Towards Electron Spin Resonance dating of anthropogenic carbonates: ESR signals of 14C-dated historical lime mortars

Anthropogenic carbonates such as lime mortars and plasters have been receiving growing attention as they are an invaluable source of information for archaeologists, conservators, and restorers of cultural heritage. Taking into account the production process, the age of mortars reflects the age of the building. Two physical dating methods currently enable us to date mortars: radiocarbon (¹⁴C) dating and optically stimulated luminescence (OSL). Fast development in ¹⁴C and OSL mortar dating naturally widens the scope of performed analysis, and promotes the search for different methods which may be applied to these materials. In this study we present the analysis of historical lime mortars in order to assess the possibility of future dating by Electron Spin Resonance (ESR) spectroscopy. ESR dating has been applied to various geological and archaeological materials, but there are virtually no examples of dating carbonates younger than 10 000 years. Since carbonate crystals are formed during the mortar production, this moment can be regarded as the zero point for the accumulation of trapped charges, and their concentration in a measured sample should reflect the age of the mortar.

Our previous works on samples from Sveta Petka church in Budinjak, Croatia, and an ancient settlement Hippos, Israel, show the growth of signals related to the paramagnetic centres with the dose of laboratory radiation. However, in order to obtain the age of the samples the natural material should exhibit measurable ESR signals of the centres suitable for dating. In the relatively young (as far as ESR dating is concerned) materials the signals are very weak, however detailed analyses showed presence of such signals in several investigated mortar samples. In this work we investigate ESR signals in natural and laboratory-irradiated carbonate lime binders from several different archaeological sites, with ages ranging from about 2000 to 500 years old. The samples have been previously dated by ¹⁴C method, which means they had undergone extensive characterisation and preparation, ensuring the selection of binder, which reflects the true age of the mortar. We analyse the spectra with the aid of ESR simulations in order to identify the paramagnetic centres present in the samples, and compare them to the centres commonly found in carbonates and used for ESR dating. The goal of this work is a qualitative analysis of the natural signals found in a variety of mortars, assessing their potential suitability for ESR dating. This study is a first part of the ongoing project focused on establishing ESR as a method of dating anthropogenic carbonates in a form of lime mortars, and comprises the preliminary analysis of the subject, which will be followed by future in-depth studies.