A STUDY ON DEVELOPMENT OF CRACKS ON PAINTINGS
SAMPLES OF fabric (Belgian linen, 500 g/sq.m, thread count 13 [warp] � 12.5 [weft]/sq.cm) were stretched on spring-loaded frames with tension 250 g per 1 cm of stretcher bar (1.4 lb/in); they were thoroughly wetted and dried under tension. Care was taken to avoid deviation of fibers from straight lines. The samples were transferred onto wooden strainers without the loss of tension and sized with 6% w/v rabbit-skin glue gel (M. Grumbacher, Inc., New York, New York 10001). The sizing was followed by 6% glue/chalk gesso, coated with a thin layer of 3% size. Dimensions of samples were 28 � 56 cm (11 � 22 in), warp direction lengthwise. Total amount of size (including the size in gesso) was approximately 0.0046g/sq. cm. The stretched samples were stored at 45%–55% RH/21�C (later referred to as normal humidity) for 1 week to 3 months prior to testing.
In certain cases, to eliminate the influence of canvas on the direction of movements, samples without fabric were prepared by applying size and gesso onto a stretched sheet of Mylar (0.5 mil, Dupont). The dried samples were protected from the front with tissue/Beva facing (Beva 371, Adam Chemical Co.). Both Mylar and the facing were removed prior to tests.
In preparation for testing, the samples were transferred onto a frame equipped with a system of pulleys and springs to achieve different tensions or onto rigid aluminum strainers. The strainers were used to investigate movements in restraint as they occur on actual paintings. Edges of the samples were attached to bars with contact adhesive, thus avoiding tacking margin effects, which were tested separately. The entire surfaces of samples were exposed from the front and back to eliminate the differential response to humidity changes of central parts of samples as compared with edges overlying stretcher bars. Tension adjustments were made in normal humidity prior to the application of the brittle coating.
The ready-made solution of the brittle lacquer (Tens-Lac, TL-500-85, Measurement Group Inc., Raleigh, North Carolina 27611) was applied by spraying, 20% v/v trichloroethylene having been added to slow evaporation and to prevent formation of air bubbles in the coating. Upon drying/conditioning, the coating became progressively more brittle. Estimation of breaking strains was made using a manufacturer-supplied calibrator and cantilever calibration bars. The bars and tested samples were sprayed with the coating together and kept in the same conditions. When the coating reached desired brittleness as tested on the bars with the calibrator (at least 22 hours), humidity was changed to induce in-plane movements in the supporting samples and cracks in the coating.
Cracks are always perpendicular to the principal direction of tensile stress and strain, and patterns of cracking represented direction of movements resulting in extension. These local displacements were not always reflected in actual enlargement of samples. They were also caused by the inability of samples to move along rigid stretcher bars and by the geometry of samples. If magnitudes of strain were equal in two perpendicular directions, random patterns occurred (Dally and Riley 1978).
Little data are available on mechanical properties of the coating. The tensile strength is generally estimated in the literature at about 200 psi in 50% RH (Kobayashi 1987), about half that of oil paint (Kilpatrick 1980). Stiffness of the coating was certainly less than the size layer, as it could not restrain movements resulting from free swelling of size in high humidities, over 93% RH.
Difficulties during tests were associated with the long time of conditioning for the coating (up to 170 hours for low rupture strains). During that time, samples underwent sufficient creep to show as cracks in the coating when conditioned in higher humidities. This effect tended to obscure cracks resulting from humidity change. Therefore, cracks had to be detected before and after the change for comparison. It was also essential that the stretched samples be even, without out-of-plane distortions that would later inevitably lead to uneven tensions and irregular crack patterns.
Cracks were visible, but to record the results they were developed with black ink. Excess ink and the coating was later removed to leave black crack lines on a white gesso background.
Cracks formed at higher humidities were always sparser but wider than cracks observed in lower humidities. In higher humidity more extension was possible, and stress could be released through extension of size within an already-formed crack. This effect, no doubt, applies to cracking of aged paint on paintings.
Approximate minimum strains are given with each movement. The strains measured with the calibrator were always smaller than the actual strains on samples due to the much longer times of loading for samples. For faster movements (up to 3 hours) the actual strains can be higher by about 30% to 100%; for slower movements (up to 20 hours), several times the given values of strain (Dally and Riley 1978). For movements in high humidities, actual values of strain could be much higher due to the extension of size in cracks.