Atmospheric Source of Atomic Oxygen for Cleaning Paintings

John H. Glenn Research Center, Cleveland, Ohio

A portable apparatus that operates at atmospheric pressure generates a beam of monatomic oxygen. The apparatus is designed to be used in a dry, noncontact process for removing organic contaminants from the surfaces of paintings. Organic contaminants that can be wholly or partly removed by use of this apparatus include some deposited in acts of defacement (e.g., lipstick and marks left by felt-tip and ball-point pens) and some deposited from fire (e.g., soot and charred binder). In some cases, this apparatus may make it possible to restore works of art that were previously counted as lost.

Figure 1. A Stream That Contains O Atoms and O- Ions is generated in a dc arc in a flowing He/O2.

The use of monatomic oxygen to remove undesired organic materials from the surfaces of paintings was reported in several articles in previous issues of NASA Tech Briefs; namely, "Atomic Oxygen Removes Varnish and Lacquer From Old Paintings" (LEW-16031), Vol. 20, No. 4 (April 1996), page 61; and "Cleaning Soot From Oil Paintings With Monatomic Oxygen" (LEW-15896), NASA Tech Briefs, Vol. 20, No. 10 (October 1996), page 110. Monatomic oxygen reacts with carbonaceous deposits on the surfaces of paintings, converting the carbon to CO and CO₂ and converting any hydrogen content of the deposit to H₂O vapor. Conventional techniques for cleaning paintings involve the use of solvents, which are not effective in some defacement cases and cases of severe fire damage. In contrast, the use of monatomic oxygen causes the removal of carbonaceous deposits at controlled rates, and cleaning can be stopped at any point.

Figure 2. The Effectiveness of Lipstick Smudge Removal on Andy Warhol's painting,

The cited prior articles describe placing each painting to be cleaned in a vacuum chamber, where it was exposed to monatomic oxygen from either a plasma or a beam source. The present atmospheric-pressure apparatus makes a vacuum chamber unnecessary. In this apparatus, monatomic oxygen is generated in a dc arc in a mixture of oxygen flowing at rate of 0.1 to 0.2 L/min and helium flowing at a rate of 4.3 L/min. The role of the helium is to inhibit the recombination of monatomic oxygen into diatomic oxygen.

The arc is struck between (1) a cathode in the form of a stainless-steel disk containing a 3.175-mm-diameter circular orifice on a truncated cone at one end of a plenum through which the gas mixture flows and (2) an anode in the form of a tungsten needle inside the plenum, 1.6 mm upstream from the orifice. The arc is powered by a 7-kVdc supply in series with a 1-MΩ current-limiting resistor. The arc is blown through the orifice, giving rise to a stream of oxygen ions and charge-exchange neutral oxygen atoms that are propelled about 1 cm downstream from the orifice. These oxygen species can react with organic materials exposed to the stream.

A painting to be cleaned is typically placed about 8 mm downstream from the orifice. Because the spot that is cleaned by exposure to the stream is only 3 to 5 mm wide, it is necessary to translate the apparatus gradually along the painting surface to clean a larger area. The apparatus was used to clean lipstick defacement from an Andy Warhol painting. As shown in Figure 2, the lipstick was completely removed.

This work was done by Bruce A. Banks and Sharon K. Rutledge of Glenn Research Center and Edward Sechkar and Thomas Stueber of Dynacs Engineering Co., Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.

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