Potassium Permanganate Bleaching of an 18th-Century Hand-Coloured Aquatint Print

Abstract

AbstractThe treatment of a large-scale late 18th-century landscape view is described, focussing on a bleaching treatment. The hand-coloured aquatint Panorama de Chaumont is owned by the Bibliothèque cantonale et universitaire in Neuchâtel, Switzerland. It had been mounted on a wooden strainer and framed without glazing; severe and uneven browning made it unsuitable for presentation. The discolouration resulted from light exposure and direct contact with a wooden strainer which released volatile organic compounds. To minimize the discolouration, the six parts of the print were dismounted and separated along the joints from each other. The pieces were deacidified in tap water. Only one segment was bleached at first with a 0.1% potassium permanganate solution. However, large areas of blue colouring were decolourized during the subsequent sodium dithionite treatment. The decolourized areas of the print recovered their previous blue tone during the following rinsing in water and drying. The unexpected effect was caused by the reduction of indigo to indigo white (leuco-indigo) and its subsequent re-oxidation to indigo in absence of the reducing agent and through impact of atmospheric oxygen. Being able to assess the variables after a first testing phase, all print segments were finally bleached with a 1% potassium permanganate solution. The sheets were deacidified aqueously, resized on the verso with a 0.8% gelatine solution and tears were repaired. The image segments were not rejoined contrary to conservation recommendations.