Self-archived version of the paper that appears in the Journal of the American Ceramic Society

Vol. 96, Issue 4, Pages 1047-1050, doi: 10.1111/jace.12262

Rapid Communication

Effect of Humidity Instability on Rehydroxylation in Fired Clay Ceramics

Jaroslaw Drelich,1* Patrick K. Bowen,

1* and Timothy J. Scarlett

2

1Department of Materials Science and Engineering;

2Department of Social Sciences

Michigan Technological University, Houghton, MI 49931, USA

Abstract

Several samples of the XIX-century Davenport pottery and XX-century structural masonry were

reheated at 500°C and then exposed to a humid gas of controlled relative humidity. Changes in

the sample masses were recorded in response to both systematic and transient step changes in

humidity. Additionally, a reheated masonry sample underwent a sequence of soaking and drying

and hundreds of hours of interactions with humid air in between these treatments in order to

examine long-term effects of extreme humidity fluctuations. All experimental results indicate

that instantaneous humidity and the sample’s hygral history have a negligible effect on the long-

term kinetics of mass gain. This important finding provides strong experimental support for the

newly developed rehydroxylation ceramic dating technique by proving that humidity affects

physically bonded water in the ceramics, but has a negligible effect on chemically bonded water.

Keywords: rehydroxylation; fired clay ceramic; humidity

*Corresponding authors; emails: jwdrelic@mtu.edu and pkbowen@mtu.edu

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Introduction

Since the introduction of rehydroxylation (RHX) ceramic dating by Wilson et al.,1 the

archaeological community has been very active in testing and validating this novel, low-cost

dating technique on samples with various geochemistries, depositional records, and firing

histories. RHX dating relies upon the well-known characteristic of fired clay objects to gain

mass continuously from the moment they are fired.2 This is a result of recombination of clay

minerals with moisture from their environment. Rehydration of ceramic samples consists of two

distinct, but concurrent processes that occur in two stages.3; 4; 5

During Stage I, clay minerals and

ceramics first gain two primary types of water: capillary water, un-bonded water that fills

macro- and micro-pores in the ceramic body; and physically adsorbed water, where water

molecules adhere to surfaces and edges of crystals, including between lattice layers. Both

capillary water and physically adsorbed water are grouped together under physical water or

physically bonded water terms in this contribution. At the nanoscale, swelling behavior is caused

by the diffusion of one to four molecular layers of water into the interlayers spacing between

intact or reconstructed clay sheets.6; 7

As liquid water penetrates the sheets of the clay minerals and surrounds metallic ions inherent to

the clays forming strong physical bonds, the chemical recombination (rehydroxylation, or RHX)

occurs during both Stage I and Stage II.8 Chemically bonded water comprises hydroxyl

molecules bound to clays and other minerals, and its bonding kinetics are recorded during Stage

II for RHX dating.9 Chemical reaction of water with minerals is a slower process than

condensation or physical sorption but also begins immediately after firing. During Stage II, the

amount of physically bonded water is believed to remain constant, and has been assumed to have

no effect on the kinetics of rehydroxylation.

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The RHX technique requires analysis of the data in Stage II where the mass gain curves appeared

linear with time1/n

. Wilson et al.1 insists that the rehydroxylation exponent (n) is universally

equal to four,10; 11

while previous work by the authors12; 13

has identified small departures from

n=4 for the Davenport pottery recovered in Parowan, Utah. Both temperature and humidity

fluctuations have been identified as important variables in mass gain versus time correlations for

the Davenport pottery. It was concluded that a tight humidity control in the laboratory setting is

critical for ensuring accurate, reproducible RHX dating results.12

Given the degree of variation

that can exist between the depositional conditions of artifacts, we also remain concerned about

the potential influences of seasonal (systematic) and daily (transient) humidity variations,

rainfall, snowfall, soil type and structure, groundwater depth, vegetative cover, etc.

It was the purpose of this study to address the influence of moisture variation on ceramic mass

gain, an issue essential to the RHX dating that has not yet been specifically addressed. It has

been asserted that Stage II rehydroxylation kinetics are not affected by ambient humidity,14

but

supporting data have not yet been presented. In this communication, evidence is presented that

transient or systematic changes in physical water saturating or surrounding the ceramic have

little, if any, effect on long-term mass gain (Stage II kinetics).

Experimental

The experiments reported in this communication have been undertaken on two sets of samples.

One set comprised sherds of Davenport ceramics from Parowan, Utah (ca. 18551888 C.E.),

which is an example of a fired-clay ceramic artifact that one may recover in the field. Kiln

wasters of the Davenport type have been thoroughly characterized and their rehydration/

rehydroxylation behaviors examined in earlier reports.12; 13

A second set comprised pieces of

broken brick recovered from the parking deck in Houghton, MI (ca. 19651970 C.E.) which

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represents modern structural masonry. Results of x-ray powder diffraction (not shown) indicated

that the primary crystalline phases present in these brick samples were, in descending amount:

quartz, hematite, rutile, and calcite. Other mineralogical constituents fell below the limit of

detection or were amorphous and thus not identifiable by x-ray diffraction.

The samples varied in size from approx. 1.41 to 2.84 grams, but only results for selected samples

are reported in this communication. All samples were first dried to remove physically bonded

water (dehydration) at 110 ± 5°C for 3050 hours, and then reheated to remove chemically

bonded water (dehydroxylation) at 500°C for 45 hours. After removal of water, samples were

placed for 1015 min in a low-humidity desiccator and allowed to cool to 80100oC. Then the

samples were placed in a constant-humidity glove box (Coy Laboratory Products; Grass Lake,

MI) equipped with a Citizen’s CM-11 microbalance (Citizen Scale Inc.) having 1 μg precision

and operated in a room with controlled temperature of 22±1°C. The glove box was equipped

with a desiccation circuit, dry N2 injection, and humidification apparatus and was presented in

our previous contribution.13

The samples remained in the glove box’s controlled humidity

environment at levels of 20, 28, or 40% relative humidity for the duration of the experiment.

The mass of each sample was measured and recorded periodically for several days, often with

multiple data points in one day. Statistical analyses of any linear data were carried out through

the regression analysis method in Microsoft Excel in conjunction with a test for equality of

slopes.9 In this report, slopes are reported as the mean value ± the 95% confidence interval.

Three experiments were key in the current evaluation of humidity effects. In the first

experiment, the influence of a single, systematic change in humidity was examined. Modern

structural masonry was reheated and then exposed to a humid gas with 20% relative humidity

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(R.H.) at 22°C for the first 620 hours. The sample was then exposed to 28% R.H. air for another

400 hours.

The second experiment utilized a reheated Davenport ceramic sample that was exposed to 20%

R.H. for 980 hours, 40% R.H. for 110 hours, and then back to 20% R.H. for another >1000

hours. This experiment allowed the authors to examine the long-term effects of a single,

transient change in humidity.

In the final selected experiment, a complex combination of water saturation and drying was used

to test the influence of humidity fluctuations at their extremes. One sample of structural masonry

underwent a series of rehydration, drying and soaking cycles as listed in Table 1.

Results and Discussion

Figure 1 shows the mass versus time (m vs. t) curve for the first experiment, which was intended

to simulate a systematic change in humidity. The m vs. t curve (Figure 1A) is of the typical shape

reported in previous studies for various ceramic artifacts (either as mass vs. time, mass gain vs.

time, or fractional mass gain vs. time curves) and reflect the kinetics of water pick up by the

ceramic from the environment.1; 5; 12; 15

The condensation and adsorption of physical water within

a porous structure of ceramic is a spontaneous process and dominates for 517 hours for the

Davenport pottery after initial exposure to moisture.13

This time is much longer—approximately

175190 hours—for the modern masonry samples, as discussed later.

The results in Figure 1A show that the rate of mass gain obtained at 20% R.H. is almost identical

to that obtained at 28% R.H. The primary difference is a 21 g shift in the location of the m vs. t

curve caused, most likely, by a small difference in the amount of capillary water that condenses

in pores within the ceramic at 28% R.H. and persists when the humidity is reduced back to 20%

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R.H. (per the Kelvin equation).16

A plot of the mass data versus (time)1/4

power law is shown in

Figure 1B. The linear regression analysis shows nearly identical mass gain rates obtained for

periods when sample was exposed to 20% and 28% R.H. Their respective slopes were measured

to be 666 ± 34 and 689 ± 20 g/h1/4

, with P values of 1.3 x 10-11

and 8.0 x 10-11

, respectively.

Low P values for both curves indicate a good correlation of mass with respect to (time)1/4

; P <

0.05 is required to claim statistical significance at 95% confidence. Testing the equality of

slopes9 for the two regions resulted in a P value greater than 0.6, indicating that there was no

significant region-to-region kinetic difference. Therefore, a systematic change in humidity did

not result in a change in rehydroxylation kinetics.

Figure 2 shows the m vs. t results for the second experiment, in which the humidity was cycled

in a transient fashion for a Davenport ceramic sherd. Similar tests were carried out for four other

Davenport sherds (not shown here) with almost identical results: m vs. t curves resumed the

original trend after a temporary increase in humidity level from 20% to 40%. A vertical shift in

the location of the m vs. t curve on the order of a few micrograms was often observed. Such a

shift could be explained as a result of hysteresis in water capillary condensation in the porous

materials.16

Small deviations from linearity in a (time)1/4

presentation were sometimes observed,

consistent with previously reported results in which the (time)1/4

power law did not hold over

longer periods of time.12; 13

This issue confounded the regression analyses; valid fits could not

be reliably obtained because of the observed nonlinearity. Nevertheless, no modification of

Stage II kinetics is apparent in these data. Transient changes in humidity, therefore, do not

appear to impact rates of mass gain in the long-term.

In the final experiment, the mass changes for a brick sample that was exposed to two different

treatments (Table 1), including both soaking in water and drying at 110oC to remove physical

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water. In between treatments, the typical rehydration/ rehydroxylation tests were carried out in

humid air (28% R.H., with exception of the initial tests done at 20% R.H.). The results are

shown in Figure 3 and Table 1. For the first 651 hours the reheated brick sample underwent a

rehydration and rehydroxylation in the surrounding gas with 20% R.H. Then the humidity was

increased to 28% for an additional 382 hours. Since the effects during these two stages were

nearly identical to those discussed in relation to Figure 1, they are not discussed further here.

After 1033 hours, the sample was dried for 26 hours at 110° C to a nearly constant mass. The

dried sample was soaked in deionized water for an hour and, after wicking away excess water, it

was placed in the glove box under constant humidity of 28% for 283 hours. Surprisingly,

although the sample picked up more than 0.2 g of water (~10% of sample mass) during soaking,

this water evaporated in a matter of hours in the glove box and the sample returned to its dry

mass. This result suggests that water did not penetrate the internal structure of the brick during

the soaking event. In total, it took about 180 hours to re-saturate the sample with water lost

during drying.

The same cycle of drying and soaking was repeated for a second time, with almost identical

results (Figure 3). Unfortunately, a small piece of the brick chipped off during the second

soaking event, and the total mass of the sample dropped by 100-110 g (estimated ex post). As a

result, the sample could not return to the maximum mass recorded before drying, even after 450

hours in humid air. Two more cycles of drying and equilibration at 28% R.H. were repeated with

an additional drying event at the end of the experiment.

During each cycle, the sample tended to restore the nominal m vs. t correlation (what was

observed before any drying or soaking events). As indicated by the data in Table 1, slopes in the

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(time)1/4

regime were similar. Large standard deviations for the values are a result of having only

a few data points from the end of each cycle to be used in the linear regression analysis. The time

required to restore the physically bonded water, removed during the drying, varied from 175 to

190 hours (Table 1). Note that this time is 1520 times longer than reported for the Davenport

pottery and expressed as the t99 value in the previous contribution,13

which may be attributed to

the different pore structure and mineralogy of the brick.

Conclusion

These results showed that both transient and systematic changes in humidity—and even soaking

in water—had no observable effect on the kinetics of rehydroxylation of the XIX-century

Davenport pottery and XX-century structural masonry. It has been confirmed that fluctuations in

humidity during rehydration/rehydroxylation of reheated ceramics is an important variable in

interpreting the mass versus time correlation, but it affects only the physical water content of the

ceramic. The physical water thus gained can be easily removed by ceramic drying at elevated

temperatures (110oC). These results are further evidence that rehydroxylation ceramic dating has

the potential to mature into a robust, inexpensive chronometric dating technique for fired clay

artifacts.

Acknowledgements

The authors acknowledge funding by the Archaeology and Archaeometry program in the

Division of Behavioral and Cognitive Sciences of the National Science Foundation (awards no.

1112327 and 1112049). The authors also express appreciation to the 2009 archeological field

team: Steve and Mark Matheson; Todd Prince, Park Manager, Frontier Homestead State Park

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

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Natural Resources for their assistance in obtaining the Davenport ceramic samples in Parowan,

Utah.

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. A., 465[2108] 2407-15 (2009).

2A. Hamilton and C. Hall, "A review of rehydroxylation in fired-clay ceramics," J. Am. Ceram.

Soc., 95[9] 2673-78 (2012).

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

to the chemical recombination of water in fired clay brick," J. Am. Ceram. Soc., 91[10]

3396-98 (2008).

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[5] 055402 (2008).

5M. A. Wilson, A. Hamilton, C. Ince, M. A. Carter, and C. Hall, "Rehydroxylation (RHX) dating

of archaeological pottery," Proc. R. Soc. A, 468[2147] 3476-93 (2012).

6H. van Olphen, "An Introduction to Clay Colloid Chemistry." John Wiley & Sons: New York,

(1963).

7E. J. M. Hensen and B. Smit, "Why clays swell," J. Phys. Chem. B, 106[49] 12664-67 (2002).

8S. 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[7] 1579-92 (1991).

10

9R. M. Bethea, B. S. Duran, and T. L. Boullion, "Statistical Methods for Engineers and

Scientists," 2nd ed. Marcel Dekker: New York, (1985).

10C. Hall, M. A. Wilson, and W. D. Hoff, "Kinetics of long-term moisture expansion in fired-

clay brick," J. Am. Ceram. Soc., 94[11] 3651-54 (2011).

11M. 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] 125503 (2003).

12P. K. Bowen, H. J. Ranck, T. J. Scarlett, and J. W. Drelich, "Rehydration/rehydroxylation

kinetics of reheated XIX-century Davenport (Utah) ceramic," J. Am. Ceram. Soc., 94[8]

2585-91 (2011).

13P. K. Bowen, J. Drelich, and T. J. Scarlett, "Modeling rehydration/rehydroxylation mass gain

curves from Davenport ceramics," J. Am. Ceram. Soc., in press (2013).

14C. Hall and W. D. Hoff, "Moisture expansivity of fired-clay ceramics," J. Am. Ceram. Soc.,

95[4] 1204-07 (2012).

15L. Tosheva, B. Mihailova, M. A. Wilson, and M. A. Carter, "Gravimetric and spectroscopic

studies of the chemical combination of moisture by as-fired and reheated terracotta," J.

Eur. Ceram. Soc., 30[9] 1867-72 (2010).

16P. C. Hiemenz and R. Rajagopalan, "Principles of Colloid and Surface Chemistry," 3rd ed.

Marcel Dekker, Inc.: New York, (1997).

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Table 1. Conditions under which the Houghton brick sample was treated and resulting end mass

and slope of the mass=f(time)1/4

linear correlation. Also times to restore the sample mass from

before soaking and/or drying (approximately equivalent to time to restore physically bonded

water) are presented in the last column.

Stage Sample Treatment Time

Sequence

[h]

Slope for

m=f(t1/4

)

[μg/h1/4

]

End Mass

[g]

Time

to

Restore

Mass

[h]

I Rehydroxylation: 22oC, 20% R.H. 0-651 350+20 2.834074 -

II Rehydroxylation: 22oC, 28% R.H. 651-1033 390+60 2.834341 -

III Drying: 110oC 1033-1059 - 2.833364 -

IV Soaking in water: 22oC 1059-1060 - 3.070583 -

V Rehydroxylation: 22oC, 28% R.H. 1060-1343 900+160 2.834448 175

VI Drying: 110oC 1343-1421 - 2.833484 -

VII Soaking in water: 22oC 1421-1422 - 3.072002 -

VIII Rehydroxylation: 22oC, 28% R.H. 1422-1873 810+50 2.834411 -

IX Drying: 110oC 1873-1945 - 2.833585 -

X Rehydroxylation: 22oC, 28% R.H. 1945-2353 770+620 2.834552 190

XI Drying: 110oC 2353-2405 - 2.833747 -

XII Rehydroxylation: 22oC, 28% R.H. 2405-2911 570+260 2.834749 180

XIII Drying: 110oC 2911-3005 - 2.834002 -

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Figure 1. Mass versus time (A) and mass versus (time)1/4

(B) data for the Houghton brick sample

(1.414906 g before reheating).

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Figure 2. Mass versus time (A) and mass versus (time)1/4

(B) data for the Davenport pottery

sample (1.710195 g before reheating).

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Figure 3. Mass versus time curves for the Houghton brick sample (2.848624 g before reheating).