Influence of moisture content on colour of granular materials. Part I: experiments on yellow ochre

The water is suspected of remobilizing pigments of prehistoric painting into small worm-like heaps named vermiculations. To preserve these artworks, the moisture content of pigments must be monitored. Due to the fragility of the paintings and the technical constraints on the in situ measurements, colour measurement is proposed as a non-invasive way to access humidity. This study aims at characterizing more precisely the influence of the moisture content on the optical behaviour of yellow ochre. The light reflected by the surface of drying ochre is measured with a gonio-spectroradiometer. A scenario is proposed: the water saturation at high moisture contents is connected to the presence of a specular radiation which brings a loss of chromatic information. The evaporation of water brings the disappearance of the specular radiation and the augmentation of the diffuse radiation. These first results are promising for the development of a non-destructive in situ method of moisture measurement.