THE EFFECT OF RELATIVE-HUMIDITY AND PIGMENT TYPE ON PAINT FILMS

The Courtauld Institute of Art in London has initiated a project to de termine the moisture content of ten differently pigmented paint films [1]. This was carried out by exposing the paint films to varying condi tions of relative humidity and weighing the paint films at selected ti me intervals until they reached equilibrium moisture content. The rati onale for this is to provide paintings' conservators with data on the moisture response of paint films. Moisture is used in conservation tre atment to remove surface deformations on paintings [2,3]. A measure of the time to reach equilibrium moisture content and the quantity absor bed in terms of weight uptake for different pigment types is of import ance. Diffusion coefficients for moisture sorption of these paint film s have also been calculated for different relative humidity values and pigment type, The aim of this paper is to use thermogravimetry (TGA) to continuously monitor the increase in mass with moisture uptake as a function of time for a given temperature and relative humidity. The p aint films were provided by the Courtauld Institute of Art. They had b een prepared in 1991 from mixtures of prussian blue and basic lead car bonate oil-based tube paints in different proportions. The paint films were conditioned at 20 degrees C and at 30%, 60%, 75% and 85% RH. The electrical properties of the same paint films were then studied by lo w frequency dielectric spectroscopy (LFDS) over the frequency range 10 (-4) to 10(5) H-2. LFDS was selected because it is non-destructive and it gives additional information on the structure and behaviour of mul ticomponent complex systems. One of the measured parameters, the criti cal frequency, was found to increase with increasing values of relativ e humidity and moisture content. It was shown that values for both the capacitance and conductance increased with increases in relative humi dity. The dielectric responses indicated that adsorbed water layers fo rmed on the interior surfaces of one of the components of the paint fi lms. The observed effect was much greater in the prussian blue than in the basic lead carbonate paint films and so it can be deduced that th e adsorbed water layers are in fact formed on the surfaces of the pigm ent grains. This was further substantiated by direct temperature-resol ved pyrolysis mass spectrometry (DTMS) where it has been shown that th ere is a reduction in the long chain fatty acids (e.g. stearic, palmit ic) in the hydrated samples. A model is presented to describe the ''ch arge hopping'' process which occurs in the adsorbed water layers and w hich gives rise to the low frequency dispersion.