An Overview of Current Scientific Research on Stone Sculpture
Scientific Research Lab,
Department of Conservation and Collections Management
Museum of Fine Arts
Scientific research on stone sculpture is focused on three major categories: determining sources of raw materials, developing methods of authenticating stone artifacts, and preservation. This paper reviews research in the first two categories. The goal of source determination is identification of the quarry for the stone used in a sculpture. In situ analytical techniques are occasionally applied, but most research involves samples. There are many approaches, ranging from petrography (study of thin sections and quantitative analysis of mineral compositions, usually on the thin sections) to elemental and isotopic analyses on drilled samples. Elemental analyses carried out by instrumental neutron activation, X-ray fluorescence, inductively coupled plasma (ICP) techniques, and others often provide the most useful information.
Authentication of stone sculpture can focus on materials, techniques by which the materials were worked, and weathering layers. The alteration of a sculptural surface and the buildup of alteration products on that surface in a burial environment can be useful as an indication of age, and as such is often studied in stone sculpture authentication projects.
Stone was among the earliest naturally occurring materials to be used to create artifacts because it was readily available and required little processing. The steps
from procurement to desired final product are few and can be very simple. The current areas of research on stone artifacts to which science has made and continues to make a major impact fall into a few broad categories: determining sources of raw materials; developing methods of authenticating stone artifacts; and preservation. This paper will focus only on the first two categories because of limitations of space. “Rock” is the correct geological term for the raw material. According to a definition used by some, “stone” refers to rock that has been intentionally shaped into an artifact. For the remainder of this paper all the material under discussion will be called rock.
DETERMINING SOURCES OF RAW MATERIALS
The geographical origin of the rock used to make an artifact is a crucial piece of information in fully understanding the artifact. “Quarry” can be defined as the precise location from which the rock originated. In most cases the rock used in a sculpture is taken from an outcrop or exposure of rock, and the location of that outcrop would be the ultimate desired goal of the source determination. Some outcrops could be quite small, such as a pit in the ground from which some blocks to make small sculptures could be taken. In other cases an outcrop could consist of a very large exposure extending hundreds of yards or more. In the case of a very large quarry, localization of the source to some smaller area within the larger area may be desired.
It is possible that a boulder or smaller chunks of rock could serve as source material, and these may have been collected from areas that are distant from the actual geographical location of the quarry. Examples include boulders moved by glaciers to pebbles moved down streambeds. One well-known example is Stonehenge, which some argue was constructed from glacial boulders that were far removed from their original source site.
What is the relationship between the quarry location and the location at which the rock was shaped into an artifact? Some artifacts could have been shaped at the quarry, but blocks of rock could also be transported from a quarry to workshops, which could be at distant locations. Pebbles or small rocks could, of course, be easily transported long distances from the points at which they were collected. This is the case, for example, with some of the jade used in ancient China to produce small objects. A very widely traded material in many ancient cultures was obsidian—volcanic glass—to which considerable scientific research on sources has been devoted. We know in the case of marble sarcophagi in Roman times that blocks were sometimes partially shaped at the quarry and then shipped to various workshops where the final carving was completed. Determining workshop locations is an aspect of research on rock sculptures that can draw on information about quarry sources.
Approaches to Determining Sources
Nondestructive methodologies, carried out directly on artifacts without sampling, are particularly valuable in the case of monuments or sculptures on monuments. One technique that has been utilized in recent studies is magnetic susceptibility, which gives a signal based to a great extent on the presence of the mineral magnetite. Sources for Roman-period gray-granite columns in the Rome region have been studied (Williams-Thorpe and Thorpe, 1993), and sources for the gray to yellow-brown sandstone blocks used in different phases of construction of the eighth- to thirteenth-century complexes at Angkor in Cambodia have also been identified (Uchida et al., 2003).
For the most part, sourcing of projects involves analysis of samples, which fall into two broad kinds: solid chips and powder. The appropriate type and size of sample depend on many factors related to the rock type, the geology of the potential quarry source or sources, and constraints on sampling of the sculpture(s) being studied.
A rock-sourcing project could involve a single artifact or a group of artifacts. Sourcing projects usually begin with a group of artifacts made from a particular type of rock material. The questions to be answered are typically: Do the artifacts all come from one quarry source? What is that quarry source?
One phase of the research involves characterizing the material used in the group of artifacts. The goal is to acquire types of data that can characterize the rock material in sufficient detail to determine whether the rock from the various objects in the group being studied can reasonably be concluded to have come from the same source, or is likely to have come from more than one source. If more than one specific source is likely, another aspect of this phase of the research is to group objects that are likely to have come from the different sources suggested by the data.
As in sourcing studies of other types of materials used in artifacts (such as ceramics), it is common practice today to use multivariate statistical analysis to evaluate analytical data, thereby establishing potentially related groups of objects on the basis of this kind of evaluation. There are different approaches to utilizing statistics for data evaluation, and some may be more appropriate in certain circumstances than others. Although beyond the scope of this paper, the examples discussed in this paper include most of the current statistical procedures.
Evaluation of data from artifacts can establish hypothetical distinct sources or quarries, but at this point in a study it is uncertain whether the groups established is this manner actually represent distinctly separate sources, or whether the artifacts within individual groups actually all originated from a single source.
A second phase of research involves characterizing potential quarry sources and then comparing these with results from the artifacts. The goal is to determine the quarries from which the artifact rock came. Assuming that potential quarry sources can be located, systematic sampling of these is crucial in order to fully
characterize them. In advance it is difficult to establish what would constitute systematic sampling. The nature of the rock being sampled is one factor that needs to be considered, and whether the rock type is present in more than one horizon (or layer) that was formed during distinctly different geological events. A single rock type usually displays compositional variation, particularly if the outcrop is quite large. For multivariate statistical analyses, it is usually stated that the number of source samples should be at least equal to the number of variables (usually elements) being analyzed in each sample. For rock quarries far more extensive sampling than this is usually an absolute necessity unless the outcrop is extremely small.
Another aspect that needs to be determined in analytical studies of artifacts (and quarries) is the appropriate sample size. A representative sample of an artifact can be defined as one whose composition reflects the composition of the artifact as a whole. In the case of a block of rock, samples of 3 to 4 milligrams taken from several areas may not have identical compositions, while samples of 1 or 2 grams may. What constitutes the minimum acceptable sample size to truly represent the whole will often vary according to which properties are being analyzed. In studies that involve elemental analysis, certain elements are often found to be good discriminators with very small samples, others may be useful but only if larger samples are studied, and still others may be so unevenly distributed that they have little utility. Research on stable carbon and oxygen isotope ratios in carbonate rocks (discussed in more detail below) has shown that a sample of a few milligrams can be considered to be representative of a large block of the rock.
Solid Chip Samples (Petrography)
Solid chip samples are utilized mainly for petrographic analysis, the well-established tool used by geologists to characterize and classify rocks. Geologists typically prepare thin sections that cover most of the surface of a standard petrographic slide (in the United States, 27 x 46 millimeters), but that would require a sample far larger than could be taken from the vast majority of sculptures. Samples that can provide a thin section about 1 centimeter across, taken with sculptors’ chisels or hollow core drills, are often more than adequate for characterizing a rock, unless the rock is relatively coarse grained (with numerous individual mineral grains exceeding about a millimeter in size, for example). The thin sections provide information on the mineral content of the rock, the grain sizes and shapes, and the interrelationships of the different mineral grains (the texture).
Geologists have long utilized the electron beam microprobe to quantitatively analyze individual mineral grains in thin sections of rocks. The microprobe can provide excellent quantitative analyses of the major and minor elements in mineral grains that are at least a few micrometers across. This quantitative information further characterizes the mineral in the specific rock being studied.
One application of the microprobe focused on a silvery gray schist used in the
ancient Gandharan region (present-day northern Pakistan) from about the first to third centuries AD. The specific find sites are unknown for virtually all Gandharan sculptures made from this easily recognized rock. Some small local sources of this type of rock have been identified, but there are undoubtedly many more than are currently known. In a pilot project (Newman, 1992) 6 to 8 grains of chloritoid, a characteristic mineral of the rock, were analyzed in thin sections from 16 sculptures. After considering relative amounts of magnesium, iron, and manganese in the mineral, chloritoid in seven of the sculptures was found to have virtually identical compositions. Separate samples from the top and bottom of one of these sculptures, a 1.5-meter-tall figure, were analyzed to give some sense of the variation in chloritoid compositions in a fairly large block of rock. The spread in the compositions of these grains in the one block was about the same as the spread shown by grains in individual thin sections. As a working hypothesis, it is possible that the rock used in the sculptures with virtually identical chloritoid compositions came from the same quarry, while the other nine sculptures were made from rock taken from other quarries. Although more sculptures could be studied by this method and more extensive chloritoid analyses carried out in each thin section, the conclusions will remain tentative until possible quarry sources can be identified and studied. The research points out a possible relationship between the seven sculptures, but the precise significance of the relationship is uncertain. Were the sculptures produced in the same workshop within a narrow time frame, or produced in a general area in proximity to a certain quarry but at different times, or was material exported to several sites from one quarry? Or are these sculptures even from a single quarry?
Another application of microprobe mineral analysis was applied to small scrapings (0.1-1.0 milligram) from predynastic Egyptian basalt vessels (Mallory et al., 1999). It has been estimated that nearly one-quarter of stone vessels from this period were made from basalt. Grains of two minerals were quantitatively analyzed for major and minor elements: plagioclase feldspar and pyroxene. After comparing the results with some analyses of samples from major Egyptian basalt sources, it was concluded that the predynastic sculptural material was all northern Egyptian basalt. Finished objects, or pieces of rock that were shaped in local workshops, were apparently shipped all over Egypt from the source quarry or quarries.
A more recently developed tool for individual mineral grain analysis in thin sections is laser ablation microprobe ICP mass spectrometry. Currently, the minimum spot size for this technique is about 20-50 micrometers, meaning that it requires larger grains than does the microprobe, but the technique has the advantage of being able to analyze trace elements more readily than does the microprobe. A recent application focused on basalts in ancient Egyptian sculpture (Mallory-Greenough et al., 1999), using thick polished thin sections. The mineral analyzed, augite (a type of pyroxene), is a major constituent of most basalts. It was found that some trace elements, which can easily be analyzed by the laser probe
procedure, are far more useful for discriminating between augites in basalts from different quarry sources than are major and minor elements. Samples studied were basalt temper from pottery. A conclusion was that some of the basalt sources utilized during the pharaonic period in ancient Egypt have yet to be identified, since the augite compositions in samples from some artifacts did not correlate well with augite from basalts from the known sources.
Both of the techniques just discussed are adjuncts to traditional petrography. There are many rocks, however, that cannot be adequately distinguished by petrography, or that contain minerals that do not show sufficient variations in major, minor, or trace element compositions in order to be suitable for microprobe or laser ablation ICP mass spectrometry.
Other analytical techniques that can be applied to solid chip samples, whether prepared as thin sections or not, include Fourier transform infrared spectroscopy and Raman spectroscopy. The latter shows particular promise as an analytical tool, for example, in a project that was concerned with Mesoamerican jadeite sources in Guatemala (Gendron et al., 2002).
There is a full battery of analytical techniques that are applied to whole-rock samples, that is, drilled (powder) samples that include all of the minerals found in a rock. Most of the analytical techniques currently applied to drilled samples determine elements (in some cases, isotopes), with the exception of X-ray diffraction (XRD).
XRD is useful for general mineral identification and, by extension, rock identification in some cases. In sourcing research projects XRD can provide a quantitative estimate of the amounts of different major and minor minerals in a rock. Lacking textural information, XRD is less useful than thin section analysis for specific rock identification and classification. XRD is probably most useful in studying certain monomineralic or nearly monomineralic rocks. Many minerals have variable compositions, and variations in composition can give rise to distinctive XRD patterns. One interesting project studied chert and chalcedony, two microcrystalline varieties of quartz used in New England archaeological sites (Pretola, 2001). XRD patterns were used to identify minor minerals in some of the source materials. In addition, calculated diffraction patterns from crystal structure parameters were fit by computer methods to observed diffraction patterns in order to determine the ratios of quartz to a silica polymorph called moganite in the samples.
Powder samples are most commonly used for elemental analysis. The full battery of modern elemental analysis techniques has been applied to rock analysis. Current widely applied techniques include X-ray fluorescence spectrometry, instrumental neutron activation analysis (INAA), ICP optical emission spectroscopy, and ICP mass spectrometry. Some of these techniques are more appropriate
for certain elements, or abundances of elements, than others. A given rock, of course, will contain dozens of elements, which are present from major to very low trace levels (parts per billion and less). Not all these elements will be useful in a sourcing project. Which elements are the most useful is not usually known at the outset of a project unless previous work on similar rocks has been carried out.
Usually the application of a particular technique or techniques in a discrete research project can easily produce data that is reliable. More difficult sometimes is comparing data acquired by other analytical techniques, or even other research groups using some of the same instrumentation. Use of common standards and frequent equipment calibrations can help to overcome this kind of difficulty, but it can prove to be a limitation in comparing data from different research groups or projects.
Both the suite of elements utilized in a sourcing study and the methods by which the data is evaluated are obviously crucial to the conclusions.
Limestone in Medieval France
One of the most outstanding research projects yet to be published on sculptural rock sources involves the limestone utilized in French medieval sculptures. The beginning of this massive undertaking goes back to a modest self-contained 1985 project published on a small group of sculptures (French et al., 1985). Nine Romanesque sculptures in four American museum collections that shared certain stylistic features were considered by some scholars to have originated from one monument in southern France. The goal of the project was to determine whether this was the case and the location of the rock source(s). Petrographic examination and neutron activation analysis indicated that the nine were probably made of rock from the same quarry. As a part of this phase of the project the representative sample size for the chosen analytical technique and elements being analyzed was determined, and the fact that a single sample could be considered representative of an entire block of rock (of the size used in the sculptures) confirmed. Possible quarry sites in the general region of origin were selected for sampling and analysis on the basis of the petrographic features of the sculpture samples. Initial neutron activation analysis of a few samples from several different geological formations narrowed down the possibility to one area. From this area over 100 samples of the same geological formation were analyzed, three old quarries in particular being extensively sampled. The neutron activation analyses coupled with petrographic data narrowed down the likely source to two quarries that were about 2.5 kilometers apart. Multivariate statistical analysis was carried out on the elemental data.
Since that study, the same research group has focused on the characterization of some of the major limestone quarries in the Paris region, a study that to date has involved some 2,300 samples from 300 quarries (Blanc et al., 2002). The research group concluded that 10 to 15 elements were required to best characterize the French limestones, as were many samples from each compositional group
(quarry), a number that actually ranged from 8 to 40 for the major Paris basin quarries. The extensive database now makes it possible to determine quarry sources for museum artifacts made from limestone that came from this general region of northern France. The database includes samples from some monuments, and this data has enabled the identification of the monument that sculptures in museum collections originally embellished. Examples are the head of an angel, the head of a Virtue, and a choir screen from three separate collections. The trace element patterns of these three objects closely matched the trace element patterns of a group of samples from 25 sculptures on Notre Dame in Paris. The stone on the cathedral and in the three sculptures may have come from one of the ancient quarry sites that the research group has extensively studied, Charenton. This is one of a number of quarries in the Paris region from which one particular type of fine-grained limestone (Upper Lutetian limestone) was taken. Trace element analyses permit rock from many of the different quarries to be distinguished, although all are indistinguishable by petrography. About 1 gram of rock is required for this analysis.
White Marble in Classical Antiquity
The most extensively studied general rock type used by any culture or group of cultures in a region are the white marbles of the Mediterranean. Bronze Age sculptors in the Cycladic islands of the Aegean used white marble extensively in Greek and Roman times and well beyond. There are less than a dozen major quarries that supplied rock that was widely exported at different periods. In addition, there were dozens of quarries of mainly regional importance.
The majority of the white marbles consist entirely or almost entirely of calcite. Their textures, grain sizes, and minor and accessory mineral compositions provide some useful properties for discrimination, but petrographic properties alone have not been adequate to clearly distinguish most of these marbles.
An important breakthrough came in 1972 with a publication by marine geochemists who applied a tool of their trade, stable isotope analysis, to a few samples from several of the major ancient Mediterranean marble quarries (Craig and Craig, 1972). The pilot project showed that simple plots of the ratios of the stable oxygen isotopes (18O and 16O) versus the stable carbon isotopes (13C and 12C), calculated with reference to a standard international reference material, the carbonate fossil Pee Dee belemnite, separated the marble from different quarries. Stable isotope analysis, which is comparatively inexpensive to carry out and requires a sample of only a few milligrams, is now perhaps the most frequently applied technique in sourcing studies of marble sculptures from this part of the world, but time has clouded the picture considerably. A recent publication showed how the isotope fields for the quarries have expanded since the 1972 publication (Gorgoni et al., 2002). It is possible to discriminate between certain quarries with stable isotope data, but many cannot be distinguished from this data alone. Quarry
fields were initially defined by drawing an outline around all data points from analyzed samples taken from a quarry, but statistical analysis is now more commonly utilized.
In studies of ancient quarries it is not always certain that samples taken from the quarry as it is known today entirely represent the quarry as it may have been known during a much earlier period. For example, it is possible that parts of a quarry may no longer be accessible, or even may have been depleted. In this case, analyses of sculptures that can reasonably be assumed to have been made from material originating in that quarry can be used to define the compositional field, instead of relying solely on newly taken quarry samples. This has recently been done for some of the major Mediterranean marble quarries. Incorporation of sculptural data has in fact somewhat expanded some of the fields.
Many other analytical procedures have been applied to the study of these white marbles. Some have been championed by particular research groups and have yet to become widely applied, in spite of their promise. A brief survey of these techniques serves as an example of how there are diverse, often equally useful approaches to a sourcing problem, and more important, how data from more than one technique may be crucial in solving a particular problem.
Although petrography was probably not considered a particularly valuable technique by most researchers for many years, within the last few years one particular property that is best determined with a thin section has been shown to be very useful: maximum grain size. Although a large thin section is required to accurately determine this, this parameter does enable some quarries whose isotope fields overlap to be distinguished. In a recent publication (Gorgoni et al., 2002), in fact, isotope maps for fine-grained and coarse-grained marbles were shown (a maximum grain size of 2 millimeters was the dividing point).
Elemental analysis has been applied by a number of research groups, which have proposed different elements. Reliable comprehensive databases have yet to be established for many quarries, and elemental analysis is probably best applied in small-scale projects. Certain elements have been shown to be useful in certain either-or questions, where the possible quarries have been narrowed down to two.
Two other techniques that have been employed only by a few researchers to date are quantitative cathodoluminescence and electron paramagnetic resonance (EPR). Qualitative cathodoluminescence carried out with a suitably equipped optical microscope and recorded on color film distinguishes certain marbles on the basis of the color they show when bombarded with an electron beam. Quantitative cathodoluminescence carried out in a scanning electron microscope with an attached spectrophotometer is potentially more valuable. In the ultraviolet/ visible region of the electromagnetic spectrum most marbles show only one or two major peaks: a peak at 620 nanometers, due to manganese (2+) substituting for calcium in the crystalline matrix and a pair of peaks at about 350 and 377 nanometers due to cerium (3+). Ratios of the two can distinguish many marbles
from one another. The orange color shown by some marbles is related mainly to trace amounts of manganese in the rock.
EPR or electron spin resonance (ESR) has been applied for many years by a small number of researchers but in the last few years has come into its own as a viable technique. One recent article evaluated a number of features of the EPR spectra of marbles, features in part attributable to the presence of manganese and iron in the crystalline lattice of calcite (Polikreti and Maniatis, 2002).
The study of Mediterranean white marbles has reached such a level of maturity that many important quarries have been well characterized and the strengths and limitations of a range of analytical techniques that can be focused on a specific sourcing problem are fairly well understood. In the near future more work will continue to center on characterizing some of the smaller quarries. Multivariate statistical analyses incorporating properties or features from two or more analytical techniques have become common practice in marble provenance studies.
DEVELOPING METHODS OF AUTHENTICATING ROCK ARTIFACTS
Authentication of stone artifacts from a scientific point of view usually focuses on one of two lines of evidence: the original materials and manufacturing procedures, and weathering or alteration of the surface after carving or other finishing.
Materials and Working Procedures
In this category would obviously be the nature of the rock itself. If a sufficiently large database of information on the rocks that presumably should have been utilized were available, detailed rock analysis may help to build a case for or against the authenticity of a problematic artifact. More often than not, the result of such an analysis even when a large database is available may lead to the conclusion that the material used in an artifact is consistent with the purported period and place of origin, which does not imply that the object is definitely authentic, only that it could be.
Working of the rock can also be important. Certain tools or manners of using a tool may be characteristic of a type of stone artifact. Some researchers have carefully compared drill marks produced by known ancient drilling procedures with marks produced by modern drills, and this information could obviously be used in certain authentication studies. Tool marks on ancient gemstones have also been extensively studied (Rosenfeld et al., 2003), with the purpose of determining the tools that were utilized and the way the tools were used. Applications of this type of information to weathered sculptural surfaces is much more difficult since many details have been worn away.
There may be residues of tools that can be identified, for example, elements from metal chisels or bits of abrasives used in polishing operations. A study of
Roman gemstones found residues of lead and tin metal as well as barite, all of which were probably associated with abrasive polishing procedures (Rosenfeld et al., 2003).
It is difficult to quantify the appearance of tool marks on a complex sculpture in such a manner that would allow the data to be readily utilized by other researchers. The use of tool marks in authentication studies may fall more into the realm of technical connoisseurship, where interpretation of significance depends on the experienced eye of a researcher who has carefully examined many tool marks on many artifacts. One example is in a paper published 13 years ago by a sculptor who has studied tool marks on Roman marble sculptures (Rockwell, 1990). He argued that although a skilled modern forger could use the same tools as an ancient Roman sculptor, utilizing them in exactly the same manner would be very difficult, and that the differing manners of use could be distinguished by careful examination of tool marks, when the appropriate marks are still present on a sculpture. A very interesting concluding remark of his was, “In reading literature on fakes and faking I find that no one questions that the faker can, if he wants to, reproduce the technique of the period being faked. It is taken as a given that a good technician can reproduce the technique of any period. I think that this is an assumption about techniques, based on the ignorance of nontechnicians, that deserves serious questioning.”
One final approach is to look closely at surface alteration. Changes that take place on a newly worked rock surface over time can involve physical as well as chemical factors. Sharp details can become rounded or abraded. Minerals at the surface can chemically break down due to interaction with the atmosphere or burial environment. The surface of an ancient rock sculpture may contain materials that in part arise from interaction of the rock surface with its environment over some period of time. “Weathering layer (layers)” will be used to refer to all such materials.
Many ancient rock artifacts will have been buried for some extended period of time, but few if any details are usually known about the burial environment. While it is quite possible that there is some correlation between thickness of a weathering layer and amount of burial time for a given type of rock and given environment, all the information that would be required to establish this is never available.
The composition and microstructure of a weathering layer are both useful. Possible variation in both composition and structure from place to place on the surface of an artifact make it prudent to examine multiple samples. Small chips that can be prepared as cross-sections are the single most useful type of sample, since compositional and structural features can both be studied on the same
sample utilizing such techniques as scanning electron microscopy with attached X-ray spectrometers.
On rocks that contain more than one mineral the microscopic structure and composition of a weathering layer will not be identical on exposed surfaces of the different minerals. Some minerals, such as quartz, are very resistant to weathering, while others are much less resistant. The processes by which different minerals break down and the products of those breakdown processes vary from one mineral type to another. Even on monomineralic rocks the nature of the weathering layer will not necessarily be uniform over the entire surface of a sculpture.
To have confidence in the application of weathering layer data to an authentication question, the weathering that takes place on the type of rock in question in the (burial) environment in which it is likely to have undergone most of its weathering should be characterized. Geological samples if appropriate or artifacts (or architectural blocks) if appropriate should be examined. A range of deterioration, on a single artifact and on different artifacts, can probably be expected. Rarely can such a database be built, however. Another option is to look for weathering features and by-products that are known to result from the deterioration of certain minerals or types of rocks under general conditions similar to those of a sculpture made from the same material.
If the question involves the possible faking of weathering layers, specific analyses that can distinguish the real phenomenon from an artificially produced emulation need to be carried out. Some researchers search for organic materials that might have served as binders used to adhere an artificial patina onto the surface of an artifact. Some organic materials will undoubtedly be present in authentic weathering layers, but the nature of these and their relative abundance can reasonably be expected to be quite different. A wide range of very sensitive techniques for organic analysis are available, such as gas chromatography/mass spectrometry, which can be applied to characterization of organic residues in rock weathering layers.
The study of weathering layers on an artifact may require analyses of a number of different kinds. To date, the weathering layers that arise on stone sculptures through long-term burial have not been very extensively studied for many classes of rocks or rock artifacts. Probably the rock to which the most work has been applied is again the white marbles of the Mediterranean region. Marble is sensitive to acidic groundwater. Weathering in a burial environment usually involves solution at the surface, and along grain boundaries, coupled with reprecipitation of calcium carbonate on the sculpture surface. The reprecipitated calcite often incorporates bits of rock from the surface and organic and inorganic materials from the soil. In the case of marble, stable isotope analysis has also been carried out, since interaction between rock and groundwater will result in a change in isotope ratios. This technique can sometimes detect extremely thin weathering layers.
Marble made of dolomite instead of the much more common calcite can undergo a very different kind of weathering. Dedolomitization is a phenomenon in which calcite replaces dolomite through interaction with groundwater in some specific circumstances (Doehne et al., 1992). To date, dedolomitization has not been able to be induced in a laboratory setting to more than a minor extent. The presence of a reasonably thick layer of this kind would provide some proof that an artifact has been buried for an extended period of time, at least given our present understanding of the phenomenon. In a study of a dolomite sculpture attributed to the Archaic Greek period, researchers observed a weathering layer that very closely resembled in structure and composition the layers seen on buried dolomite marble that had undergone weathering for nearly two millennia (Newman and Herrmann, 1995).
Most rocks are not as simple in mineralogy as marble, nor as reactive to groundwater. The weathering layers on these can be more complex to analyze and interpret. One recent example involves the volcanic tuff utilized in eastern Java during the Majapahit period (1293 to about 1520). In a study of eight statuettes researchers concluded that certain minerals form in small depressions on the sculpture surface during long-term burial (Duboscq, 1989-1990). The depressions are sites where certain minerals that made up the original rock were located (the depressions were formed in geological time, not during burial). The research identified several minerals coating the surfaces in these depressions. It was concluded that these minerals (zeolites, clay, and monazite) should be present in similar depressions on authentic artifacts made from this type of rock. Scanning electron microscopy with energy-dispersive X-ray fluorescence was used to characterize the weathering products, using either cross-sections or, more typically, scrapings from the insides of depressions on the surface of a sculpture. Although the conclusions regarding the value of this type of evidence were too stridently stated given the scope of the project, the research is an example of the use of detailed compositional and structural information associated with weathering.
Weathering layers are certainly one of the best pieces of evidence in authentication studies of rock artifacts, but the application of this type of evidence can be said to be in its infancy for the vast majority of rock types. More extensive research will be required to add certainty to conclusions based on this approach.
The study of marble weathering layers can be taken as an example of the increasing sophistication applied to the problem as time goes by. A little over 30 years ago, when the use of weathering layers on marble as an authentication tool was first suggested, thin sections examined under a polarizing-light microscope were the only tool applied to their study.
Another type of evidence that for many decades has been taken to indicate an ancient weathered surface on a marble artifact are root marks, bits of plant roots that become cemented to the surfaces of sculptures that have been buried. In recent years the very important role of biodeterioration in the weathering of rocks has been the subject of much research, most of it focused on monuments and
architecture. Some of the molecular characterization techniques that have been applied to the study of biodeterioration in aboveground settings may also be of value in characterizing weathering in burial environments, where biological agents may also be at work.
CONCLUSIONS: THE FUTURE
The major categories of research noted in this paper have long been areas of research and will continue to be very important in the future. As examples in this paper have shown, sourcing projects that can be expected to produce worthwhile results are often time consuming. There are many sourcing projects on rock sculptures that remain to be undertaken, projects that can potentially be of great importance to art historians and archaeologists. Authentication of rock sculptures has also long been an active area of research, in which advances are continuously made. The future will hold more of the same.
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