Infrared spectra of mixtures of heated and unheated clay: Solving an interpretational conundrum

Isaac Ogloblin Ramirez, Zachary C. Dunseth, Dina Shalem, Ruth Shahack-Gross

Research output: Contribution to journalArticlepeer-review

Abstract

Fourier transform infrared (FTIR) spectroscopy is frequently used for archaeological studies related to fire, allowing, among other things, researchers to distinguish between unheated and heated clay minerals. However, heat signatures are not always clear-cut in infrared spectra of bulk sediments, as spectra occasionally appear with ambiguous absorbance bands attributed to hydroxyl (OH) in clay minerals. This paper presents an experimental study addressing this interpretational problem by considering the effect of mixtures of heated and unheated clay, a phenomenon expected in archaeological sites. After creating experimental mixtures and testing them using bulk FTIR spectroscopy, our results indicate that even a relatively small amount of unheated clay—only ca. 5%–10% mixed into a fully heated deposit—will result in ambiguous infrared spectra that are difficult to interpret. For comparison, ambiguous bulk FTIR spectra from two archaeological contexts—an ashy fill within a pit installation and a hearth—were studied with FTIR microspectroscopy, which demonstrated the presence of unheated clay within a largely heated deposit. Micromorphological observations explain the mixed nature of the investigated archaeological contexts, in this case, primarily via bioturbation. Our results thus emphasize the importance of microcontextual analysis of clay minerals. Furthermore, these results indicate that heated deposits are likely missed altogether in some archaeological contexts where only bulk FTIR analyses have been conducted.

Original languageEnglish
Pages (from-to)822-829
Number of pages8
JournalGeoarchaeology - An International Journal
Volume38
Issue number6
DOIs
StatePublished - 1 Nov 2023
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2023 The Authors. Geoarchaeology published by Wiley Periodicals LLC.

Funding

The authors thank the Israel Antiquities Authority for supporting I. O. R.; the cost of analyses was covered by the Laboratory for Sedimentary Archaeology (headed by R. S.‐G.) at the University of Haifa. The mFTIR equipment was available for this research thanks to the support of the Israel Science Foundation (mid‐career equipment grant to R. S.‐G.). The authors wish to thank two anonymous reviewers for their thoughtful and constructive comments on this manuscript. The authors thank the Israel Antiquities Authority for supporting I. O. R.; the cost of analyses was covered by the Laboratory for Sedimentary Archaeology (headed by R. S.-G.) at the University of Haifa. The mFTIR equipment was available for this research thanks to the support of the Israel Science Foundation (mid-career equipment grant to R. S.-G.). The authors wish to thank two anonymous reviewers for their thoughtful and constructive comments on this manuscript.

FundersFunder number
Israel Antiquities Authority
Laboratory for Sedimentary Archaeology
Israel Science Foundation
University of Haifa

    Keywords

    • FTIR microspectroscopy
    • bulk FTIR
    • clay alteration
    • postdepositional processes
    • spectrum interpretation

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