Stone
Microanalysis provides an in-depth study of stone objects. Absolute dating methods (carbon 14 – thermoluminescence) cannot be applied to mineral materials, so the search of age criteria provides convincing evidence of an object’s authenticity.
Nature of the rock
From surface replicas or micro-samples of the stone, it is possible to determine the nature of the rock (marble, sandstone, serpentinite, jades, basalt, etc.) and check the constituency with the supposed origin of the object.
Egyptian bioclastic and chalky limestones for instance, have characteristic compositions: fossils, microcrystalline calcite, dolomite, opal lepispheres, coccoliths, etc.
Study of weathering processes
Identifying the natural and long-term degradation of an object or revealing artificial alteration.
Optical microscopy, coupled with scanning electron microscopy, can highlight specific weathering phenomena of the stone:
- Dissolution along the crystallographic planes of the mineral phases
- Micro-fragmentation
- Exfoliation of micas
- Crystallisation of neoformed phases
- …
Microsection of a sample from a Roman marble bust showing superficial dissolution of the calcite phases of the stone.
Microsection of a sample from a Khmer sandstone sculpture showing the exfoliation of a mica on the surface of the stone.
Among the weathering processes specific to the stone, sandstone are often affected by a preferential dissolution of the intergranular cement leading to the appearance of surface porosity (arrows), associated with a discoloration of the material on the surface of the sculpture.
A superficial chromatic change in the stone may also be the result of surface treatment of the object.
Khmer sculpture cleaned with hydrochloric acid resulting in superficial browning of the stone and the formation of an artificial« silica gel» patina (arrows).
Identification of the conservation environment – Analysis of deposits
Surface deposits are characteristic of a conservation environment. Long term deterioration of an object in a burying context leads to interaction between the weathered surface of the stone and its burying environment, resulting in, often clayey, crusting deposits.
Among the characteristic deposits observed on the surface of stone objects, biofilms and traces of mineralised roots are often preserved on ancient sculptures found in their natural burying environment.
TRACEOLOGY
Study of the tool traces ensures verification of the use of traditional techniques for the making of the sculpture, in accordance with the presumed period of the object.
Tools marks on the surface of an ancient stone object appear dull with variations in direction, thickness and width (arrows), consistent with a hand-made operation using natural minerals tools and abrasives.
The observation of fine, parallel polishing striae with sudden changes in directions (arrows), on the other hand, indicates the use of a modern rotary tool.
