1 1. INTRODUCTION

Research into using atomic oxygen as a treatment technique started through inquiries by the Conservation Department of the Cleveland Museum of Art as to available NASA technologies that could be used to remove urethane varnish. Presentation of the results achieved with removal of various types of varnish, and discussion with conservators from other organizations, led to investigating the technique for removing soot and char from the surface of paintings and other fine art. Conservators believed such cleaning to be a problem area in need of some additional tools.

Atomic oxygen is present in the earth's atmosphere at altitudes where satellites typically orbit. It has been shown to react chemically with surface coatings or deposits that contain carbon (Banks et al. 1988; Banks and Rutledge 1988). The reaction converts the carbon to carbon monoxide and some carbon dioxide. Water vapor is also a by-product of the reaction if the surface contains carbon-hydrogen bonds. Depending on the material's chemical structure, the reaction can proceed through hydrogen atom abstraction, oxygen addition to form excited radicals followed by hydrogen atom elimination, oxygen insertion into the C-H bonds, and replacement by formation of alkoxy radicals (Banks and Rutledge 1988; Dever 1991). The majority of the reaction products are volatile species. The rate of the reaction will vary with the surface chemistry, atomic arrangement, temperature, energy of the oxygen atoms, and presence of electrons and other species. Materials already in a high oxidation state, such as metal oxides, are not affected by atomic oxygen.

Exposure to atomic oxygen can be harmful to a satellite if enough carbon-containing materials critical to its operation are removed. Due to the importance of the problem and the need to test potential solutions for protecting surfaces from reaction, facilities have been developed for producing atomic oxygen on Earth (Banks and Rutledge 1988; Banks et al. 1989). Radio frequency (RF) and microwave radiation or electron bombardment has been used to dissociate molecular oxygen into atomic oxygen. Either these atoms can be directed at a surface as a gentle flow of gas, or the surface can be immersed in the gaseous atomic oxygen. The exposure is typically performed in a vacuum chamber where pressures can range from 1.3 � 10−4 Pa (1.93 � 10−8 psi) to 13.3 Pa (1.93 � 10−3 psi), depending on the dissociation process used. This condition is defined as a rough vacuum and requires only a mechanical vacuum pump to achieve. The atomic oxygen reaction is confined to the surface because the oxygen atoms have a high reactivity, usually reacting with what is directly in their path on either first or second contact. Recombination into relatively inactive molecular oxygen becomes more likely with multiple collisions, so reactions deep into the surface are very rare unless a lot of large, straight, open pores extend deep into the material.

Atomic oxygen is of interest for cleaning paintings because the process is in the gas phase, there is no mechanical contact, and the reaction is confined to the surface, which reduces the risk of damaging the underlying paint or canvas. The atomic oxygen cleaning technique has been demonstrated to be effective for removing soot from small sample sections of canvas, acrylic gesso, and an unvarnished oil painting (Rutledge and Banks 1996). This article investigates its usefulness in removing soot from acrylic gesso and ink on paper, and in removing soot and thermally damaged binder and varnish from an oil painting. The process, which has been patented by NASA, is intended not to be a replacement for conventional techniques but to be an additional tool for use where conventional techniques may not be effective (Banks and Rutledge 1996).