Comparative study of washing treatments for pastel drawings

A treatment procedure for pastel drawings developed at the Portuguese Institute of Museums and Conservation (IMC), consisting of alkaline blotter washing in a humidifying chamber, is compared with three other methods described in the literature. These include immersion in cold water, a combination of water mist and suction table treatment and ultrasonic mist in combination with a dry blotter support. The four treatments were applied to non-aged and aged test samples consisting of three types of paper and four pastel sticks, totalling 120 samples. Effects caused by the afore mentioned treatment methods were evaluated using six semi-quantitative parameters determined by visual observation (i.e. lateral pigment movement, paper staining, pigment loss, decrease in colour intensity, darkening and change of hue), as well as three semi-quantitative parameters obtained through optical microscopy and scanning electron microscopy (pigment agglomeration, compacting and penetration into the paper support). The results showed that the IMC method is not suitable for treating pastel drawings executed on papers with low porosity, but, of all tested methods, it is the most advantageous technique for treating pastel drawings executed on velour paper - the prevalent type of carrier for pastel drawings treated over the years at IMC.