Journal of the American Ceramic Society 94(8)(2011)2585-2591.

Rehydration/Rehydroxylation Kinetics of Reheated XIX-Century

Davenport (Utah) Ceramic

Patrick K. Bowen1†, Helen J. Ranck1, Timothy J. Scarlett2, and Jaroslaw W. Drelich1

1 Department of Materials Science and Engineering

2 Department of Social Sciences

Michigan Technological University, Houghton, MI 49931

Abstract

Rehydroxylation Dating (RHX dating) has recently been proposed as a new chronometric

dating tool for use on archaeological fired-clay ceramics. The technique relies upon the well-

known characteristic of reheated porous ceramic vessels to regain water, the kinetics of which

has been shown to follow a (time)1/4 power law at temperatures of 13-50°C . In this study,

experiments were conducted in which the mass measurements taken from 19th-century ceramic

artifacts revealed a deviation from the (time)1/4 power law over a wide range of temperatures.

This finding has led to the formulation of an empirical equation which describes the observed

ceramic’s rehydration and rehydroxylation behavior as an additive process in which the long-

term mass gain is dictated by a (time)1/n power law. As part of this study, the mineralogy of the

ceramics and their thermal properties have been evaluated. The instantaneous effect of humidity

on mass measurements was demonstrated to be the principal source of error.

Financial support for this study was provided by the Charles and Carroll McArthur Research

Internship program, the Michigan Space Grant Consortium undergraduate fellowship, and the

Summer Undergraduate Research Fellowship at Michigan Technological University.

†Author to whom correspondence should be directed. e-mail: pkbowen@mtu.edu

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1. Introduction

Archaeologists currently have only a limited number of techniques with which to date

ceramic artifacts including thermoluminescence dating, archaeomagnetic dating, relative dating

through use of superposition and association, analyzing stylistic elements, and, in limited cases,

carbon-14 (radiocarbon) dating of associated organic residues. The only lab techniques

applicable to most pottery sherds and archaeological ceramics are thermoluminescence and

archaeomagnetic dating. Both techniques require advanced analytical tools, trained personnel,

and therefore are expensive and not affordable to every archeological laboratory.

Recently, Wilson et al. proposed a new, simple chronometric dating technique called

fired-clay Ceramic Rehydroxylation Dating (RHX dating).1 It relies on monitoring the mass gain

of reheated porous clay objects exposed to water vapor. This low-cost technique, if it can be

successfully applied to archaeological samples, offers significant advantages over many

established ceramic dating techniques. Therefore, it is crucial that the kinetics for fired-clay

ceramics to regain water be well understood. Questions remain regarding the effects of

mineralogy, humidity, and temperature on water absorption behavior. It is the purpose of this

study to address some of these questions.

1.1. Rehydroxylation Dating

Wilson et al. observed that fired clay objects quickly regain waters of hydration and

slowly undergo rehydroxylation process in a manner that follows a (time)1/4 power law.2,3,4

Waters of hydration are physically bonded—water that is adsorbed as well as that which causes

clay interlayer swelling—while hydroxyl water is returned to the clay mineral during

reconstruction after firing in the form of structural OH groups.

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Wilson’s research team determined that the rate of mass gain and volume expansion for

fired-clay ceramic artifacts was governed by a (time)1/4 power law,1-4 described by Equation 1:

m(t) = γt14 + δ (Eq. 1)

The constant "γ" was previously called the "Rehydroxylation Rate Constant." Equation

1 also accounts for some initial mass after firing, "δ." This 1/4 power law was evident when

examining the long-term mass increase of samples at a constant temperature in the range 13-

50°C.

Using the (time)1/4 power law in conjunction with an artifact’s measured mass gain

behavior (mass gain since the sample was last fired) and the mean lifetime temperature, one is

able to calculate a calendar date for manufacture. Wilson’s team published accurate dating

results on ceramic fragments ranging in age from approximately 200 days to nearly 2000 years.

They argue that the dating technique should work over millennial timescales.

Temperature is the sole significant factor influencing the rate of water absorption by

heated porous ceramic vessels. In the initial study, the authors derived a mean temperature

calculated from temperature records from several stations in southern England.5 Wilson’s team

expressed their concern about the uncertainty that unknown temperature variations produced in

their age calculations.

1.2. Effect of Mineralogy

When fired to sufficient temperatures many clay minerals, such as illite, undergo

reversible dehydration and dehydroxylation along with other reversible and irreversible physical

and chemical changes.6,7 Other clay minerals, such as kaolinite, undergo irreversible

dehydroxylation at different temperatures as they vitrify and enter an amorphous glass phase.8

Throughout the process of firing, ceramic samples undergo chaotic, uneven changes along the

4

entire thermal cycle, particularly for pottery (fired at low temperatures, < 900°C) and ceramic

(fired at higher temperatures, < 1200°C) artifacts that include building materials like brick and

tile. The conversion of crystalline clay minerals into anhydrous materials is rarely complete,

however. Wilson et al.1 suggest that RHX dating could be used on ceramic materials with a

wide range of chemical compositions and crystalline structures produced using a wide range of

firing temperatures.

Because RHX dating procedures require measuring each sample's characteristic

rehydroxylation behavior, the technique requires no comparative standardization or

foreknowledge of the sample’s mineralogy. Wilson et al. noted that the complexities of

molecular- and nano-scale processes of diffusion that govern rehydroxylation are not fully

understood. Because of the self-calibrating nature of the technique, the unknown nanoscale

processes should not hinder the application of RHX dating.

1.3. Goal of This Study

In this study, a general equation that describes rehydration/rehydroxylation behavior of

archaeological ceramics is developed and its dependence on temperature analyzed. The impact

of humidity on instantaneous mass measurements is also addressed. The Davenport ceramic is

characterized both structurally and thermally in this study in order to identify the temperature at

which the sample experiences dehydroxylation and attempt to relate it to its constituent clays.

2. Materials and Methods

2.1. Pottery Samples

Samples for this study came from an excavation at the site of a 19th century pottery shop

owned by the Davenport family, located in the Parowan valley in Utah. The samples obtained

from this fieldwork presented an ideal opportunity to apply RHX dating because the archaeology

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team had extraordinary chronological control of the site’s features and stratigraphic deposits.

The chronology was exclusively relative, however. The development of a refined expression for

rehydration/rehydroxylation behavior would allow a dramatic enhancement of the

anthropological research questions currently being posed at the site.9-12

Ceramic sherds were recovered from numerous stratigraphic levels at the excavation site.

They were then washed with tap water and soft-bristle brushes to remove debris, dried, and

stored in an open-air container. A large (>200 g) sample was selected and broken apart into

smaller, individual pieces. Fragments of pottery selected for the rehydration study each weighed

between 32 and 96 g, while other fragments were used in various analyses to characterize the

ceramic. Table 1 shows the fragments of the large sample and their uses. Note that four of the

samples were used in rehydration/rehydroxylation experimentation.

The mass of sample 1 was close to capacity of the balance, so it was used only for a

single trial due to concerns about the accuracy of the balance when it was loaded close to

capacity. Samples 2-4 were smaller, and they were used repeatedly in order to ensure that all of

the samples had been fired at the same temperature, were precisely the same age, and had

uniform mineralogy.

2.2. X-Ray Diffraction Analysis

A sample was prepared from a fragment of pottery which was ground in an agate mortar

until fine. X-ray diffraction (XRD) experimentation was performed using a XDS-2000 (Scintag

Inc., Cupertino, California) diffractometer with a copper target (λKα = 1.540562 Å) and a

graphite monochromator. The scans were performed continuously from 10o to 85o in 2θ at a

speed of 0.2o per minute with a step size of 0.03o. The XRD instrument was run with a potential

difference of 45 kV and a current of 35 mA. The divergence slit widths were 1 mm and 2 mm

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and the receiving slits were 0.3 mm and 0.5 mm. Peaks on the collected spectrum were

identified by using the DMSNT (Scintag, Inc.) software package in tandem with the Powder

Diffraction Database Search (PDDS) software (Scintag, Inc. and Radicon, Ltd.). The PDDS

software was used to search the Joint Committee on Powder Diffraction Standards-International

Center for Diffraction Data (JCPDS-ICDD) database. The spectrum was smoothed by using a

10-point average, in an attempt to minimize the appearance of background noise.

2.3. Thermogravimetric Analysis

The thermogravimetric analysis (TGA) / differential thermogravimetric analysis (DTGA)

were performed using a Q600 (TA Instruments, New Castle, Delaware) instrument equipped

with simultaneous DSC/TGA capabilities. The instrument was evacuated and purged with

nitrogen gas before the experiment began, and the gas was circulated through the instrument at

100mL/min during the experiment. The samples were placed in alumina pans contained inside

the TGA instrument. Data was collected from room temperature up to 750oC at the rate of

2oC/min.

2.4. Drying and Re-Hydration Procedures

Removal of physically bonded absorbed water (dehydration) was carried out in the dryer

at approximately 110oC until the mass of ceramics sample had stabilized, typically after 20-30

hours. The samples were weighed using an Ohaus (Pine Brook, NJ) Adventurer AR2140

balance with 0.1 mg precision, and then placed in the furnace at 500 oC (sample 1) or 650oC

(samples 2-4) to remove chemically bonded water (dehydroxylation) for 4 (sample 1) to 12 hours

(samples 2-4). The samples were removed from the furnace after dehydroxylation, stored in the

desicator, and weighed soon after the samples had cooled to room temperature.

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The freshly dehydrated/dehydroxylated and massed samples were placed on glass Petri

dishes and exposed to air with a relative humidity of 20-28% in either: ambient laboratory

conditions (22±2oC), a drier (~80oC), or a refrigerator (low temperature between -2 and 3oC).

Both temperature and relative humidity were continuously monitored using an indoor La Crosse

(La Crosse, WI) unit (model WS-9410TWC) with temperature and humidity precisions of 0.1oC

and 0.6%RH, respectively. Temperatures and relative humidities used in rehydroxylation

experimentation are listed in Table 2 along with their standard deviations.

The samples stored in the room temperature environment were covered to prevent the

deposition of dust. Elevation of the cover allowed laboratory air to circulate freely around the

samples to maintain a constant humidity level. In selected experiments the samples remained

inside the balance for the entire time that their masses were being monitored. In these

experiments, the top cover of the balance remained closed, but the side doors were opened

slightly to allow free circulation of air. The weight of samples during their exposure to ambient

laboratory conditions was typically monitored for two to three weeks.

2.5. Statistical Analysis

Because analysis of the data required fitting it to a complicated, non-linear function, the

MATLAB nlinfit function was utilized. This function utilizes the Levenberg-Marquardt

algorithm13 to generate a non-robust, least-squares functional fit to the data. A 95% confidence

interval was generated for all of the fit parameters by using the nlparci function in conjunction

with the Jacobian matrix generated by the nlinfit function. Some data sets held at room

temperature experienced large, systematic humidity fluctuations which skewed the data in such a

way that a fit could not be performed. Note that no individual data points were eliminated during

this analysis; hence all of the data sets used in this study were analyzed in their entirety.

8

Verification of a linear relationship (a relationship with a nonzero slope) was performed

by using a two-tailed test for significance. In this test, the t-distribution was utilized, and the null

hypothesis was slope equal to zero (standard for a linear significance test). The resulting t-

statistic was then correlated to the appropriate p-value to obtain the fit significance. All

calculations of standard deviation were done in the style of a sample standard deviation, "s," (as

opposed to a population standard deviation, "σ").

3. Results and Discussion

3.1. Mineralogical Study of Davenport Pottery

X-ray diffraction analysis performed on the pottery revealed a complex spectrum,

presented in Figure 1, which is composed of many phases. Extremely large peaks were observed

at 20.9o and 26.6o. These were attributed primarily to large amount of quartz present in the

pottery sample. Interestingly, a comparison of the Parowan Valley clay deposits by Watkins14

indicated that the maximum quartz content of most raw clays in the region is 20-30%. The large

amount of quartz is likely due to either the addition of sand temper, used to change the thermal

properties of the clay15 or a byproduct of the washing/settling process used to prepare the raw

clay for use. Watkins also indicated that many Parowan Valley clay formations contain a large

(>50%) amount of plagioclase (soda-lime) feldspar. From this description, it was probable that

high-concentration anorthite (ICDD-JCPDS No. 89-1462) and low-concentration albite (84-

0752), the end compositions of plagioclase feldspars,16 could be identified. Their presence is

confirmed by the presence of characteristic peaks at 21.9o, 22.0o, and 28.0o.

After verifying the presence of plagioclase feldspar(s) and quartz , an intensified

literature search yielded a significant amount of information about other possible constituents.

High-feldspar formations in the Parowan valley were likely to contain biotite, hornblende,

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sanidine, pyroxene, and miscellaneous Fe/Ti oxides.17,18 Sanidine (77-0981) was readily

identifiable, given the high visibility of characteristic peaks at 29.8o, 27.0o, and peak asymmetry

present at 21.0o. Biotite (88-2192) was identified by the presence of small peaks at 34.1o and

41.5o. The small size of the peaks indicates that the amount of biotite is likely close to the

detection limit of the XRD instrument, which is typically on the order of 0.5%.19 The iron and

titanium oxides, hematite (87-1166) and rutile (87-0710), respectively, were also identified in

roughly the same quantity as biotite, as approximated by their characteristic peak heights.

Mullite (83-1881), an aluminum silicate, was identified by the presence of a small characteristic

peak at 16.5o. The other common constituents, hornblende and pyroxene, did not appear to be

present in this sample.

The common minerals calcite (5-586) and diopside (11-654) are also present in this

spectrum, as evidenced by the composite peak at 29.7o, the strong diopside peak at 31.3o, and the

calcite peaks at 39.3o and 43.0o. The occurrence of calcite in fired-clay pottery could be

explained by either (i) gradual weathering processes that transform dehydrated, partially

decomposed anorthite into calcite or (ii) recarbonation of calcite that was decarbonated during

the original firing process.20 Diopside was likely evolved in a complex series of mineralogical

transformations that take place at temperatures ranging from 900oC to 1100oC, as shown in the

study by Trindade et al.21

Watkins indicated that one particular clay deposit, described as "the Bauers Tuff Member

of the Condor Canyon Formation," located in the southwestern part of the Parowan Valley,

contains plagioclase feldspar (55%), sanidine (35%), biotite (7%), iron/titanium oxides (3%), and

a trace amount of pyroxene.14 This clay formation could be the one from which this pottery was

manufactured.

10

A qualitative identification of lower volume fraction clay minerals is difficult due to the

XRD detection limit, however it appears as if one minor constituent could be illite (35-0652), as

indicated by the presence of a peak at 25.7o and another, smaller, peak at 17.3o. Another possible

minor constituent is metahalloysite (74-1023), the dehydrated companion of halloysite,22 as

determined by the small peak at 20.1o and the size of the composite peak at 24.0o. This

particular meta-clay is quite rare, and the identification is tentative at best as there is no concrete

evidence that the source clay contained halloysite. Given the small amount of clay present in

these samples relative to non-hydrating or amorphous material, it is reasonable to conclude, as

Wilson et al. did, that the maximum expected mass gain over the lifetime of such a sample

would not exceed 1-2%.

3.2. Thermal Study of Davenport Pottery

The TGA and DTGA curves from a sample of Davenport pottery are presented in Figure

2. There appear to be two, distinct regions in this curve. The first is a slow, relatively constant

loss of mass that begins just after 27°C and continues to about 540°C. This loss in mass can

likely be attributed to the loss of capillary water and physically bonded water in the sample, as

well as a small amount of dehydroxylation. The major loss of mass occurs at approximately

642°C as determined by the minimum in the DTGA curve.

At 642°C, it is assumed that all of the weakly-bonded water has been removed, and the

remaining mass loss occurs as the hydroxyl water escapes from the clay minerals. This result

agrees well with the location of the second stage of mass loss peaks in several studies utilizing

TGA23 and differential thermal analysis (DTA).24,25 The DTA study of clays by Grim and

Bradley25 indicates that when many types of clays (including calcareous montmorillonites, some

illites, and many kaolinites) are fired and left to age for a period weeks (or sometimes months),

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the shape of the DTA/TGA takes on a form similar to what was observed in this

experimentation. In other words, the only significant thermal activity is observed at

temperatures around 700°C. It should be noted that decarbonation of calcite in the sample could

contribute to the sharp drop at ~642°C, although the results of Grim and Bradley point to mass

loss dominated by the escape of hydroxyl water.26,27

It is difficult to determine exactly which clay causes this type of thermal behavior. The

dehydroxylation temperature of 642°C could be due to the presence unfired halloysite, kaolinite,

pyrophyllite, or any number of montmorillonites.25 Additionally, a significant shift in

dehydroxylation temperature is often observed after clays are fired for the first time and left to

age,27 so this particular temperature may not be representative of the raw material.

The 500oC temperature used by Wilson et al. would not be sufficient to completely

dehydroxylate the samples used in this study, so a higher firing temperature was required.

Calcite, when heated to high temperatures, tends to decompose to calcium oxide by releasing

carbon dioxide. Thus, care was taken to ensure that the selected dehydroxylation temperature

would minimize decomposition. After careful consideration, a dehydroxylation temperature of

650oC was selected to precede the rehydration/rehydroxylation experimentation.

Slow dehydration/dehydroxylation behavior was observed over the course of about 2.5

weeks at a temperature just above the boiling point of water (105°C). Samples 2, 3, and 4 were

placed in the dryer prior to firing in experiment 2, and the resulting set of normalized mass-loss

data is presented in Figure 3. These data show remarkable consistency, even between samples

of very different masses. The power-law fit analysis of these data yields an average power of

0.102 ± 0.005 (95% confidence).

3.3. An Empirical Model for Rehydration/Rehydroxylation Behavior

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Rehydration/rehydroxylation behavior was observed to be similar to that presented by

Wilson et al.1 Mass gain curves for a sample held at a low temperature as well as one held at

room temperature are presented in Figure 4. The fractional mass gain of the low temperature

sample (sample 2 from experiment 4) at 250 hours is ~0.4%. In comparison, the fractional mass

gain of the room temperature sample (sample 4 from experiment 3) is ~0.6%; an increase of

50%. The drastic increase in mass gain from low to room temperature indicates that there is a

certain temperature dependence. This temperature dependence was explained by Wilson et al.1

to be thermally activated in the temperature range of 13-50°C, but this study failed to reproduce

those findings at different temperatures.

These mass gain curves in Figure 4 represent neither solely rehydroxylation (described by

the power law of Wilson et al.) nor rehydration behavior, but rather a combination of the two.

Rehydration/rehydroxylation of a ceramic sample, after it is removed from the furnace, consists

of two, distinct steps.3,4 The first event is fast penetration by atmospheric water vapor, which

adsorbs onto the surface of the pores and meta-clay, and ultimately causes the clays present in

the sample to swell. At the nanoscale, swelling behavior is caused by the diffusion of between

one and four molecular layers of water into the interlayer spacings between the clay sheets.28,29

After liquid water penetrates the sheets of the clay minerals, the chemical recombination

(rehydroxylation) begins to take place. The interlayer water in the clay sheets slowly begins to

diffuse into the nano-scale pores within the clay sheets, with diffusion likely falling into either a

Knudsen30 regime, a single-file31 regime, an anomalous diffusion (subdiffusion) regime,32 or

some combination thereof. It is unlikely that traditional (Fickian) diffusion33 would take place at

these small scales.

13

Thus, any complete empirical model attempting to quantify mass gain behavior must

consider both the short-term mass gain caused by interlayer swelling and the long-term mass

gain presumably caused by rehydroxylation. It was found through careful analysis that the initial

rehydration behavior is described rather well by the integral form of the Lagergren rate

equation.34 Use of this particular function gives the beginning of the mass gain curve the form of

inverted exponential decay. The long-range term was constructed such that the power associated

with it is treated as a fitting parameter, and therefore has not been constrained to 1/4.

The resulting expanded empirical model for rehydration/rehydroxylation is presented as

Equation 2:

m(t, T, H) = β(H)(1 − exp(−αt)) + γt1

n(T) + δ (Eq. 2)

This equation has five constants (four if considering normalized mass values, as δ=0) and

three independent variables, although two are considered to be constant during the fitting

process. The constants are defined as: α (hr-1), a kinetics constant for the Lagergren rate

equation term, β (g), a scaling factor for the first term which varies directly with humidity, γ

(g/hr1/n), which was dubbed the "rehydroxylation rate constant" by Wilson et al. and describes

the magnitude of the rehydroxylation activity, δ (g), the inherent (dehydroxylated) sample mass,

and n, which is a temperature-dependent exponent which describes the kinetics of

rehydroxylation. We will refer to n as the "rehydroxylation exponent" in this discussion.

The additive behavior of this equation is illustrated in Figure 5. Note that for the vast

majority of the data, the first (rehydration) term essentially acts as a constant. However, when

the few data points taken early in the experiment are considered, as in the Figure 5 inset, the first

term provides a rather good fit to the data. This is true of all mass gain data to which we have fit

14

Equation 2 (using nlinfit in MATLAB). The normalized fit parameters resulting from the nlinfit

analyses for all mass gain data are presented in Table 3.

3.4. Temperature and Humidity Dependence of the Empirical Model

From the fit parameters in Table 3, it can be surmised that the values of "n" (the

rehydroxylation exponent) are not constant across all temperatures. When the rehydroxylation

exponent is plotted against temperature, as in Figure 6, a strong, direct relationship becomes

apparent. When a detailed regression analysis is performed on these best-fit n and temperature

data, one can state that a linear (or some similar) correlation exists with confidence greater than

99.9%. The resulting regression line gives n(T) = 0.0386 T - 7.912, where T is the temperature

in Kelvin. This means that, according to the analysis performed, n=4 at 35 ± 2°C (standard

error) which agrees reasonably well with the conclusion drawn by Wilson et al. that n=4 at room

temperature.

This temperature-dependent behavior of the rehydroxylation exponent adds another

dimension to the RHX dating technique. If ceramics (or other fired-clay items) are stored in an

extreme-temperature environment, outside the range of temperatures that have been considered

in other studies, then a variation in "n" could have a large impact on the dating results. In

addition, we wonder about the role played by high pressure in rehydroxylation of ceramics from

underwater archaeological sites, particularly those located in deepwater maritime environments

or those deposited in terrestrial contexts where the average ambient temperature underground

varies considerably from the average atmospheric temperature.

Changes in humidity produced instantaneous fluctuations in the measured mass, and are

demonstrated here to be the principal source of error in mass measurements. This is illustrated

qualitatively by Figure 7, which contains the mass and humidity data collected for sample 2 in

15

experiment 2. When the residual mass values from the latter part of the curve are plotted against

the humidity deviation (departure from the mean humidity value), as in the Figure 7 inset, a

linear relationship is observed. This shows that tight humidity control in the laboratory setting is

absolutely essential for RHX dating to become a viable option for the dating of ceramics.

When a linear regression analysis is performed, a nonzero slope is confirmed with 90%

confidence, which is sufficient for a simple analysis. This analysis gives the fractional mass

change per percent relative humidity to be 4.92x10-5 %RH-1. A linear relationship would imply

that only a first-degree term, such as β, is affected by these humidity fluctuations. Thus, the

error at 95% confidence associated with any instantaneous mass measurement becomes ±1.96s,

where s is the sample standard deviation (if humidity is treated as a nonvariant statistic). In this

case, any measurement can then have an associated deviation in humidity of ±2.74 %RH, or

(using the result from the linear regression) an error in fractional mass of ±1.35x10-4 (about 2.3%

of the total fractional mass gain).

4. Conclusions

The results of this study indicate that temperature, humidity, and mineralogy must all be

taken into account when quantifying the kinetics of rehydration/rehydroxylation. Experimental

results collected from samples that were rehydrated/rehydroxylated over a wide range of

temperatures have shown that the 1/4 power law of Wilson et al. does not accurately describe

rehydroxylation kinetics at temperatures significantly different than room temperature. It has

also been demonstrated that the proposed expanded empirical model for

rehydration/rehydroxylation provides a good fit to data over time periods of up to three weeks.

The effect of humidity on the instantaneous mass of the ceramic sample appears to be

significant. The mass of a typical ceramic sample appears to increase by about 0.005% per 1%

16

increase in relative humidity. Thus, exceptionally precise humidity control must be exercised

when rehydroxylation dating is applied to archaeological ceramics, and any systematic humidity

fluctuations must be accounted for during subsequent error-propagation analyses.

Dehydroxylation is impacted significantly by the sample’s mineralogy, so care must be taken to

select a temperature that will remove all of the hydroxyl water contained in the sample while

maintaining the integrity of all of the other constituents. This can be accomplished by

performing TGA analysis.

Future work will be conducted under tightly controlled humidity and temperature

conditions to assess: (i) the temperature and humidity dependence of the Lagergren kinetics

constant and scaling constant and (ii) the temperature and humidity dependence of the

rehydroxylation rate constant when the power is not fixed at 1/4. After the temperature

dependence of these parameters is quantified, the empirical model presented in this study should

be expanded in order to accommodate a more general description of temperature.

5. References

1M. A. Wilson, M. A. Carter, C. Hall, W. D. Hoff, C. Ince, S. D. Savage, B. McKay, and I. M.

Betts. "Dating fired-clay ceramics using long-term power law rehydroxylation kinetics," Proc. R.

Soc. London, Ser. A, 465 (2108), 2407–2415 (2009).

2M. A. Wilson, W. D. Hoff, C. Hall, B. McKay, and A. Hiley, "Kinetics of moisture expansion in

fired clay ceramics: A time^1/4 law," Phys. Rev. Lett., 90 (12), 1–4 (2003).

3S. D. Savage, M. A. Wilson, M. A. Carter, B. McKay, and W. D. Hoff, "Mass gain due to the

chemical recombination of water in fired clay brick," J. Am. Ceram. Soc., 91 (10), 3396–3398

(2008).

17

4S. D. Savage, M. A. Wilson, M. A. Carter, W. D. Hoff, C. Hall, and B. McKay, “Moisture

expansion and mass gain in fired clay ceramics: a two-stage (time)^1/4 process,” J. Phys. D.:

Appl. Phys., 41, 055402 (2008).

5D. E. Parker, T. P. Legg, and C. K. Folland, "A new daily central England temperature series,"

International Journal of Climatology, 12, 317–342 (1992).

6L. Heller, V. C. Farmer, R. C. Mackenzie, B. D. Mitchell, and H. F. W. Taylor, “The

dehydroxylation and rehydroxylation of trimorphic dioctahedral clay minerals,” Clay Minerals

Bull., 5, 56-72 (1962).

7K. Traore, F. Gridi-Bennadji, and P. Blanchart, “Significance of kinetic theories on the

recrystallization of kaolinite,” Thermochim. Acta, 451, 99-104 (2006).

8W. E. Worrall, Clays and ceramic raw materials, Elsevier Applied Science Publishers, 2nd

edition, 1986.

9J. Montcalm, A Burning Question: Archaeology at the Davenport Pottery and Technological

Adaptation in the Mormon Domain, MS thesis, Michigan Technological University, Houghton,

2010.

10T. J. Scarlett, Potting In Zion: Craft and Industry in Utah, 1848–1930, PhD thesis, Department

of Anthropology, University of Nevada, Reno, 2002.

11T. J. Scarlett, Trade and Exchange: Archaeological Studies from History and Prehistory,

"What if the Local is Exotic and the Imported Mundane? Measuring Ceramic Exchanges in

Mormon Utah," Springer Verlag, New York, NY, 2010.

12T. J. Scarlett, R. J. Speakman, and M. D. Glascock, "Pottery in the Mormon economy: an

historical and archaeometric study," Historical Archaeology, 41 (4), 70–95 (2007).

13G. A. F. Seber and C. J. Wild, Nonlinear Regression, Wiley-Interscience, Hoboken, NJ, 2003.

18

14C. N. Watkins, Parowan pottery and fremont complexity: Late formative ceramic production

and exchange, Master’s thesis, Brigham Young University, 2006.

15A. O. Shepard. Ceramics for the Archaeologist. Carnegie, Washington, D.C., 1985.

16A. N. Winchell, "Studies in the feldspar group," The Journal of Geology, 33 (7), 714–727

(1925).

17J. J. Anderson and P. D. Rowley, "Cenozoic stratigraphy and of southwestern high plateaus of

Utah," Special Paper 160, Geologic Society of America, 1975.

18P. L. Williams, Stratigraphy and petrography of the Quichapa group, southwestern Utah and

southeastern Nevada, PhD Thesis, Unpublished, University of Washington, Seattle, 1967.

19F. H. Chung and D. K. Smith, Industrial applications of X-ray diffraction, CRC Press, 1999.

20S. Shoval, P. Beck, Y. Kirsh, D. Levy, M. Gaft, and E. Yadin, "Rehydroxylation of clay

minerals and hydration in ancient pottery from the ’Land of Geshur’" J. Therm. Anal., 37, 1579–

1592 (1991).

21M. J. Trindade, M. I. Dias, J. Coroado, and F. Rocha, "Mineralogical transformations of

calcareous rich clays with firing: A comparative study between calcite and dolomite rich clays

from Algarve, Portugal," Appl. Clay Sci., 42, 345–355 (2009).

22G. W. Brindley, K. Robinson, and D. M. C. MacEwan, "The clay minerals halloysite and meta-

halloysite," Nature, 157 (3982), 225–226 (1946).

23A. F. Gualtieri and S. Ferrari, "Kinetics of illite dehydroxylation," Phys. Chem. Miner., 33,

490–501, (2006).

24R. E. Grim and R. A. Rowland, "Differential thermal analysis of clay minerals and other

hydrous materials," Am. Mineral., 27 (11,12), 746–761, 801–818 (1942).

19

25R. E. Grim and W. F. Bradley, "Rehydration and dehydration of the clay minerals," Am.

Mineral., 33 (1), 50–59 (1948).

26S. Shoval, “Using FT-IR spectroscopy for study of calcareous ancient ceramics,” Optical

Materials, 24, 117-122 (2003).

27S. Shoval, M. Gaft, P. Beck, and Y. Kirsh, “The thermal behavior of limestone and

monocrystalline calcite tempers during firing and their use in ancient vessels,” J. Therm. Anal.,

40, 263-273 (1993).

28H. vanOlphen, An Introduction to Clay Colloid Chemistry, John Wiley & Sons, 1963.

29E. J. M. Hensen and B. Smit, "Why clays swell," J. Phys. Chem. B, 106 (49), 12644–12667

(2002).

30K. Malek and M. O. Coppens, "Knudsen self- and Fickian diffusion in rough nanoporous

media," J. Chem. Phys., 119 (5), 2801–2811 (2003).

31D. G. Levitt, "Dynamics of a single-file pore - non-Fickian behavior," Pys. Rev. A, 8 (6), 3050–

3054 (1973).

32P. Castiglione, A. Mazzino, P. Muratore-Ginanneschi, and A. Vulpiani, "On strong anomalous

diffusion," Phys. D (Amsterdam, Neth.), 134 (1), 75 – 93 (1999).

33D. R. Gaskell, An introduction to transport phenomena in materials engineering, Macmillan

Pub. Co., 1992.

34S. Lagergren, "Zur theorie der sogenannten adsorption geloster stoffe," K. Sven.

Vetenskapsakad. Handl., 24 (4), 1–39 (1898).

6. Acknowledgements

We would like to thank Lakshmi Krishna and Ed Laitila for their assistance with the x-

ray diffraction analysis, Lei Zhang for her help in performing the TGA experimentation, and

20

David Hand for an intellectually stimulating discussion on the topic of adsorption/diffusion.

Thanks, also, to Kim Bowen and Emily Shearier for their time spent editing and proofreading.

We would also like to acknowledge the 2009 archaeological field team; Steen and Mark

Matheson of Parowan, Utah; Todd Prince, Park Manager, Frontier Homestead State Park

Museum and Karen Krieger, Deputy Director Administration, Utah State Parks, Department of

Natural Resources for their assistance in obtaining the samples and information necessary to

perform this study.

21

Fig. 1. X-Ray diffraction results for the Davenport pottery sample. Peaks are labeled by the

corresponding phase: Al-albite, An-anorthite, B-biotite, C-calcite, D-diopside, H-

metahalloysite, He-hematite, I-illite, M-mullite, Q-quartz, R-rutile, and Sa-sanidine.

Fig. 2. Thermogravimetric analysis and differential TGA curves for a 19th century

archaeological (Davenport) ceramic sample.

Fig. 3. Observed dehydration mass curve for samples 2 (square), 3 (circle), and 4 (diamond) for

drying performed prior to experiment 2. The inset (same axes and units) is a log-log

presentation of the data, which uses absolute fractional mass. These mass-loss data

exhibit both dehydration and slow dehydroxylation behavior, as evidenced by their non-

asymptotic behavior.

Fig. 4. The observed rehydration/rehydroxylation mass gain curve for sample 4 in experiment 3

(triangle, solid line), which was held at room temperature and sample 2 in experiment 4

(circle, dotted line), which was held at a low temperature.

Fig. 5. Illustration of the additive behavior of the empirical equation developed to describe

rehydration/rehydroxylation behavior of sample 2 in experiment 4. The solid line

represents the total mass predicted, the dashed line is the first term describing rehydration

behavior, and the dashed-dotted line is the second term, describing rehydroxylation. The

inset image (same axes and units) illustrates the behavior of the empirical equation and its

components over short time period.

Fig. 6. Temperature dependence of the rehydration/rehydroxylation exponent, n, in

rehydroxylation experimentation. Circles indicate rehydroxylation processes, and

triangles represent dehydration. Note that n~4 at room temperature (298 K, or 25°C).

22

Horizontal error bars represent one standard deviation in temperature and vertical error

bars represent the 95% confidence level of the rehydroxylation exponent.

Fig. 7. Visualization of the correlation between fluctuations in humidity and in observed mass

for sample 2 from experiment 2. The solid vertical and dashed lines represent peaks and

depressions in mass and humidity, respectively. The inset figure illustrates the effect of

humidity fluctuations on the deviation of the instantaneous from the mass predicted by

using Equation 2.

23

Table 1: Samples used for experimentation and characterization

Sample Purpose Mass (g)

Sample 1 Rehydration/Rehydrox. 96.0

Sample 2 Rehydration/Rehydrox. 58.7

Sample 3 Rehydration/Rehydrox. 34.5

Sample 4 Rehydration/Rehydrox. 32.5

XRD Fragment Powder Diffraction ~5

TGA Fragment TGA Experimentation 0.024

Table 2: Experimental and environmental parameters for each sample

Experiment Sample T Range DHX T (°C)

Mean T ± s (°C)

Mean RH ± s (% Rel. Humidity)

1 1 Room 500 ~23† -

2 - DHX 2 Dryer - 105 ± 7.00‡ -

2 - DHX 3 Dryer - 105 ± 7.00‡ -

2 - DHX 4 Dryer - 105 ± 7.00‡ -

2 - RHX 2 Room 650 22.7 ± 0.18 21.0 ± 1.4

2 - RHX 4 Room 650 22.6 ± 0.24 20.0 ± 1.1

3 2 High 650 80 ± 7.00‡ -

3 3 Low 650 -1.3 ± 0.73 27.7 ± 2.6

3 4 Room 650 22.9 ± 0.15 24.7 ± 2.8

4 2 Low 650 3.4 ± 0.32 27.6 ± 0.3 † Estimated temperature ‡ Estimated associated error

24

Table 3: Normalized (fractional) rehydration/rehydroxylation fit parameters for all valid

mass gain data. Errors for n are at 95% confidence.

Experiment Sample T Range α (hr-1) β (g/g) γ (hr-1/n) n

1 1 Room 0.3496 0.0007 0.0003 3.5 ± 0.8

2 - DHX 2 Dryer 0.0101 0.0004 -0.0032 9.7 ± 0.8

2 - DHX 3 Dryer 0.0087 -0.0001 -0.0030 9.5 ± 0.3

2 - DHX 4 Dryer ~0.0000 ~0.0000 -0.0032 10.3 ± 0.4

2 - RHX 2 Room 0.2786 0.0011 0.0002 3.0 ± 1.0

2 - RHX 4 Room 0.7108 0.0010 0.0003 3.8 ± 1.6

3 2 High -† -† 0.0004 5.7 ± 0.4

3 3 Low 0.3347 0.0010 0.0002 2.9 ± 0.4

3 4 Room 0.0039 0.0041 0.0008 3.8 ± 1.1

4 2 Low 0.4601 0.0013 0.0003 2.3 ± 0.3 † Sample 2 from experiment 3 required a fit with 2 alpha and beta parameters

25

Figure 1

Figure 2

5

15

25

35

45

10 12 14 16 18 20 22 24 26 28 30 32 34

Inte

nsity

(CPS

)

Double Angle, 2θ (o)

Q Q

An/AlM I

An H

SaA

n Al

Sa/A

nSa

/An H/A

l

IM

Sa

R

C/D

R/A

n/A

l

Sa

An

D

SaQ/He

B

-0.0006

-0.0004

-0.0002

0.0000

-0.04

-0.03

-0.02

-0.01

0

0.01

350 450 550 650 750 850 950

Diff

eren

tial M

ass (

K-1

)

Nor

mal

ized

Mas

s, ∆m

/m0

(g/g

)

Temperature (K)

DTGA

TGA

26

Figure 3

Figure 4

-0.006

-0.005

-0.004

-0.003

-0.002

0 50 100 150 200 250 300

Frac

tiona

l Mas

s, ∆m

/m0

(g/g

)

Dehydration/Dehydroxylation Time (hr)

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 50 100 150 200 250 300 350 400 450 500

Nor

mal

ized

Mas

s, ∆m

/m0

(g/g

)

Time from Dehydration/Dehydroxylation (hr)

Sample 4, Experiment 3Room Temperature (22.8°C)

Sample 2, Experiment 4Low Temperature (3.4°C)

0.102 ± 0.005

0.001

0.010

1 100

|∆m

/m0|

27

Figure 5

0.000

0.002

0.004

0.006

0.008

0 300 600 900 1200 1500 1800

Nor

mal

ized

Mas

s, ∆m

/m0

(g/g

)

Time from Dehydration/Dehydroxylation (hr)

0.000

0.001

0.002

0.003

0 10 20 30

28

Figure 6

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

265 280 295 310 325 340 355 370 385

Reh

ydro

xyla

tion

Exp

onen

t, n

Temperature (K)

Rehydration/Rehydroxylation

Dehydration

29

Figure 7

6

9

12

15

18

21

24

27

57.80

57.84

57.88

57.92

0 50 100 150 200 250 300 350

Rel

ativ

e H

umid

ity (%

RH

)

Sam

ple

Mas

s (g)

Time from Dehydroxylation (hr)

4.92E-05 %RH-1

-0.00015

0.00015

-1 0 1 2

Frac

. m R

es.

Humidity Deviation (%RH)

Mass

Humidity

30