Obsidian hydration dating by infrared spectroscopy: method and calibration

Low temperature (90–190 °C) hydrothermal experiments have been conducted on seven obsidians where composition of the glass varies significantly in the concentration of structural water within the unhydrated bulk material. Infrared transmission spectroscopy was used to track the diffusion of molecular water into the glass surface as a function of time and temperature. Long-term (60–360 days) hydration sequences at 90 °C show a t^0.6 time dependence for the mass uptake of molecular water that forms the hydration layer. The structural water concentration of the unhydrated bulk obsidian is highly correlated with the pre-exponential and activation energy and may be used to estimate the Arrhenius constants. In addition, secondary ion mass spectrometry (SIMS) hydrogen profiling of Napa Glass Mountain obsidian hydrated at 90 °C reveals that the early stages of diffusion exhibit a dynamic behavior that includes a fluctuating hydrogen concentration and a changing diffusion coefficient that slows with time.     

A NEW DATING METHOD FOR HIGH‐CALCIUM ARCHAEOLOGICAL GLASSES BASED UPON SURFACE‐WATER DIFFUSION: PRELIMINARY CALIBRATIONS AND PROCEDURES*

The first European settlers came to North America in the early 17th century using glass in the form of containers and decorative objects. Thus, glass is a horizon marker for all historic period settlements and a potential source of chronometric dates at archaeological sites belonging to the historic period in the Americas. We have developed a new absolute dating method based upon water diffusion into the surface of manufactured glasses that predicts diffusion coefficients based upon variation in glass chemical constituents. Low‐temperature (< 190°C) hydration experiments have been performed on a set of five high‐calcium (21.7–28.3%) glasses that were used to manufacture wine bottles from the 17th?19th centuries. Infrared spectroscopy and secondary ion mass spectrometry was used to model the water diffusion/alkali exchange process. The ability of the model to accurately predict archaeological ages was evaluated with artefacts recovered from ceramic‐dated contexts at Thomas Jefferson's plantation known as Monticello.     

Obsidian Diffusion Dating by Secondary Ion Mass Spectrometry: A Test using Results from Mound 65, Chalco,Mexico

Secondary ion mass spectrometry (SIMS) was used to measure hydrogen and other elemental concentrations as a function of depth in ten obsidian artifacts (Pachuca Source), each with a well-constrained ¹⁴C date, from Mound 65, Chalco, Mexico. Hydrogen depth profiles for the different artifacts all display a characteristic S-shape, and increasing maximum hydrogen content in each profile and profile depths are well correlated with time. These data are used to investigate the potential use of Obsidian Diffusion Dating by SIMS (ODDSIMS) for both extrinsic and intrinsic dating of obsidian artifacts. Using “characteristic points” on the hydrogen profile (half-fall depth, inflection point depth), simple hydration rate equations were evaluated against time constraints provided by associated ¹⁴C dates. We demonstrate that neither the traditional OHD equation for depth (x) as a function of the square root of time (t^1/2) nor a linear function (t¹) fit the data. Solving the more generalized tⁿ function provides an excellent fit between characteristic point depths and ¹⁴C dates (for n≈0·75), and meets the constraint that at time equal to zero, the depth of the hydration profile must also be zero. However, this may be an average coefficient over the range of ages available, and may not accurately reflect rates at shorter or longer times. Using only two obsidian samples and their associated ¹⁴C dates, a calibration curve can be derived that provides ODDSIMS dates for the other pieces that are in excellent agreement with associated¹⁴ C dates, indicating that empirical application of the technique is potentially feasible, at least at individual sites.The underlying processes governing hydrogen transport into the obsidian were also investigated by using finite difference modelling to reproduce the shape of the hydrogen depth profile. Excellent fits were obtained by assuming concentration-dependent diffusion, and dates that agree well with associated ¹⁴C dates can also be extracted from the finite difference profiles. Although considerable additional work needs to be done, the success of the finite difference modelling suggests that development of an independent, intrinsic ODDSIMS model may be possible.     

ARCHAEOLOGICAL VOLCANIC GLASS FROM THE SITE OF ROCCHICELLA (SICILY,ITALY)*

The site of Rocchicella, near Catania, in eastern Sicily, has yielded important archaeological evidence from prehistoric times to the Middle Ages. Extensive archaeological investigations of cultural layers dating from the Palaeo‐Mesolithic to the Copper Age have recently been undertaken, and volcanic glass, mainly obsidian, has been collected in the course of excavation. To determine the provenance of this volcanic glass, a non‐destructive elemental analysis was carried out to measure the concentration of characteristic trace elements. The analysis was carried out using a new XRF spectrometer equipped with a beam stability controller and a quantitative method developed at the LANDIS laboratory of the INFN–CNR Institutes of Catania. In addition to the obsidian, it was demonstrated for the first time that a local vitreous material similar to obsidian, but displaying a completely different composition, was used during all the investigated periods. This material was identified as a basaltic glass, characterized by a superficial product of devitrification called palagonite. Analysis of the obsidians has led to the identification of the island of Lipari as the provenance source. High‐ and low‐power microscopic use‐wear analysis on obsidian and basaltic glass artefacts indicated that soft wood and plant matter might have been processed at the site.     

Effective hydration temperature of obsidian: a diffusion theory analysis of time-dependent hydration rates

An estimate of effective hydration temperature (EHT) is needed for chronological use of obsidian hydration data. This paper describes a method for calculating EHT by the practicing archaeologist, replacing three techniques that are in general use today: estimates based on mean temperature, numerical integration of models of diurnal and annual temperature variations, and use of temperature cells. The hydration (or diffusion) coefficient of obsidian is a function of temperature and thus is time varying, while the classic quadratic law of hydration is not valid for time-varying diffusion coefficients. This paper presents a mathematical solution to the case of a time-varying hydration coefficient, based on diffusion theory, with a concise definition of EHT. It is shown that the results are not affected by concentration dependence in the diffusion coefficient. A computer program to compute the rigorous solution is described, and data are presented to explore the resulting range of variation. That use of the Lee equation to compute EHT is not appropriate for obsidian hydration studies is evident from the data presented. The effects of paleoclimatic variation are estimated, and an algebraic best fit equation and worksheet are provided as practical aids to the archaeologist.     

THE MELTING FURNACE OF THE DERRIÈRE SAIROCHE GLASSWORKS (COURT,SWISS JURA): HEAT‐INDUCED MINERALOGICAL TRANSFORMATIONS AND THEIR TECHNOLOGICAL SIGNIFICANCE*

Seventy‐one oriented specimens from the pre‐industrial reverberatory melting furnace of the Derrière Sairoche glassworks (Swiss Jura) were analysed. During its period of activity (1699–1714), this glassworks produced wood‐ash glass of liquidus temperatures up to 1400°C. Textural and mineralogical features point to temperatures around 1500°C in the melting chamber. The inner structure of the furnace shows chemical contamination due to ash, molten glass and furnace gases. These results reveal the pyrotechnological know‐how of the glassmakers and prove that the thermal performance of this kind of furnace has been under‐evaluated in the past.     

An archaeologically validated protocol for computing obsidian hydration rates from laboratory data

This paper reports the computation of hydration rate for Topaz Mountain obsidian from laboratory data, and a comparison with archaeological data from a well-dated site, Camels Back Cave, in western Utah. Topaz Mountain obsidian is found to be slow-hydrating, with a rate of 0.071 ± 0.021 μ/yr^½ at an effective hydration temperature of 16.01 °C. This rate agrees with a rate developed from archaeological data from Camels Back Cave within ∼6%. Activation energy of Topaz Mountain obsidian is 10370 ± 544 K, and its diffusion constant is (1.87 ± 9.13) × 10¹³ μ²/yr, both of which are independent of temperature. Its intra-source variability in hydration rate is very low (CV < 0.01), implying a low variability in intrinsic water. We present a model of chemical erosion which shows why earlier laboratory-determined rates were incorrect, and discuss the implications of our findings on the determination of experimentally derived rates in obsidian hydration dating.     

Obsidian hydration dating: accuracy and resolution limitations imposed by intrinsic water variability

Obsidian hydration dating typically yields a range of ages for a single chronometric measurement, even after controlling for source chemistry and effective hydration temperature. Previously published data suggest that this range is due to hydration rate variations caused by variability in the concentration of intrinsic water, and specifically hydroxyl ions, in the obsidian. Such variability exists within any given obsidian source, and even within any particular specimen. This paper analyzes the effect on age estimates of intrinsic water variability and concludes that if a controlled sample of obsidian yields a range of ages, there is no way to tell whether the range is due to long site use or to variations in hydroxyl concentration. At present there is no robust and cost-effective protocol for measuring hydroxyl concentration. Suggested guidelines for obsidian hydration dating are developed using a case study. These findings do not invalidate obsidian hydration as a chronometric technique, but they do suggest limits to the temporal resolution achievable.     

ROMAN GLASS‐MAKING AT COPPERGATE,YORK? ANALYTICAL EVIDENCE FOR THE NATURE OF PRODUCTION*

Ceramic vessels and associated vitreous debris, excavated at Coppergate, York, UK, have been interpreted as the remains of Roman glass‐making from the raw materials. This paper reports the results of analysis of this assemblage by XRF, ICPS, XRD, SEM– EDAX and thin‐section petrography. These findings suggest that some ceramic vessels, used as crucibles, have been subjected to temperatures up to 1200°C, well above the firing temperatures of the local domestic assemblage from which they were selected. Analysis of quartz‐rich debris, intimately mixed with glass in some samples and in others interleaved with glassy phases, indicates partially reacted glass‐making raw materials, with α‐quartz, tridymite and cristoballite phases represented. This could represent evidence either of a failed attempt to frit the raw materials, or a batch that had not fully fused. Variability in the composition of glass at the site is viewed in the light of glass‐making technology, and possible interpretations concerning this episode are discussed in the light of the archaeological evidence.     

Trade,tribute, and neutron activation: The colonial political economy of Xaltocan,Mexico

In Trade, Tribute, and Transportation, Ross Hassig argues that indigenous towns in the northern Basin of Mexico during the colonial period were largely self-sufficient. They traded with Mexico City mostly in elite goods, but for the most part they produced for their own subsistence or traded with nearby towns. Chemical characterization by instrumental neutron activation analysis (INAA) and portable X-ray fluorescence (pXRF) of ceramics and obsidian from post-conquest contexts in Xaltocan, a site in the northern Basin of Mexico, reveals that Hassig’s model is partly correct for describing Xaltocan. The town focused on trade with nearby towns and it produced some ceramics for local consumption. However, Xaltocan was hardly isolated and self-sufficient in the post-conquest period. Instead, the data suggest that the people of Xaltocan also obtained ceramics and obsidian from a greater variety of sources than under Aztec domination. Rather than being an isolated rural site, Xaltocan either increased its external connections and number of trading partners after the Spanish conquest, or it managed to obtain a greater variety of products than before through a bustling market system.     

Prehistoric settlement chronology on Rapa Nui,Chile: obsidian hydration dating using infrared photoacoustic spectroscopy

The prehistoric Polynesian inhabitants of Rapa Nui (Easter Island) utilized obsidian for nearly 700 years in many activities connected with daily life. The near ubiquitous occurrence of the natural glass in both domestic residences and religious structures makes the application of obsidian hydration dating highly suitable for the investigation of cultural change. We have applied previously developed calibrations that estimate hydration rates for obsidian based upon the structural water content of the glass as determined by infrared spectroscopy. The archaeological ages estimated by this method were compared with accelerator mass spectrometry (AMS) dates using short-lived woody species endemic to the island. The convergence between the two dating methods is strong and we suggest that obsidian hydration dating may be used on Rapa Nui to reliably date contexts where suitable material for AMS dating may not be available.     

Sources of archaeological volcanic glass in the Primorye (Maritime) Province,Russian Far East*

Geochemical studies of volcanic glasses (obsidians and perlites) from geological outcrops (N = 80) and archaeological collections (N = 110) were performed in order to determine source provenance in Primorye (Russian Far East), using neutron activation analysis and X‐ray fluorescence spectrometry. Three major sources of archaeological volcanic glass were identified, two relatively local and one more remote. Several minor sources detected in the archaeological assemblage have not been located. This study suggests that long‐distance obsidian exchange between Primorye and adjacent North‐East Asia has existed since c. 10 000 bp .     

SIMS‐SS,A NEW OBSIDIAN HYDRATION DATING METHOD: ANALYSIS AND THEORETICAL PRINCIPLES*

The water diffusion mechanism in obsidian has been revisited with the advent of a new dating approach employing secondary ion mass spectrometry (SIMS). The water diffusion SIMS(+) profile and the concept of a surface saturation (SS) layer in obsidians both provide a sound basis for the new diffusion age equation (SIMS‐SS) and are supported by dated world examples ( Liritzis et al. 2004 ). Here, we present the basic physical–chemical analysis of the mass transport phenomenon on which the new dating method is based. The crucial age parameter of the SIMS‐SS dating approach for archaeological obsidians—that is, the surface saturation (SS) layer—is supported by both theoretical and experimental data.     

SCIENTIFIC ANALYSIS OF SEVENTH‐CENTURY GLASS FRAGMENTS FROM THE CRYPTA BALBI IN ROME*

Inductively coupled plasma emission spectroscopy, reflectance spectroscopy and X‐ray diffraction were used to study seventh‐century AD glass fragments from the Crypta Balbi in Rome. All the samples were found to be silica‐soda‐lime glasses. Iron determines the colour of blue‐green, green and yellow‐green transparent glasses; chemical composition suggests deliberate addition of iron and/or manganese in about half the samples. Copper was found as the main colourant in red, pale blue and blue‐green opaque fragments; elemental copper acts as an opacifier in red glass, and calcium antimonate in white, pale blue and blue‐green glasses. Detection of antimony in transparent fragments suggests recycling of opaque mosaic tesserae.     

Field data validation of an algorithm for computing obsidian effective hydration temperature

This paper describes an analysis to validate the effective hydration temperature (EHT) algorithm for obsidian [Rogers, A.K., 2007. Effective hydration temperature of obsidian: a diffusion-theory analysis of time-dependent hydration rates. J. Arch. Sci. 34, 656-665], using hourly temperature data from the Amargosa Desert Research Site near Beatty, NV. These data were used as input to a numerical model of the temperature-dependent diffusion process, and EHT was calculated yearly and for the aggregate. The same temperature data were processed to extract input parameters for the climatic model reported and used as input for EHT computation. EHT as computed from recorded hourly data was found to be within 1 °C of that computed from the climatic model. It is also shown that computed EHT differences between sites are not sensitive to whether air or surface temperature data are used, as long as they are used consistently; however, surface temperature data must be used if depth corrections are to be made.     

GLASS BY DESIGN? RAW MATERIALS,RECIPES AND COMPOSITIONAL DATA*

Compositional analyses of ancient and historic glasses have often been interpreted in terms of the use of specific raw materials in glass manufacture. However, the known inhomogeneity of many glass‐making raw materials and the insolubilities of some compounds make any explanation of compositional data problematic. This paper looks at three glass‐making alkalis with a view to understanding how the compositions of these raw materials are carried through to the final glass. The chemistry and variability of the raw materials are discussed, as is their contribution to the final glass composition. In addition, the choices and decisions made by glassmakers are acknowledged in the final glass compositions. This combination of factors addresses the complexity of predicting the use of specific raw materials from the finished glass composition.     

Book Reviews

Abstract

Obsidian studies play an integral part in archaeology around the world, particularly in the Americas, but few archaeologists have employed obsidian studies to understand Native American life at historical archaeological sites. Yet, obsidian sourcing and hydration analysis can provide critical insights into site chronology and use, lithic recycling, and procurement and trade at contact and colonial sites. Obsidian geochemical sourcing and hydration analyses of a 19th-century rancho site in northern California have revealed new information on Native Americans who labored there in the second quarter of the 19th century. The obsidian data indicate a significant amount of lithic manufacture and use, a change in obsidian procurement in the 1800s, and an unprecedented number of obsidian sources represented on-site. The implications for general obsidian studies, as well as for regional archaeological issues, concern the problems with popular sourcing methods in northern California and the need to revisit current understandings of the first micron of hydration rim development.     

Potassium–Calcium Glass: New Data and Experiments*

The chemical composition of potassium–calcium ‘wood‐ash’ glass reflects the elemental pattern of the involved non‐volatile base materials in quartz sand, wood ash and possibly potash. The essential elemental ratio K₂O/CaO of wood ash varies between 0.2 and 0.8, and depends on the habitat and geological substratum of the wood rather than on the tree species; ratios between 1.0 and 3.0 in wood‐ash glass are only possible when potash is added as a third base material. Melting temperatures of wood‐ash glass sensu stricto, termed K–Ca‐2, produced with the two raw materials quartz sand and wood ash, are between 1250°C and 1400°C, while those of three‐component‐glasses, termed K–Ca‐3, are between 900°C and 1250°C, according to the amount of added potash. Experimentally produced glass displays different hues, from colourless to brown, olive‐green and pink, according to the chemical composition of the wood ash. Elevated MnO concentrations between 0.5 and 3 wt% may originate from wood ash and are hence not necessarily an indicator of colour‐inhibiting additives. Phosphate stemming from wood ash is an essential discriminator between wood‐ash glass and potash–lime glass. Because wood ash contains only minor amounts of sodium, wood‐ash glass with equal concentrations of potassium and sodium is a hybrid glass type, where besides quartz sand, wood ash, possibly potash and also soda‐rich cullet have been applied for glass production.     

Quantitative analysis of COH fluids synthesized at HP–HT conditions: an optimized methodology to measure volatiles in experimental capsules

The quantitative assessment of COH fluids is crucial in modeling geological processes. The composition of fluids, and in particular their H₂O/CO₂ ratio, can influence the melting temperatures, the location of hydration or carbonation reactions, and the solute transport capability in several rock systems. In the scientific literature, COH fluids speciation has been generally assumed on the basis of thermodynamic calculations using equations of state of simple H₂O–nonpolar gas systems (e.g., H₂O–CO₂–CH₄). Only few authors dealt with the experimental determination of high‐pressure COH fluid species at different conditions, using diverse experimental and analytical approaches (e.g., piston cylinder + capsule piercing + gas chromatography/mass spectrometry; cold seal + silica glass capsules + Raman). In this contribution, we present a new methodology for the synthesis and the analysis of COH fluids in experimental capsules, which allows the quantitative determination of volatiles in the fluid by means of a capsule‐piercing device connected to a quadrupole mass spectrometer. COH fluids are synthesized starting from oxalic acid dihydrate at P = amb and T = 250°C in single capsules heated in a furnace, and at P = 1 GPa and T = 800°C using a piston‐cylinder apparatus and the double‐capsule technique to control the redox conditions employing the rhenium–rhenium oxide oxygen buffer. A quantitative analysis of H₂O, CO₂, CH₄, CO, H₂, O₂, and N₂ along with associated statistical errors is obtained by linear regression of the m/z data of the sample and of standard gas mixtures of known composition. The estimated uncertainties are typically <1% for H₂O and CO₂, and <5% for CO. Our results suggest that the COH fluid speciation is preserved during and after quench, as the experimental data closely mimic the thermodynamic model both in terms of bulk composition and fluid speciation.     

FURTHER STUDIES IN THE COMPOSITIONAL VARIABILITY OF COLOURLESS ROMANO‐BRITISH VESSEL GLASS*

Previous studies of colourless Romano‐British vessel glasses have suggested that, regardless of vessel type, they show considerable compositional homogeneity. Intriguing differences in variability (as opposed to mean composition) have, however, also emerged. This paper reports on a compositional study of 243 vessels, that is larger and more carefully controlled than in previous studies of this kind. Unexpected compositional differences have been found both between and within the four vessel types studied. We discuss the implications of these results in the context of different models that have been proposed for glass‐making and glass‐working in the Roman world.