2 2. PROCEDURE

2.1 2.1 TEST OBJECTS

A commercially available artist's stretched canvas (cotton canvas primed with acrylic gesso, approximately 90 � 120 cm) was used to perform the initial test of the large-area atomic oxygen treatment system. The stretcher was held above a wax candle flame, and the candle was moved back and forth underneath the stretcher to create soot streaks on both the paintable surface and the back of the canvas.

Two fire-damaged works of art were also used in testing the atomic oxygen cleaning technique. The first was a Roy Lichtenstein (b. 1923) ink drawing on paper (untitled abstract from 1950, approximately 21.6 � 27.9 cm). The work had been heavily smoke-damaged and partially thermally decomposed in a fire. The McKay Lodge Fine Arts Conservation Laboratory in Oberlin, Ohio, had previously tried float washing in alkaline water (8.0 pH ammonium hydroxide) for a few hours followed by immersion in a sodium borohydride solution (< 1% v/v), but these processes had a minimal effect on the appearance of the drawing.

The second work was a copy of the Raphael (1483–1520) painting Madonna of the Chair. It was painted by Bianchini of the Studio Viale Petrarca in Florence, Italy, and it was originally displayed in St. Alban's Episcopal Church in Cleveland, until an arson fire destroyed the church in June 1989. This painting was given to the Cleveland Museum of Art for treatment. The Madonna painting, a varnished oil painting approximately 74.3 � 74.3 cm, was heavily smoke-damaged and partially charred. A section of the painting was initially treated at the museum with acetone, then with methylene chloride and some additional solvents. Some of the soot and varnish were removed by these techniques, but the surface was still very dark, and features were difficult to distinguish. The Madonna painting was considered to be unsalvageable and was donated for testing of the atomic oxygen treatment process.

2.2 2.2 APPARATUS FOR ATOMIC OXYGEN CLEANING

Cleaning of the test objects was performed in a large vacuum chamber that could hold a stretched painting roughly 1.5 � 2.1 m. The vacuum chamber was not specifically designed to hold paintings, but was modified to be able to accommodate paintings of this size suspended in a vertical position inside.

The vacuum in the chamber is provided by conventional mechanical vacuum pumps, which can be used with traps but for these tests were not. Pressures during treatment range from 0.13 Pa (1.93 � 10−5 psi) to 0.667 Pa (9.67 � 10−5 psi). Two large aluminum parallel plates inside the chamber are used as electrodes to create the atomic oxygen through radio frequency (RF) excitation. One plate is connected to an RF power supply manufactured by RF Power Products Inc. operating at roughly 400 watts; the second plate is at ground potential. From the ground plate protrude several bolts from which test objects to be cleaned can be suspended directly or by hanging with fine wire. The objects are hung so that the ground plate is in contact with the back of the object, thereby shielding the back side from the atomic oxygen during cleaning. A controlled entry of air into the chamber at rates between 50 and 280 standard cm3 per minute provides the source of the atomic oxygen. Radio-frequency oscillating voltage between the two plates causes dissociation of the molecular oxygen and nitrogen in the air into atomic species, creating a plasma discharge with a pink glow between the plates. The atomic nitrogen has been found not to have an effect on carbon removal, and to date no detrimental effects, such as changes in coloration, have been observed on any of the test objects because of its presence. An automated timer and NASA-designed and -constructed controller on the system allows the cleaning to proceed unattended over a desired time frame, and will turn the system off if a loss in vacuum, water cooling to the pumps and power supply, or a drop in plasma intensity is detected.

2.3 2.3 MONITORING AND ASSESSMENT

Monitoring and assessment of the treatment process were performed by removing the canvases from the vacuum chamber and measuring in air the diffuse reflectance from selected portions of the test objects at periodic intervals in the cleaning process. A quartz halogen microscope light set at full intensity, with a color temperature of 3200 K (blackbody peak wavelength of 900 nm), was mounted on an aluminum beam so that the light could hit the surface of a painting at roughly a 45� angle. A photodiode detector was placed near the light source. The equipment was placed in a dark room to minimize effects from stray light. The reflectance from a magnesium oxide-coated glass slide was used to correct the data from the detector to eliminate drifts in the intensity of the light source between measurements. The area that could be illuminated was approximately 1.91 cm in diameter, so it was necessary to select areas from the test articles that were both uniform over this size range and had the potential for changing the most (largest contrast) during the cleaning process. In this way, the end point of the cleaning could be determined by looking for a leveling-off of the reflected light signal, indicating that no further change was taking place.