Relative humidity and the stress/strain response of canvas paintings: uniaxial measurements of naturally aged samples

Five naturally aged painting fragments and primed canvas samples from dates between c. 1825–1912 were examined to determine their mechanical behaviour during RH cycling. The tests were performed mostly at room temperature and the samples were tested uniaxially. All of the samples showed the same basic response pattern. Below a relative humidity of about 80% restrained specimens gained tension during desorption and lost tension during absorption. Above 80% RH the opposite occurred, to a greater or lesser degree, depending on the shrinkage tendency of the fabric support. The form of the recorded response curves matched that predicted by Mecklenburg who superimposed the behaviour of individual layers so as to model whole painting response. Free-hanging specimens contracted in conditions when the restrained specimens gained tension and enlarged when they lost tension. Shrinkage movements as large as 1% were recorded for some samples at 98% RH. Beeswax impregnation was found to suppress the tension variation of a sample exposed to RH cycling. An old glue/paste lining on one of the samples was reformed by exposure to high RH resulting in increased tension values in dry conditions.