Using Raman spectroscopy to estimate the dates of carbon-based inks from Ancient Egypt

• Published in: Journal of cultural heritage 2019-07, Vol.38, p.106-117
• Main Authors: Goler, Sarah, Hagadorn, Alexis, Ratzan, David M., Bagnall, Roger, Cacciola, Angela, McInerney, James, Yardley, James T.
• Format: Article
• Language: English
• Subjects: Ancient inks; Carbon-based pigments; Papyrus; Raman characterization
• ISSN: 1296-2074; 1778-3674
• DOI: 10.1016/j.culher.2018.12.003

[Display omitted] •Raman spectra for inks of Egyptian papyri dated 400 BCE and 1000 CE vary systematically with manuscript date.•Statistical analysis can provide date estimates for similar papyri to ± 120 years with 68% confidence.•Intrinsic variations in Raman spectra for different manuscripts limit accuracy of estimated dates.•Gaussian Mixture Modeling provides a statistical means for identifying carbon inks of similar character.•This methodology can constitute a strong indication of the antiquity and rough period for ancient inks. In the first study of its kind, we previously investigated Raman spectra of 17 papyri from the Columbia University Libraries collection, written in carbon-based black ink with well-established dates between 400 BCE and 1000 CE. Using a simple two-peak spectral model for the Raman spectrum of carbonaceous materials, we discovered small but statistically significant systematic variations in the spectral parameters with manuscript date. The purposes of this report are to present: (1) more-powerful statistical examinations of the correlation based on the two-peak spectral model and on a new three-peak model which includes spectral effects for amorphous carbon; (2) new data for additional ancient manuscripts, broadening the investigation; (3) a method for potentially exploiting the observed correlation to predict the dates of undated manuscripts of similar character; and (4) a hypothesis as to the underlying chemical basis of the observed spectral changes. We apply both linear and multiple linear regression analyses to the spectral models. We show that the three-peak model fits the experimental data much better than the two-peak model. Through the use of multiple linear regression, the three-peak model reduces significantly the overall correlation error. We find that our ability to predict dates for manuscripts that are similar to those of the study over the date range 400 BCE to 1000 CE is limited by intrinsic variations in the character of the pigments to about ± 118 years with 68% statistical confidence (or ± 236 years with 95% confidence). We confirm the observed correlation and prediction capabilities through a blind study of six additional ancient manuscripts and two late 19th- or early 20th-century fakes in the Columbia University collection. The data suggest that chemical degradation of amorphous carbon and chemical modification of graphitic crystals may both contribute to the observed spectral differences. This would also