Volume 18, Number 1.... January 1996

Technical Exchange

Cleaning Paintings with Atomic Oxygen

NASA's study of how materials react in low Earth orbits may provide the conservation field with a non-contact cleaning system.

Scientists have been placing test materials on satellites for some time now in order to investigate the longevity of protective surface coatings in low Earth orbits. The research found that polymer films containing hydrogen/carbon chains do not fare well in this environment due to interaction with highly reactive atomic oxygen. The conditions of low Earth orbit are now reproduced in laboratories on the ground for more economical study.

Sharon Rutledge, a research engineer in the Electro-Physics Branch at NASA, had asked Bruce Christman, chief conservator of the Cleveland Museum, about some of the difficult problems conservators encounter in their work. Christman said, "it would be great if you had something to remove polyurethane". Rutledge replied,"we can do that". That was the beginning of what led to a series of experiments by the Electro-Physics Branch into removal of coatings with atomic oxygen. The experiments began with the removal of soot which also contains hydrogen and carbon chains. A joint presentation by Sharon Rutledge and Bruce Banks, given at the Midwest Regional Guild Meeting this October, provided an overview of the atomic oxygen process, as well as a description of several experiments dealing with the removal of soot from painted surfaces. The soot deposits were produced by placing gessoed test strips on a wall inside a controlled fire structure used by the Cleveland Fire Department.

Another sample, from an actual oil painting, was placed inside a monitored enclosure and subjected to soot from burning motor oil. These samples were then cleaned with the atomic oxygen process for about a two hour period. The initial visual evaluation appeared quite promising, however conservators have many questions about the long term effects of such a process.

The use of atomic oxygen as a cleaning method has limitations, but it also has tremendous advantages. One of the biggest drawbacks is that the object must be placed into a vacuum of .008 inches of mercury at room temperature. This removes most of the moisture from the surfaces in the vacuum chamber, which in the case of a painting may not be the optimum environment. On the other hand, the cleaning is achieved as the plasma generator produces a soft rain of oxygen atoms over the surface, slowly attacking carbon and hydrogen bonds on the surface without too much penetration. Care must be taken to seal the edges and reverse of a painting, as leaks could cause oxidation of the supporting canvas fibers.

As the atomic oxygen combines with soot or polymers, it is converted into water vapor, carbon monoxide, and other gases, which are instantly removed by the vacuum pump. The soot and varnish layers are simply eroded away without human contact. One stationary plasma generator can be adjusted to cover a maximum radius of approx. 12". Therefore, cleaning larger surfaces would require some innovative engineering. A chamber necessary to accommodate a 30" x 40" would be so large that it would be impractical as a mobile unit.

Overexposure can cause a type of damage to a paint layer which is similar in appearance to solvent abrasion. However, the reaction can be stopped instantly by flipping off the switch to the plasma generator. The reaction can be blocked with substances which could be applied in clear coatings, for example, metal oxide or salt preparations.

The researchers at NASA have said they would engineer and build an atomic oxygen chamber large enough to accommodate an average size painting if they could be reimbursed for the cost of the materials, about $75,000.00.

As with any technique or process, the skill and experience of the operator is a large factor in determining the success of a given treatment. There are clearly many advantages to atomic oxygen cleaning, especially if the surface will not permit contact with water, solvents, or the touch of a swab. There may be cases, particularly smoke damaged modern paintings, which may justify larger scale testing when other options have been exhausted. Clearly, more samples, conditions, and contact times need to be tested before we can fully evaluate atomic oxygen cleaning as a viable technique.

Dean Yoder

Kiwi LTH-8K Data Logger

A Massachusetts-based company, The Kiwi Group, designs, manufactures, and distributes miniature data loggers, handheld data collectors/computers, and desktop acquisition (DA) systems. The company is currently preparing a handheld logger/barcode reader for release in January 1996, as well as a comprehensive environmental management system for the PC, scheduled for release in March.

The Kiwi LTH-8K is a miniature data logger that can record light, temperature, and humidity verses time, in a single unit which measures 1 3/4" x 1 1/2" x 3/4". Sample rates can be programmed from 1 second to 1 month intervals. Records can be viewed as spreadsheets or plotted in graph formats. The unit runs on replaceable lithium batteries which last for a minimum of 3 years or 2 million samples. Temperature range is -40 to 85 degrees C , plus or minus 2%; relative humidity range is 0 to 100%, plus or minus 3% to 5% (avg.); light range is 0.02 to 20,000 foot-candles, plus or minus 5%. Other ranges are available by special order.

Pricing for the Kiwi LTH-8K is $249 in single quantities and $49 for the Kiwi Logger software and serial cables. Volume pricing is available.

For more information contact: John Hotchkiss, The Kiwi Group, PO Box 1048, North Falmouth, MA 02556 Phone/Fax: (508) 563-2824

Conservation Support Systems' new enlarged catalog is now available. Call: 1(800) 482-6299

Conservation Materials now carries Rhoplex 1950 which, according to Rohm and Haas, is virtually identical to the discontinued Rhoplex LC-67.