The 7th International Conference on Science and Technology in Archaeology and Conservation, Petra, Jordan

Hamdallah Bearat's picture
Research Title: 
A Study of Cinnabar Blackening: New Approach and Treatment Perspective
Vanessa Terrapon
Hamdallah Béarat
Tue, 2009-12-01
A_Study_of_Cinnabar_Blackening_New_Approach_and_Treatment_Perspective.pdf627.89 KB
Research Abstract: 

Blackening of cinnabar or vermilion has always been a dilemma. The hexagonal mercury sulfide pigment, often of mineral origin, has an intense red color. For long, cinnabar has generated large interest among mural painters, artists, and alchemists in antiquity as well as nowadays conservation scientists and restorers, who are anxious to restore the color of paintings seriously affected by blackening of the pigment, often qualified as dramatic and irreversible. However, if particular attention has been given to the study of such phenomenon since the beginning of the 20 th Century, conservation of this color still presents numerous difficulties. Light has for long time been considered the cause of the color change. The Romans recognized this color change and utilized the pigment for inside painting away from direct sun light. The principal factors to cause or accelerate this specific alteration are solar radiation, atmospheric agents and associated pollutants, relative humidity as well as soluble salts and organic compounds. Yet, no viable solution of preventive conservation at excavations or restoration of altered vermilion has been proposed. Lately, research has shown that blackening is associated with an amorphous phase, forming as a coating on the exposed grains, and not due to transformation of cinnabar into the black cubic sulfide phase. This study aims at isolating the factors responsible of blackening in order to evaluate the most significant effects on the preventive conservation of the pigment. In this perspective, artificial weathering (aging) experiments were carried out by exposing modern and archaeological samples to direct sun light, or in the shade, with distinct conditioning. All were covered with optical filters transmitting only certain frequencies of light spectrum to determine the wavelengths the most detrimental on the stability of pigment and which conditions mostly favor the alteration. Progress of alteration was monitored and analyzed prior and after irradiation, using scanning electron microscopy, energy dispersive spectroscopy, particleinduced X-ray emission spectroscopy, and colorimetric methods. Data obtained show that the different wavelengths transmitted have different effects on cinnabar. The chromatic change can be accelerated also by higher relative humidity and by certain atmospheric pollutants, which may induce formation of gypsum by sulfate attack of calcium-rich phases present. This study, while providing a better understanding of the mechanism of alteration process of pigment under external conditions, allows for a cost-effective method of preventive conservation during excavation wherever cinnabar is being used as pigment in wall paintings.