reproducibility in rehydroxylation of ceramic artifacts

The following is a self-archived preprint version of S. Zhao, P. K. Bowen, J. W.

Drelich, and T. J. Scarlett, Reproducibility in rehydroxylation of ceramic

artifacts, J. Am. Ceram. Soc., DOI: 10.1111/jace.13745.

Reproducibility in Rehydroxylation of Ceramic Artifacts

S. Zhao,1 P. K. Bowen,

1, J. W. Drelich,

1* T. J. Scarlett

2

1 Department of Materials Science and Engineering

2 Department of Social Sciences

Michigan Technological University

Houghton, MI 49931, USA

Abstract

Since its introduction in 2009, application of the rehydroxylation (RHX) technique for dating

fired-clay ceramics has been controversial, with very few satisfactory dating results collected in

the interim. The stability and efficiency of this technique has been called into question by several

investigators in the last few years, who have struggled to reproduce and validate this new dating

method. Based on our new mass gain measurements for ca. 2000-7000 years old ceramic

artifacts, the reproducibility in the RHX process rate is analyzed and discussed. Timespan

analysis was performed, and age uncertainty related to RHX dating technique was evaluated by

considering the error propagation. The results show poor reproducibility of the RHX process in

the samples of the same origins, which give new evidence for a revision of the RHX protocols.

Keywords: rehydroxylation; fired clay ceramic; reproducibility; data scatter

*Authors to whom correspondence should be addressed, email: [email protected] and

[email protected]

2

Introduction

Rehydroxylation (RHX) dating was proposed in 2009 as an easy and reliable technique in dating

sherds of unglazed archaeological pottery. 1 It relies on monitoring the rate at which water is

regained by refired ceramics through rehydration (Stage I) and rehydroxylation (Stage II)

processes, 2 and then modeling the Stage II process by a power law-correlation:

1

4, where

is the change in sample mass and t is the time after firing or reheating of the ceramic. 3, 4, 5

The mass gain term, m, can be defined in terms of measured mass (in grams) or as a fractional

mass gain value relative to the starting mass of the sample (g/g); the latter is used here. The

roots of this power law correlation have been proposed to originate from a single file diffusion

mechanism 6 for water molecules migrating through the clay and meta-clay structures in

ceramics. Under these assumptions, the time elapsed since the last firing of the ceramic artifact

can be estimated from the accurate measurement of Stage II mass-gain kinetics. 1

Since its introduction, great effort has been invested in this new dating method by several

archeological and materials research groups. Unfortunately, only ten fired clay fragments have

been convincingly shown to be successfully dated, 7 which is a surprisingly small number.

Attempts by others have failed, 7, 8, 9, 10, 11

and major concerns about this new dating technique

continue to include: i) the stabilization of sample mass after the removal of physically bonded

water; ii) the universality of reheating conditions used to remove chemically bonded water; iii)

the stability and self-calibrating ability of the technique; 8 and iv) the validity of the single file

diffusion mechanism, which provides a basis for the 1

4 power law. 12

Very recent studies

demonstrate that appropriate chemical pre-treatment of archaeological material and/or the

theoretical limits to rehydroxylation dating could be helpful in optimizing instrument design. 13

Additionally, the relative insensitivity of the rehydroxylation rate to short-term humidity

3

fluctuations was demonstrated, providing strong, but still qualitative, experimental support for ceramic

artifacts dating through examination of the rehydroxylation process. 14

To succeed with the RHX dating method, at a minimum, the following conditions need to be met:

a) the mass of non-refractory component from as-received samples must be negligible to

avoid the release of non-water constituents upon refiring;

b) excess capillary water is removed during drying prior to re-equilibration at

experimental conditions specified under c;

c) the sample is well equilibrated (attains a constant mass) in a constant humidity at a

temperature that represents effective lifetime temperature (ELT) of the artifact at the

archeological site;

d) all water, including the hydroxyl water that is chemically bonded to clays, is totally

removed during refiring;

e) the water gains are obtained for refired ceramic under constant humidity and

temperature (the same as in step d); and

f) the rate water pick up fits the power law 1

4 .

Although these conditions are quite well recognized by the research community working on the

RHX dating method, little was done to critically examine the effects these conditions have on

dating. Most of the research followed blind protocols of set time and temperatures during the

experimentation suggested in original publication by Wilson et al., 1 and publications that

followed from the same research group. 3, 4, 5, 15, 16, 17, 18

Because no other research team was able

to successfully date ceramic artifacts through the protocols proposed by Wilson et al., the RHX

dating method remains controversial. Only poorly reproducible results were reported so far, 8, 9, 10,

4

14, 19 with many requiring variations on the original

1

4 power law. In our previous contributions,

we have used a generalized power law (1

) with a variable exponent (n), to describe the RHX

behavior of ceramics without identifying their ages. 10

The vs. 1

curves produced for several

samples demonstrated improved linearity in RHX Stage II. 9, 12

The purpose of this contribution is to examine the reproducibility of the Stage II kinetics in the

RHX dating protocol for a group of small sherds from the same parent fragment after refiring at

500oC. The intragroup repeatability was examined for four such sets of ceramic artifacts, all

treated identically. Understanding the reproducibility of RHX dating raises new questions about

the validity of the dating protocol suggested in the past.

Experimental

In our study, four sets of sherds of fired clay ceramics from Mediterranean locations were

examined (Table 1). Ceramic samples were collected and provided for this research by

Prof.Kostalena Michelaki from Arizona State University. The sherds were cleaned to remove

loose debris and dirt, and washed with deionized water. Larger fragments, typically square-

shaped cross sections, were cut into approximately 10 10 5 mm pieces with a water-cooled

diamond saw. Masses of the final samples varied from 0.4 to 1.0 g. One fragment of each set of

samples of the same origins was dried at 110C for 5 to 7 days as a control sample. Four to six

other samples were furnace dried overnight at 300C. This elevated drying temperature was used

here to speed up the drying process. We found previously (unpublished) that some ceramic

artifacts hold some amount of physically bonded water very strongly and drying them at 110-

120oC is not always sufficient to remove the totality of physically bonded water.

5

After drying, all samples were placed in a glove box (Coy Laboratory Products, Grass Lake, MI)

equipped with a Citizens CM-11 microbalance (Citizen Scale Inc., Edison, NJ) having 1 g

precision. The glove box was set for 202% RH (relative humidity) for the duration of the whole

experiment and located in a lab with an average ambient temperature of 202C. The mass of

each sample was recorded for several days until it reached its equilibrium mass, previously

defined as 2 by Wilson et al., 15

characterized by measurement-to-measurement mass changes

no larger than 0.01%. Then all samples were fired in a box furnace at 500C for 1 to 3 days and

placed back into the glove box, again at 20C and 20% RH. Samples were labeled with their

drying temperature and refiring times; for example, 300C-2a would mean that the sample was

heated at 300C, equilibrated to find 2, and then fired for two days at 500C. The refired

ceramic mass 0 was critical to all other experimentation, as it was the value to which changes

in mass were normalized to yield a fractional value of . Sample weight was monitored

periodically after refiring; every 15 min in the first couple of hours, then every 1-2 hours, and

finally at least once a day for several weeks. X-ray diffraction (XRD) measurement was

performed on a small sample surface by using Scintag XDS2000 / X-ray diffractometer with

Cu K radiation ( = 1.540562 ). The scans were performed continuously from 15 to 70o in 2

at a speed of 0.6o/min with a step size of 0.02

o. The XRD instrument was operated at a tube

potential of 45 kV and 35 mA filament current. Peaks were identified using the DMSNT (Scintag

Inc.) software package in concert with the Powder Diffraction Database Search (PDDS) software

(Scintag Inc. and Radicon Ltd., St. Petersburg, Russia). The PDDS software was used to search

the Joint Committee on Powder Diffraction Standards-International Center for Diffraction Data

(JCPDS-ICDD) database.

6

Statistical Analysis

Linear regression analysis was used in this study to analyze the linear, Stage II portion of the

versus 1

4 curves.

Timespan analysis was performed by comparing the RHX rate () against the 1

4. The RHX rate

() was calculated by the slope of the fractional RHX mass gain with respect to 1

4, and was

calculated at intervals that began with the last data points and was gradually expanded to include

data back to t = 0 (using the slope from the end approach). 7 In this presentation, values

calculated near the end of the experiment experience large perturbations due to the small number

of data points considered; as the timespan analysis approaches the transition to Stage II mass

gain the value of becomes more statistically robust.

Random error propagation analysis 20

was used to estimate the error associated with the RHX

dating outcome. It was assumed that the archaeological age of the sample (ta), is described by

Equation 1:

= (24

)

4 (1)

where = 2 4, 2 is the equilibrium fractional mass (normalized with respect to 0),

4 is the y-intercept of the linear segment of the vs. t1/4

plot according to Wilsons et al.15

Then, the uncertainty associated with ta was estimated by the root of the sum of the square of

each terms associated standard error multiplied by the partial derivative of ta with respect to the

same term. This is shown in Equation 2:

= (

)2 2 + (

m4)2 m4

2 + (

m2)2 m2

2 (2)

7

The standard error values for the coefficients and 4 were determined by using the "LINEST"

function in Microsoft Excel. Standard error for 2 was estimated as the fractional measurement-

to-measurement deviation of the equilibrium mass (0.01%). Sources of systematic-type error

are not included in this approach.

Results and Discussion

All ceramic artifacts selected were multiphase. Mineralogical analysis via XRD revealed quartz

as the dominant phase and at least 4-5 secondary phases (Figure 1). For all samples, the peaks of

quartz are high and narrow (especially at 2=20.90 and 26.66), indicating that a relatively large

amount of highly crystalline material was present. The clay mineral illite-2M1 (JCPDS-ICDD

No. 26-0911) and illite-2M2 (43-0685) were confirmed by the presence of characteristic peaks at

19.8, 23.9, 29.9 and 30.3, 34.7, 37.4 respectively. Plagioclase feldspar was recognized by the

presence of its two end phases: albite (09-0466) and anorthite (41-1481), which were identified

by small peaks at 22.0, 23.5, 27.9 and 27.8, 33.8, 35.7, respectively. Apart from the quartz

phase, sample RLG 007 showed large amount of calcium carbonate (85-1108) identified by the

peaks at about 29.5, 39.5, 47.6, 48.6. Since our sample is not in the pulverized form so there

might be the possibility of accidentally deposition of a large amount of weathering

product/concretion on the surface. From XRD results, it can be seen that the high crystallinity of

ceramics coming from the non-RHX active components (quartz, feldspars) while there was

relatively little evidence of crystalline RHX-active components (i.e. illite).

As shown in Figures 2 to 5, results from all four sets of samples illustrate different RHX

behaviors. The values of linear regression variables describing Stage II RHX fractional mass

gainthe intercept, 4, and the RHX rate, are summarized in Tables 2 to 5.

8

Seven fragments from sample RLG 019 show a typical two-stage progression of mass with t1/4

.16

Neither drying at 300C, nor prolonged firing time showed a significant effect on the RHX rate.

The timespan analysis of the RHX curve (shown in solid lines) indicates that a trend is present

for the RHX rate () throughout the experiment. Instead of reaching an ideal plateau near the

Stage II transition, appears to increase systematically as the interval of analysis increases up to

the time where Stage II presumably begins. The observed scattering and fluctuations of over

the entire timespan analysis could be argued to increase uncertainty in the calculated age.

For sample RLG 024, evolution of fractional mass gain does not strictly follow the two-stage

pattern; it displays both concave and convex behavior at different times. As the observed

curvature for all six specimens does not occur at the same time, it is likely that the disturbance

comes from the intrinsic instability of the sample and the RHX technique. It is rather unlikely

that this could be attributed to perturbation of the environment, as all samples within the set were

simultaneously submitted to concurrent temperature and humidity fluctuations. With respect to

the RHX rate over time, samples 300C-1c and 100C-1 show a plateau in from 3.0-4.0 and 3.5-

4.2 hr1/4

, respectively, while the others do not appear to stabilize at all in the timespan analysis.

Because external conditions were identical, the different RHX behavior of samples from the

same sherds [300C-1 (a, b, c, d, and e)] indicates the poor reproducibility of the technique.

Overall, the RHX rates of RLG 024 specimens appear to scatter more than those from RLG 019,

and lead to larger average age uncertainties. The vast scattering can also be evidenced from the

wide range of 4 and value listed in Table 3. The higher drying temperature, again, had no

discernable effect on the RHX rate. The change in drying conditions appeared to both increase

the RHX rate from 96 g/h1/4 to 400 g/h1/4, as well as decrease it to 47 g/h1/4. The fact that

9

values nearly ranged over a full order of magnitude is worrisome; in principle, this value should

be practically constant.

Fractional mass gain for sample RLG 007 resembles sample RLG 019, in that both sets of

samples exhibit a typical two-stage progression of mass with time1/4

. The timespan analysis over

the entirety of Stage II (Figure 4) shows RHX rates that are relatively flat with a narrow

spreada better outcome than for other samples. Also like RLG 019, the value of in the

timespan analysis appears to depend strongly on the time at which Stage II is assumed to begin.

The timespan and linear regression analyses indicate that the scattering for a family of specimens

cut from the same larger sherd differs from sherd-to-sherd, which is in agreement with recent

results shown by LeGoff and Gallet. 7 Separation in 4 between 300C-1a and 300C-1c is as

wide as 25%, but samples 100C-1 and 300C-1e, they exhibit nearly identical RHX behavior.

Taken in whole, results in RLG 007 again indicate poor stability in the RHX process.

For sample RLG 014, a plateau in the RHX rate () was observed in Stage II at 2.5-3.5 h1/4. Even

though relatively constant values were observed, their magnitudes vary widely, by as much as

82%. Again, the poor sample-to-sample consistency cannot be explained by random error or

fluctuations in either temperature or RH conditions. The poor reproducibility instead points to

intrinsic instabilities in the RHX process. It should be mentioned that samples 110C-1 and 300C-

2a exhibited an unexpected mass decrease near the end of the experiment. It is thought that this

was a result of sample disintegration.

For all four sets of samples, the scattered nature of the RHX process introduce 10-40% of

random uncertainty in the calculated ages and the estimated ages are much younger than their

documented ages (Table 1). (However, in the case of RLG 007, the absolute calculated dates

10

should be much shorter due to RHX measurements being performed at temperatures substantially

higher than the parent sherds ELT. In this case, the sample ages cannot be compared to the

documented age.) This consistent outcome points to some source of systematic error (>99%) in

the RHX dating process that caused a mismatch between the calculated and archaeological ages.

There was also a noticeable amount of scatter within each family of samples, especially RLG

007 and 014, which had calculated ages that spanned two orders of magnitude. The scatter could

be explained, in part, by the pronounced scatter behavior of in Stage II as well as the

subjectivity involved in selecting a Stage II transition in samples that did not show a RHX rate

plateau. If one desired, dates could be made to appear older or younger by arbitrarily selecting a

cut-off point along the timespan analysis that gave the desired RHX rate () and value of 4.

The data illustrate the troubling scatter in RHX rates and calculated dates between samples cut

from the same parent sherds, and processed in an identical manner.

Conclusions

By examining the RHX rate for all the four series of samples, few examples of consistent RHX

rates, with respect to both time and other samples within a family, were identified. Replicate

experiments show striking scatter at two drying conditions (110C and 300C) and different

refiring conditions (500C for 1 to 3 days), with no discernable effect of either condition on

RHX kinetics. High intragroup variation of RHX rate within a batch of specimens cut from the

same parent sherd is indicative of poor intrinsic Stage II stability when analyzed in the t1/4

power

law framework. The poor stability and the scattered nature of the RHX process introduce 10-40%

of random uncertainty in the calculated ages. This is in addition to unidentified systematic errors

(in some cases corresponding to >99% error) that result in shorter-than-actual calculated ages for

11

all specimens examined here. All the results are indicative of inherent, unexplained complexities

in applying the RHX dating technique to archaeological fired-clay ceramics.

Acknowledgements

We thank Kostalena Michelaki from Arizona State University for providing the fragments used

in the current study, and our collaborator Carl Lipo and Elizabeth Niespolo from California State

University at Long Beach for helpful discussions over the course of RHX experiment. This

project was supported by an Archaeometry Research Award from the United States National

Science Foundation (No.1219540).

References

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12

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2585-91 (2011).

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fired-clay ceramics," Izv. Phys. Solid Earth, 49[1] 105-12 (2013).

12. M. Le Goff and Y. Gallet, "Experimental variability in kinetics of moisture expansion and

mass gain in ceramics," J. Am. Ceram. Soc., In press (2014).

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Rehydroxylation in Fired Clay Ceramics," J. Am. Ceram. Soc., 96[4] 1047-50 (2013).

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dating of archaeological pottery," Proc. R. Soc. London, Ser. A, 468[2147] 3476-93

(2012).

13

16. M. A. Wilson, S. Clelland, M. A. Carter, C. Ince, C. Hall, A. Hamilton, and C. M. Batt,

"Rehydroxylation of Fired-Clay Ceramics: Factors Affecting Early-Stage Mass Gain in

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Fired-Clay Brick," J. Am. Ceram. Soc., 94[11] 3651-54 (2011).

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account for the effect of cooling on post-reheating mass gain," J. Archaeol. Sci., 40[10]

3596-603 (2013).

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Massachusetts, Palo Alto, London, (1957).

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excavations: Preliminary Report. ," pp. 33-35. in. Department of Archaeology, University

of Cambridge, The soprintendenza archeologica della Calabria., 2003.

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archaeological landscapes: an example from Calabria, Italy," J. Archaeol. Sci, 39[2] 234-

46 (2012).

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HumanLandscape Interactions: a Ceramic Taskscape Perspective," J. Archaeol. Method

Theory (2014).

14

24. J. B. Auban, T. O. Kohler, A. D. Castillo, M. G. Puche, and F. J. M. Hernandez, "Mas dIs

(Penguila, Alicante): aldeas y recintos monumentales del Neoltico Inicial en el valle del

Serpis," Trabajos de Prehistoria, 60[2] 39-59 (2003).

25. J. B. Auban and T. O. Kohler, "Mas d Is (Penaguila, Alicante): un recinto monumental del VI

milenio cal BC," pp. 485-96. in. Universidad de Cantabria, Santander,Espaa., in Actas

del III Congreso del Neoltico en la Pennsula Ibrica, 2005.

15

Table 1. Sample Information

Sample Site and

Archaeological

Context

Location ELT/ oC Age:

Achaeometric

or

Calendrical

and RHX

target in Years before

2014

Main Phases

RLG007 Mas D'Is; D4;

Sherd #100-

405

Penguila,

Alicante,

Spain

10.9 Neolithic

5,590 +/-40

BP,

TRHX: 5,614-

5,694

Quartz, illite-

2M1,

calcite

RLG014 Umbro Greek/

T2 N73 E45

Context 237

Bag 2348

Bova Marina,

Calabria, Italy

19.5-

20.5

Classical

Greek

425-350 BC

TRHX: 2,439-

2,364

Quartz, illite-

2M1,

anorthite, albite,

RLG019 Umbro

Neolithic

T6 N6 E59

Sherd #1421-

58

Bova Marina,

Calabria, Italy

19.5-

20.5

Early to

Middle

Neolithic

5,500-5,000

Cal BC

TRHX: 7,514-

7,014

Quartz, illite-

2M1,

anorthite, albite

RLG024 Penitenzeria;

T4; E40 N46

Sherd #2038-

66

Bova Marina,

Calabria, Italy

19.5-

20.5

Early/Middle

Neolithic

5,500-5,000

Cal BC

TRHX: 2,439-

2,364

Quartz, illite-

2M1(2),

anorthite, albite

Sample dates include expected dates expressed as calibrated or uncalibrated radiocarbon dates or using

the known date range of cultural phases at the specific site. These dates are then converted into RHX

Target Age (TRHX) understood as years before A.D. 2014. Samples RLG014, RLG019, and RLG024

were recovered during excavations by the Bova Marina Archaeological Project team. Details about the

excavation contexts for the sample sherds have been reported elsewhere 21, 22, 23

. Sample RLG007 is from

the site of Mas DIs which has also been described in the literature 24, 25.

16

Table 2. Time RHX parameters for RLG 019

Sample

RLG 019

Nominal

Mass m410

5[g/g] 105[(g/g)/h1/4] Age/year

100C-1 0.780520 1053 9 160 2 2.1 0.2

300C-1a 0.693347 932 10 156 3 1.6 0.1

300C-1b 0.762487 956 9 161 2 1.3 0.2

300C-2a 0.520244 936 10 163 3 2.2 0.2

300C-2b 0.814421 949 9 157 2 1.7 0.2

300C-3a 0.895630 929 11 156 3 2.1 0.3

300C-3b 0.504413 873 1 156 3 1.7 0.2

17


Sample

RLG 024

Nominal

Mass m410

5[g/g] 105[(g/g)/h1/4] Age/year

100C-1 0.80787 698 7 10 2 7 4

300C-1a 0.684012 807 14 5 4 3 1

300C-1b 0.624774 664 16 19 4 1.3 0.4

300C-1c 0.92682 849 10 20 3 0.5 0.1

300C-1d 0.886827 610 8 31 2 1.4 0.5

300C-1e 0.811570 834 10 40 3 4 3

18


Sample

RLG

007

Nominal

Mass m410

5[g/g] 105[(g/g)/h1/4] Age/year

100C-1 0.635143 864 11 130 3 38 1

300C-1a 0.591287 990 13 135 3 0.5 0.1

300C-1b 0.750737 782 10 137 3 24 2

300C-1c 0.622529 739 9 133 2 23 2

300C-1d 0.603122 754 11 119 3 290 30

300C-1e 0.577538 870 11 121 3 33 4

19


Sample

RLG 014

Nominal

Mass m410

5[g/g] 105[(g/g)/h1/4] Age/year

100C-1 0.659297 252 8 55 3 685 27

300C-1a 0.622409 257 5 59 1 34 9

300C-1b 0.656815 336 12 35 2 9 1

300C-2a 0.458795 237 7 59 2 600 200

300C-2b 0.682371 200 6 62 2 15 3

300C-3a 0.629391 242 6 11 4 157 23

20

Figure 1. X-Ray diffraction results for each parent sherd.

21

Figure 2. RHX mass gain and RHX rate of RLG 019. After drying at 110C (100C-1) and 300C,

they were fired at 500C for 1, 2, or 3 days samples denoted as 300C -1a(b), 300C -2a(b), 300C -3a(b), respectively.

22


they were fired at 500C overnight samples denoted as 300C-1 (a, b, c, d, e)).

23


they were fired at 500C overnight, which was denoted as 300C-1 (a, b, c, d, e)).

24

Figure 5. RHX mass gain and RHX rate of RLG 014. After drying at 110C(100C-1) and 300C,

they were fired at 500C for 1, 2, 3 days, which was denoted as 300C -1a(b), 300C -2a(b), 300C

-3a respectively.

reproducibility in rehydroxylation of ceramic artifacts

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