National Gallery of Canada / Musée des beaux-arts du Canada

Bulletin 2 (I:2), December 1963

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From the Laboratory of the National Gallery

by Nathan Stolow, 
Chief Conservation and Scientific Research Division

Résumé en français

Pages  1  |  2  |  3  |  5 

In the post-war period there has been an increasing interest in the application of scientific method to the examination and preservation of works of art. A recent compilation of UNESCO (1) reveals the existence of at least 133 museums and related institutions in 38 countries in which conservation laboratories have been established. In major centres scientific research is also carried out. (2)

Scientific interest is much in evidence at the National Gallery of Canada as recent Annual Reports (3) show. The following case histories and reports discuss in some detail most recent conservation work and, as well, the research undertaken in the fields of deterioration and environmental control by the Conservation and Scientific Research Division. (4)

Le Pont de Narni

By Corot

This painting was acquired by the National Gallery in 1940. No previously recorded technical history was available, though it was fairly obvious from a cursory examination of its condition that it had been restored before, at least once, between the estimated date of execution (c. 1826-27) and the date of its accession by the National Gallery. Superficial examination had led one to believe that the overall yellowish tone was not of Corot's doing but was more likely that of discoloured varnish. The opportunity to examine this painting for possible restoration and conservation treatment confirmed this. Examination of Figs. 1, 3, and 4, shows the presence of the 'masking yellowed varnish'. In addition, the disfiguring crack formation on the left side of the canvas (Fig. 2 - as seen by raking light), could now be properly assessed in terms of the cleaned picture, once freed of restorations and over-painting.

The painting had been relined in the past, and considerable pressure must have been applied as seen in the fabric imprint (Fig. 4 at A). The adhesive used was probably animal glue with a resin additive. A study of the weave pattern along the edges of the painting showed the absence of the expected 'scalloped' patterning along the left and right sides. This suggested the trimming of the painting some time in the past. Examination of the paint surface showed the typical wrinkling of Corot's oil-rich paint, especially in the roadway (Fig. 1). Cleaning tests with a variety of solvent mixtures indicated that the varnish layer was of the natural resin type, (5) and could actually be safely removed with shell sol, (6) acetone, and cellosolve (ethylene glycol monoethyl ether) in the proportions 2:1:1. From these tests it was established that the overall tone of the picture would be cooler, and that there would be a greatly increased feeling of space and depth throughout. Also indicated in the cleaning tests were the existence of previous restorations, abrasions, and areas of overpaint ( see Figs. 3 and 4). The location and extent of many of these areas could be determined by means of binocular micro-scope examination of the surface, by means of ultra-violet fluorescence, and by infra-red photography.

On the basis of the condition noted it was decided to clean the painting entirely, reline it, re stretch it on a new stretcher frame, compensate losses by filling and in-painting, and apply a final protective varnish. A summary of the various steps in the treatment follows: (7)

The reverse of the canvas was vacuum cleaned to remove surface dirt and accretions. The varnish was then removed with solvent mixture previously determined. At the same time most of the previous restorations and retouches were dissolved away. In some areas the retouches were completely removed by application of a scalpel. The painting was freed from its stretcher frame by removing nails from edges. Paper strips were glued to the four edges of the freed painting and were attached while damp to a larger temporary stretcher. (This method is referred to as the 'Dutch method' and enables the canvas to be temporarily held in position in a manner permit ting access to both front and back of painting.)

The surface of the painting was faced with a type of wet-strength paper using beeswax and carbon tetra-chloride mixture as the adhesive, applied while warm. After cooling the painting was reversed and the old relining canvas was carefully separated from the original canvas by mechanical means. The remains of old adhesive were then carefully scraped away by means of a scalpel. This proceeded very slowly owing to the extent of penetration of the adhesive into the weave of the original canvas. It was discovered that previous tears and holes in the original were filled with a white lead composition and shellac. These areas had to be worked clean of extraneous matter.

The cleaned back of the canvas was locally reinforced with strips of wet-strength paper teased at edges using LePage's Bond-Fast glue (a type of polyvinyl acetate emulsion adhesive) in the regions of tears, holes, etc. Following this the entire back was impregnated with a wax-resin composition using an electric hand iron with thermostatic control. The composition of this adhesive was: beeswax 75%, damar resin (Singapore) 20%, and gum elemi 5%, melting range approximately 60 to 70 degrees centigrade. After impregnation the painting was turned face up and the facing paper and beeswax adhesive were removed by application of a low boiling hydrocarbon (petroleum benzine) solvent. At this time the paper supporting strips (Dutch method) were removed.

The relining, or reinforcing canvas selected was cotton of smooth texture and weave, similar to that of the original, having 27 horizontal threads and 31 vertical, double woven in both directions. The relining canvas was placed on an expandable temporary stretcher, and was pre-shrunk with water, and when dried, restretched. It was then impregnated with the same composition of wax-resin adhesive as noted above, employing the heated surface of a thermostatically controlled hot table. The relining process was then carried out on the same hot table using a rubber latex sheet with vacuum pressure to bind the new canvas to the original while the correct degree of heating was applied to distribute the wax-resin adhesive into and through the surfaces to be bonded together. The duration of the warming - infusing - cooling cycle was approximately 2 hours, the temperature ranging upwards to 60 degrees, and the applied pressure (at the edges of the rubber latex enclosure) between 1/2 and 2/3 of an atmosphere. Descriptions of the hot table vacuum relining technique have been reported elsewhere. (8, 9, 10) The relined, cooled painting was restretched on a new 4-member stretcher frame with tite-joint fasteners and metal dowels installed in the mitred corners. This differed from the type known as the LeBron stretcher in that only part of the tite-joint fastener was used. (11) Thus instead of using the normal retaining ring of the tite-joint fastener a 1/4" hanger bolt was used, allowing for more positive tightening and loosening of the corners of the stretcher than was formerly possible. Following this the excess film of wax-resin on the surface of the painting was removed with petroleum benzine solvent.

Remains of old cracked fillings were mechanically removed and new fillings made with a paste of LePage's Bond-Fast glue and calcium carbonate. The appearance of the painting after cleaning, relining, filling, but before in-painting is shown in Fig. 5. The entire painting was then brush-coated with Acryloid B72 resin in xylene solvent. (12) After drying the in-painting was commenced using permanent powder colours ground in a solution of duPont Lucite 44 (normal butyl methacrylate polymer) in xylene. In this way the filled areas were made to match in colour the surrounding paint. Following this the painting was sprayed with a 10% solution of the same resin in xylene, and additional in-painting was carried out where necessary. Finally a second spray coat of the resin was applied.

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