1. Carl P. Lipo, Hector Neff, and Jacob Kovalchik
The Promise and Practicalities of Rehydroxylation Dating for Prehistoric Ceramics
Carl P. Lipo, Hector Neff, and Jacob Kovalchik
IIRMES and the Department of Anthropology, California State University Long Beach
Introduction
RHX Method
Results
Of all the measurements that one can make in the archaeological record, recording attributes that allow one to relate a set of attributes for an object in time is perhaps the most important. Over the years, archaeologists have made use of a wide array of dating techniques: radiocarbon (14C) dating, luminescence dating, tree-ring dating, obsidian hydration, electron spin resonance, uranium/thorium series dating and so on. Each of these techniques has applicability in dating past events in some area of the archaeological record though they are generally limited in the range of time for which they are applicable or the material upon which they can be used.
Recently, a group of material scientists (Wilson et al. 2009) has demonstrated the potential of a new technique -- rehydroxylation -- for dating ceramics. The technique (here referred to as RHX dating) uses the measurable rate of hydroxyl (OH molecules) absorption into the fabric of ceramics as a means of dating the last high temperature (> 500 degrees C) firing event, which is usually the event of manufacture. Preliminary results indicate that this technique is potentially able to produce relatively high precision dates yet requires only relatively simple measurements and instruments. Configured for large numbers of simultaneous measurements, RHX dating has the potential to radically increase our ability to date ceramics by dramatically reducing the per sample cost and by making the technology accessible for a wide range of laboratory settings.
Here, we describe our efforts to develop instrumentation and protocols that are optimized for efficient production of RHX measurements on ceramics at low cost and with high throughput. Our goal is to replicate existing research and to determine the boundary conditions on sample size, weight precision, temperature, and humidity control. In parallel, we are developing an instrumented system that combines the essential aspects of the RHX method yet maximizes the rate of sample throughput and minimizes the cost of the analyses.
Figure 2: Schematic of Rehydroxylation Process
As a result of hydration and rehydroxylation, RHX is a two-stage process. The first (fast) begins immediately after firing and takes a day or two. During the first stage, clay regain water through hydration. This hydration results in a relatively rapid weight gain. If humidity is held constant, weight gain due to hydration will cease once the sample is in equilibrium.
Once hydration is complete, weight gain of a sample is dominated by just rehydroxylation. The rate of this process is dependent on temperature. As long as some water is available, rehydroxylation will occur at a constant rate. The weight gain caused by rehydroxylation is linear with time1/4. If one can measure the rehydroxylation rate at the same mean temperature at which the sample was deposited/buried, then that rate can be used to calculate the age of the sample since firing.
Figure 7: RHX Data for Plumbate Sherd (Guatemala, Terminal Classic, ~ AD ). RHX Age = AD 1195 AD
Figure 6: RHX Data for Late Mississippian Shell Tempered Sherd (NE Arkansas, AD ). RHX Age = AD 1490
RHX Dating is based on the process of rehydroxylation:
Both processes commence immediately after firing.
Rehydroxylation rate linear with time1/4.
Stage 1 (rehydration) ends after a few hours or days.
Stage 2 (rehydroxylation) continues indefinitely.
Stage 1 is physical adsorption (Ince 2009).
Stage 2 is chemical reaction dependent on temperature (Ince 2009). Thus, temperature must be held constant during measurement of rehydroxylation rate.
Rehydroxylation rate is not dependent on absolute levels of humidity though humidity must be held constant during measurement to hold hydration in equilibrium.
removes hydroxyls and returns fabric to as-fired
Age calculated on the basis of weight loss from dehydroxylation during firing and the measured rate of weight gain due to rehydroxylation.
Figure 8: RHX Data for Plumbate sherd (Guatemala, Terminal Classic, ~AD 800 – 1000). RHX Age = AD 842
In our initial experiments, we have been able to successfully date several sherds. Dates are consistent with expectations.
Not all experiments, however, have been successful. In many cases, we have obtained dates that are far too young.
Principles
Figure 9: RHX Data for Late Mississippian shell tempered sherd. RHX Age = AD 1939.
Muscovite KAl2AlSi3O10(OH)2 O
OH- (Hydroxyl)
AI+3
Si+4
Kaolinite Al2Si2O5(OH)4
OH- (Hydroxyl)
Mg+2
Hydroxylation is a chemical process in which OH molecules bind to the crystal lattice of clay particles (Figure 1). When clay is fired, hydroxyls are driven off (dehydroxylation). Some of the dehydroxylation is nonreversible. As the temperature of the firing increases, the extent of the reversible dehydroxylation decreases and the extent of the nonreversible dehydroxylation increases. At high firing temperatures, the clay crystals break down and the structure becomes amorphous – at this point the dehydroxylation is permanent.
However, for relatively low fired ceramics such as those found in most prehistoric contexts, much of the clay structure is retained and over time OH molecules will rebind to the crystal structures. This process is rehydroxylation.
Rehydroxylation is distinct from rehydration. Rehydration is a physical process in which water (H2O) is absorbed by the clay matrix (in 2:1 clays this causes interlayer swelling).
Table 1: Steps in Rehydroxylation Dating of Ceramics
Complications
Step
Description
1. Clean and dry ceramic sample.
Brush sherd clean. Heat sherd to 105 °C and hold for > 1 hour. Removes dirt and liquid water from sample to provide consisting starting condition. Using the VTI-SA, samples can be up to 2 grams in weight. In our environmental chamber, samples as large as 50 grams can be measured. Generally, the larger the sample, the better.
2. Condition sample at set relative humidity (RH) and temperature.
Relative humidity (RH) can be established at any level (e.g., 30%) though must be held constant (with variance <1%) over the course of measurements. Temperature is set at the same estimated for the deposition conditions. This is usually the mean annual temperature for the deposit location (e.g., 28 °C for Guatemala, 16.8 °C for NE Arkansas).
3. Record initial weight.
Equilibrate sample sherd at the mean annual temperature from the location in which the sherd was found. Relative humidity must be constant.
4. Dehydroxylate (Firing).
After sherd stops gaining weight during equilibration, it is ready to be fired at 500 degrees Celsius to burn off the water hydroxyls. Sample is fired for at least four hours in furnace. Higher temperatures and longer firing times may be warranted (Figure 10)
5. Rehydroxylation.
Return sample to conditions with relative humidity and temperature as used previously. Record weight gain until Stage 2 slope and intercept can be estimated. Measurement time ranges from 2 to 14 days. During this time, it is vital that the temperature and humidity remain constant.
Composition
One factor that likely contributes to erroneous results is due to composition of the ceramics, specifically in calcareous ware (e.g., Mediterranean tableware, amphorae, architectural tiles) and ceramics with shell temper.
In calcareous ware and shell temper, complications come from the “lime cycle”
Decarbonation ~650° (CaCO3 + heat  CaO + CO2)
uptake of water (CaO +H2O  Ca(OH)2 + heat)
reformation of calcite (Ca(OH)2 + CO2  CaCO3)
removes OH from fabric
Above ~900o, CaO acts as flux, promoting formation of amorphous (glass) material and high-temperature Ca-silicates, resulting in loss of potential for rehydroxylation
Two-stage time1/4 behavior seems to apply to calcareous fabrics, but our results so far don’t agree with presumed age
This may be due to initial decarbonation resulting in excessive weight 500°C.
This effect would produce dates with overestimated ages.
Firing Temperature and Time
While Wilson et al. (2009) report the need to fire ceramics for 4 hours, continued weight loss is observed when firing is extended to >12 hours. This effect would result in underestimated ages.
Also possible that samples need to be fired to temperatures higher than 500 degrees °C to remove OH to pre-fired state (Bowen et al. 2011).
However, as temperatures approach 650 degrees, there is significant weight loss due to decarbonation.
Temperature and Humidity Control
Rehydroxylation is very sensitive to variability in temperature. Even small changes in temperature will result variable rates of weight gain and thus dates (Figure 11). While changes in humidity does not affect the rate of hydroxylation, it will affect immediate weight of sample and thus must be controlled. Bowen et al. report 0.05% change in weight for each 1% difference in RH. Care must be especially taken when measurement times are long.
Hydration
While measuring changes in weight due to rehydroxylation can be done more precisely with larger samples, it may take much longer (4+ days) for samples to reach hydration equilibrium. This overlap requires greater measurement time since weight gain due to rehydroxylation cannot be measured until hydration has completed (Figure 12). Some type of post-fire quenching prior to measurement may be required.
200
400
600
800
1000
Temperature (°C)
Figure 10: TGA Data from Marseilles roof tile from late 19th century hacienda, Merida, Yucatan. Note how weight loss continues past 500 °C. In this sample, dehydroxylation may overlap with decarbonation of CaCO3.
Figure 1: Hydroxyl binding locations in the crystal structure of two kinds of clay: muscovite (illite) (top) and kaolinite (bottom). The red circles represent the location of OH molecules.
Instrumentation
Our primary experimental instrument is a TA Instruments VTI-SA vapor sorption analyzer. Vapor sorption analyzers are designed to measure weight changes under precise temperature and humidity conditions and are common instruments in material science and pharmaceutical industries. The VTI-SA uses an aluminum block to maintain constant temperature (within ±0.1°C) by Peltier control elements. Humidity is controlled using a chilled mirror dew point analyzer. The VTI-SA balance is sensitive to 0.1 microgram and has a 5g capacity. We also employ a TA Instruments Q50 Thermogravimetric Analyzer (TGA) for measuring dimensions of ceramic pyrotechnology (e.g., firing temperature).
RHX Research
Figure 11: The rate of rehydroxylation is very sensitive to variation in temperature. Instruments must be carefully isolated from external temperature changes.
RHX Environmental Chamber
Our research project includes the development of a means for simultaneous making multiple RHX measures. To do this, we have constructed an environmental chamber that maintains constant temperature humidity and that houses a high-precision balance and an auto-sampler. Work on the environmental chamber is in progress.
Mettler-Toledo XP56 Microbalance, 52-gram capacity, repeatable measurements to 1 µ grams
Humidifier/Dehumidfier (<1% RH stability)
Five TAC Peltier coolers/heaters (12-35 °C, stability)
Humidity, temperature, pressure monitoring
Custom XYZ robot for automatic sample handling and weighing
Custom computer logging and control program (Python)
Groups
University of Manchester/University of Edinburgh group (Wilson, Hall and colleagues
Original pioneers.
Validation studies and continued study of underlying process
Wilson et al. (2009
Tel Aviv University group (Moinester, Piasetzky, and colleagues)
Validation and development for archaeological research in Israel
IIRMES, CSULB (Lipo, Neff)
Validation and development for archaeological research in New World
Michigan Tech University (Bowen, Ranck, Scarlett, and Drelich)
Experimental study of effects of mineralogy, humidity, and temperature on rehydration/rehydroxylation process
Bowen et al (2011)
History
RHX Dating based on earlier work (Wilson et al. 2003) that showed moisture expansion in fired-clay bricks to be governed by a time1/4 law.
Savage et al. (2008) showed that mass gain is the more fundamental process
Ceren Ince (Ph.D. Thesis, University of Manchester, 2009) provides detailed history and discussion of the two-stage, temperature-dependent process.
Wilson et al demonstrate potential of method by dating historic tiles and bricks ranging from 200 days to 2000 years in age.
Figure 3: (Left) The VTI-SA vapor sorption analyzer. (Above) Measurement chambers and samples hanging from balance (left side) with counter weight (right). At the bottom are polonium anti-static modules.
Figure 12: Slow hydration rates may overlap with rehydroxylation and cause overestimated rates and underestimated ages.
Figure 4: Configuration of high throughput rehydroxylation dating system
References Cited
For references cited and for more information, please see:
Figure 5: (Left) Mettler-Toledo XP56 Microbalance. (Center) Custom XYZ robot. (Right) Environmental chamber.
Research was made possible by funding from NSF (BCS )