Hydrated amorphous silicon dioxide (Si02.nH-,O), or opal-A, is deposited naturally
from seepage and runoff water as white or brown rock surface coatings, called 'skins',
that often partly obscure rock paintings and engravings, but occasionally, a thin
translucent silica skin can form a protective film over rock art. White lustrous silica
skins, less than 1 mm thick, occur where seepage water regularly flows from bedding
and joint planes, whereas much thinner brown skins form on the sides of boulders and
cliffs where runoff water periodically flows. To find the degree of silica skin variability
and to determine how climate and rock type affect the properties of silica skins I
collected samples at seven Australian and two Canadian rock painting sites that were
located in temperate, tropical and sub-arctic regions. The skins had developed on
sandstone, quartzite, schist, gneiss and migmatite. I studied the effects of the skins on
rock art stability, documented their compositions, textures and structures to establish
their common properties, and searched for a way to date the silica which would provide
an indication of the minimum age of the underlying art. 1 also made replication
experiments to determine factors that influence the properties of artificial silica skins
and the rates of their precipitation so that I could propose a mechanism for natural silica
skin formation, and ascertain whether an artificial silica skin could act as a protective
rock art conservation measure.
I was able to subdivide the analysed samples into silica skin Types I, II and III on the
basis of their colour (translucent, white or brown), composition (SiO2, Al2O3 and
absorbed water contents) and texture (smooth vitreous or vermiform). I propose that
silica skins initially begin to form on stable rock surfaces by a process involving a
combination of evaporation- and ionic-induced polymerisation of silicic acid in seepage
and runoff water. Condensation reactions, random clustering of small silica spheres
and deposition of the resulting aggregates eventually produce a thin surficial silica film.
Deposition of silica often traps micro-organisms that live in the damp seepage and
runoff water zones, and these fossils in finely laminated skins enable the radiocarbon
dating of silica deposition, and therefore the dating of rock paintings enclosed by silica.
Micro-excavation of silica layers associated with rock art combined with accelerator
mass spectrometry gave preliminary radiocarbon determinations that were either
consistent with, or contradicted, prevailing opinions about the antiquity of the rock art
at selected sites. Experiments using a laser technique for combusting fossilised microorganisms
in finely laminated skins were unable to generate sufficient carbon for
dating. Catalysis of a mixture of equal proportions of methyl-trimethoxy silane and
water produces a translucent stable film that may be suitable as a consolidant, whereas
other artificial silica skins made from silica glass and tetra-ethoxy silane develop microfractures
on drying, and these are unsuitable as rock art consolidants.
| Identifer | oai:union.ndltd.org:ADTP/219436 |
| Date | January 1996 |
| Creators | Watchman, Alan Leslie, n/a |
| Publisher | University of Canberra. Applied Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Alan Leslie Watchman |
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