Minerals provide tints and possible binder/extender in pigments in san rock paintings (South Africa)

Abstract

Minerals such as iron oxides and clays provide high tinting strength and improve the adhesive properties of pigments. In this study, we investigated the mineral composition of pigments from samples of San rock art. We used X-ray diffraction and scanning electron microscopy to determine the mineral composition and micromorphology of pigments. Results showed that the major minerals in pigments in San rock art are whewellite, quartz, gypsum, hematite, and various alumino-silicate minerals. The red hue in the pigment is due to hematite; gypsum and clays provide the white coloration, whereas black might be due to amorphous manganese compounds. We believe that whewellite with globular habit was extracted from plant sap (e.g., aloe vera) and added to the pigment, perhaps as binder, extender, or whitener. Whewellite with needle-shaped morphology was present in cracks that developed in pigments and indicated an early stage of deterioration of the rock art. We propose that conservationists should seriously evaluate any change in the environmental conditions at the art site (e.g., removal of vegetation to improve touristic view) because such changes might significantly increase thermal fluctuations in pigments and promote crack formation and hence the decay of the San rock art.

Light penetration into Clarens sandstone and implications for deterioration of San rock art

“…Thus, this seeming absence of endoliths associated with the rock art remains an enigmatic question, the answer to which may be important. Arocena, Hall, and Meiklejohn (2008) did, however, show the presence of hyphae in cracks within the clay ground and postulated that some of the recorded whewellite may result from a biological origin. As oxalates (e.g., the whewellite) are certainly associated with rock painting surfaces and may have a biological origin (e.g., Hess et al, 2008), they may also be produced subsurface if endolithic activity is able to occur there.…”

“…However, recognition of the light-transmissive nature of the sandstone substrate and the resulting thermal differentiation between it and the painted areas leads to the need for more detailed study and modeling of just such a situation. This need is particularly important in regard to understanding the location and nature of any cracking (including flaking), the more so as Arocena, Hall, and Meiklejohn (2008) have shown that cracking of the clay ground beneath the pigments may allow the growth of endoliths.…”

“…Recent studies of San rock art (e.g., Hall, Meiklejohn, & Arocena, 2007a;Hall et al, 2007b;Arocena, Hall, & Meiklejohn, 2008;Meiklejohn, Hall, & Davis, 2009) refer to the surface modifier effect of applied pigments (Bullet & Prosser, 1983); with respect to the rock, the pigments also effectively act as a barrier (Thomas, 1991). As a surface modifier, the pigments impact a host of thermal attributes (Hall, Meiklejohn, & Arocena, 2007a;Hall et al, 2007b).…”

“…The application of non-light-transmissive ground (a smooth clay base on which some paintings are located rather than being put directly on the sandstone) and pigments (e.g., hematite, clays, etc. ; see Hall et al, 2007b, Arocena, Hall, & Meiklejohn, 2008 by the San to the sandstone surface spatially alters the thermal gradients between pigments and their sandstone substrate (Hall, Guglielmin, & Strini, 2008a). Although the pigment(s) may inhibit light penetration, there can still be transmission due to scattering around the edges of the pigments, thereby facilitating subsurface colonization below the edges of paintings.…”

“…A general lack of understanding of rock weathering mechanisms is considered to be the major reason for the inadequacy of previous research into rock art conservation and preservation in southern Africa (Meiklejohn, Hall, & Davis, 2009). The most recent research (see Hall, Meiklejohn, & Arocena, 2007a;Hall et al, 2007b, Arocena, Hall, & Meiklejohn, 2008, Prinsloo et al, 2008 has shifted the focus away from investigating weathering of the rock on which the art is located, to the interface between the rock and the pigments and that of pigment-to-pigment within the paintings themselves. Improved techniques from weathering research are able to measure conditions at the rock surface where the art is found (Hall, Meiklejohn, & Arocena, 2007a).…”