USE OF A PRESSURE-SENSITIVE ADHESIVE TO FACILITATE THE TRANSFER OF A SEVERELY TENTED PAINTING
GIANFRANCO POCOBENE, & IAN HODKINSON
6 CONSERVATION TREATMENT
Preliminary investigations into the problem made it clear that methods usually employed to treat this type of severe damage would prove inadequate and result in further loss. Only after considered reflection and experimentation was a treatment procedure devised.
In keeping with the principal of minimal intervention, options were considered that would allow retention of the original canvas support. Attempts were made to plasticize the structure by further controlled humidification to allow expansion of the canvas and re-laying of the detached paint. However, the magnitude of shrinkage and brittleness of the paint layers made this option not only hazardous but impossible. The paint and ground layer remained brittle in spite of slowly raising the RH to 78%. The canvas also began to tighten noticeably at this humidity level.2 Raising the RH any higher would return the painting dangerously close to the conditions that caused excessive crimping of the canvas in the first place. It was concluded that any attempt to stretch the canvas with the aid of moisture—by means of the Dutch stretcher method for example—would result in additional cleavage and paint loss. Furthermore, because shrinkage had occurred in a heterogeneous manner, forming tents perpendicular to each other at the edges and corners of the badly degraded canvas, simple outward expansion of the canvas would not reverse the complex distortion patterns. In the end, to save the whole image, it became clear that transfer would be the only option.
Even after the decision was made to transfer, a number of perplexing problems remained. Given the condition of the painting, the most important part of the procedure was the choice of facing materials and technique. Attempts to face the paint film in this state using traditional techniques would have resulted in overlapping of most of the tented areas. The problem with traditional facing systems of aqueous and synthetic-resin adhesives lay both in the inherent characteristics of the materials and the manipulative techniques required in their use. These systems were discarded because they exhibited one or more of the following deficiencies:
- The surface tension and fluidity of aqueous- and solvent-based adhesives would cause paint islands to shift.
- Shrinkage of aqueous adhesive facings on drying would cause compression, leading to overlap and shattering of the brittle paint film.
- Given the highly reactive nature of the canvas to moisture, use of aqueous-based adhesives could cause further localized shrinkage.
- Most synthetic resin adhesives in solvent systems would prevent the facing tissue from conforming to the distorted paint surface.
- The pressure required to effect adhesion of facings would be too great and result in further fracturing of the paint film.
- The tented paint layers would be bonded to the canvas in their deformed state, leading to overlapping.
- Conventional facing systems lacked the flexibility for the manipulation of the paint back to plane.
As we continued to reflect on the problem of devising a suitable facing procedure, the use of pressure-sensitive adhesives in the lining of paintings came to mind (Fieux 1984; Gransow 1984). Pressure-sensitive adhesives in the form of precast films on a tissue substrate were contemplated, but they were rejected because of foreseeable problems in their manipulation during application. If the tissue were to inadvertently adhere in the wrong position, for example, it would have been extremely difficult to remove it from the fragile paint.
After considerable testing, a novel facing procedure was devised that provided maximum control and met all of the necessary requirements. This procedure involved the use of Rhoplex N-580, a pressure sensitive adhesive, and tengujo, a Japanese paper. Rhoplex N-580 is an aqueous dispersion of butyl acrylate copolymerized with acrylic acid (CCI 1981; Haywood 1988). When applied to a surface, it drys to a transparent tacky film. Bonding of a material to a surface coated with the adhesive is effected by means of gentle pressure. The tengujo paper was chosen for its fineness, strength, and ability to conform to the distorted topography of the damaged paint surface.
Preliminary tests on Rhoplex N-580 indicated that it possessed the necessary adhesive strength for the facing procedure. Tests to determine its stability during and after heating were also carried out. Heating the adhesive to 70°C did not reduce its adhesive strength, and after heating it remained soluble in mixtures of alcohol and ketone solvents, previously determined as safe for the paint layers. Therefore, the adhesive would not release during subsequent treatment on the hot table and could later be removed with little difficulty.