ELECTROSTATIC HOLD AS A PRESSURE SOURCE IN THE LINING OF PAINTINGS
ROBERT E. FIEUX
Lining is a process intended to have an influence essentially upon the physical structure of a painting. Beneficial in purpose, the history of lining reveals ample evidence of severe loss of artistic values caused by damaging lining methods. The introduction of the vacuum hot table was primarily intended to reduce the texturally damaging effects of former methods and to improve the efficiency of the process.
In the thirty years since vacuum has been used as a pressure source in lining there has grown increasing realization that structural disfigurement of the aesthetic surfaces of paintings due to vacuum is in some instances worse than those caused by former methods and more difficult to prevent. Two international conferences have been held, one in 1974 and the other in 1976, devoted to lining problems. Several papers were explicit in description of textural deformation caused by vacuum pressure.2
The controlled caution possible during solvent varnish removal in which all areas of a painting can be tested in advance, with treatments then varied from one part of a painting to another as may be required, reflects an entirely different approach from that of lining in which the entire painting is committed to a treatment that once started must be carried on to its conclusion. There is no possibility of knowing in advance if a lining treatment will result in an alteration of artistic textures other than by empirical means. Studies were begun here after the 1974 conference to learn more about vacuum behavior upon warm, flexible paint films and to find a way to eliminate the hazard of vacuum.
By employing a surface-reading vacuum gage, it was determined that vacuum pressures previously believed to be in the “safe range” of 5 inches of mercury or less are not necessarily “safe” at all. In tests it was found that vacuum pressure of sufficient effectiveness to hold work flat imparts an imprint of a canvas texture on 1/2 mil polyester film or on 4 mil polyvinylchloride film that is permanent. The implications as to the effect on a warm, delicate paint film are obvious. Further tests using the surface vacuum gage revealed alarming irregularities in vacuum pressures between the surface and the gage built into the system near the pump; and pressure differences were found at various points on the surface of paintings being lined, depending upon distance from vacuum ports or size of the painting.
Due to the vulnerability of some paintings to any vacuum pressure, and the general irregularity of surface vacuum pressures, a technique was devised using standard vacuum hot table equipment with slight modification that totally eliminates vacuum hazard while providing sufficient “hold down” action for most lining needs. The method takes advantage of the electrostatic properties of polyester film by passing a constant air flow between two polyester membranes. An electrostatic charge is created and maintained resulting in a definite attraction toward the top of the hot table.
Hot table modification consists of disconnection of the vacuum system from the vacuum ports in the table top and running the vacuum lines to four points, two on each of the longer sides with 1/2″ copper tubing. The ends, turned upward with an elbow, are fitted with vacuum quick couplings. Four copper tubing arms are prepared in the shape of an elongated∪; one end fitting into the quick couplings, the other end having rubber suction cups made from rubber kitchen faucet sprays. This arrangement permits bringing the vacuum lines up over the table edge and placing the vacuum arms at any position on the table top beside a lining regardless of size or shape. The rubber suction cups are self-sealing to a covering vacuum membrane that is punctured at the selected points, and the vacuum brought in this manner directly to the point of work is more uniform overall and can be more effectively controlled.
To set up for the electrostatic hold technique, it is necessary that vacuum be taken off from above rather than through the table top as most systems do, and it is accomplished in the following manner. The painting to be lined is prepared in the usual manner with whatever adhesive, release sheets, etc., are required on the top of the hot table. A piece of one-half mil polyester film at least twelve inches larger than the lining is first placed over the top of the work as would be done in a usual vacuum lining. Over this is placed a piece of fine fiberglass at least six inches larger than the lining but smaller than the first membrane. A second one-half mil polyester membrane is cut about 1 inch smaller than the first one and placed over the fiberglass. Each layer is brushed out smooth with a wide brush. The four vacuum arms are swung over to points above the fiberglass near the corners, and the upper membrane only is then punctured at these points and the suction cups are affixed (Figure 1).
Modified vacuum system with regular vacuum lining set-up.
When vacuum is turned on, a flow of air is established through the fiberglass sheet between the two membranes establishing and maintaining an electrostatic charge, causing the combined covering layers to press downward against the table top. As long as the lower membrane is carefully kept intact, there is no possibility that vacuum will be able to act directly upon the painting, since it is exclusively between the two membranes. Although very slight, the downward holding action is sufficient for most lining needs.
Tests similar to those using vacuum at very low levels that resulted in imprints on one-half mil polyester film under vacuum as low as two inches of vacuum, when conducted using electrostatic hold, proved to be almost completely free of any impression and far less than imprinting by the least adjustable vacuum pressure. Critical examination of delicate paintings lined under vacuum pressure, regardless of how low, has shown a definitely discernible alteration of the surface texture. Since development of the electrostatic hold technique, many such paintings have been lined without discernible surface change. Yet, paintings with heavy paint films having cupping and other deformations have been lined with equally good results when in conjunction with a suitable vapor treatment.
A complete description of tests, vacuum control methods, and hot table modification is the subject of a paper in progress, to be submitted at a later date.
Fieux Restoration Laboratory, Cedar Street, West Barnstable, MA 02668.
Conference on Comparative Lining Techniques, April 23–25, 1974; National Maritime Museum, Greenwich, England.
Conference: “Lining of Paintings—A Reassessment,” April 6–8, 1976; National Gallery of Canada, Ottawa, Canada.