The effect of relative humidity and pigment type on paint films

Abstract

The Courtauld Institute of Art in London has initiated a project to determine the moisture content of ten differently pigmented paint films [1]. This was carried out by exposing the paint films to varying conditions of relative humidity and weighing the paint films at selected time intervals until they reached equilibrium moisture content. The rationale for this is to provide paintings' conservators with data on the moisture response of paint films. Moisture is used in conservation treatment to remove surface deformations on paintings [2,3]. A measure of the time to reach equilibrium moisture content and the quantity absorbed in terms of weight uptake for different pigment types is of importance. Diffusion coefficients for moisture sorption of these paint films 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 paint films were provided by the Courtauld Institute of Art. They had been prepared in 1991 from mixtures of prussian blue and basic lead carbonate oil-based tube paints in different proportions. The paint films were conditioned at 20°C and at 30%, 60%, 75% and 85% RH. The electrical properties of the same paint films were then studied by low 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 multicomponent complex systems. One of the measured parameters, the critical frequency, was found to increase with increasing values of relative humidity and moisture content. It was shown that values for both the capacitance and conductance increased with increases in relative humidity. The dielectric responses indicated that adsorbed water layers formed on the interior surfaces of one of the components of the paint films. 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 the adsorbed water layers are in fact formed on the surfaces of the pigment grains. This was further substantiated by direct temperature-resolved pyrolysis mass spectrometry (DTMS) where it has been shown that there is a reduction in the long chain fatty acids (e.g. stearic, palmitic) in the hydrated samples. A model is presented to describe the “charge hopping” process which occurs in the adsorbed water layers and which gives rise to the low frequency dispersion.