. For our purposes, ionic concentration can be thought of as the amount of salt in the surface dirt on Floor Cake. The salt content of the surface dirt plays a large part in how hygroscopic—or able to absorb and retain moisture—the dirt is. The salt wants to take on water, which attracts more particulate matter. This water can also swell the acrylic, causing the dirt to further embed into the paint.
To test the pH and conductivity of Floor Cake‘s dirt, we tested each of the five layers of the work in three separate locations for both pH and conductivity. The average pH of all these locations was 6.19, and the conductivity was .42 µs/cm. We can now use these readings to make a cleaning solution with a similar ionic strength or conductivity to the dirt, as well as with an appropriate pH, to ensure that unwanted material becomes soluble and can be removed.
Come back in a few weeks and we will write about performing both wet and dry cleaning tests, to see which method works best at picking up dirt.