Historic Lime-Mortar ¹⁴C Dating of Santa María La Real (Zarautz, Northern Spain): Extraction of Suitable Grain Size for Reliable ¹⁴C Dating

Abstract

ABSTRACT. This paper describes a method for effective separation of the pure binder fraction of lime mortars for reliable radiocarbon dating. The methodology allows removal of the detrital carbonate fraction and the unburnt limestone particles, obtaining particles of under 1 µm. The extracted fraction ensured that all carbonate has been generated by slaked lime carbonation. Consequently, the measured carbon corresponds to atmospheric carbon. The proposed method allows to obtain pure datable binder, simplifying considerably the performance of radiometric measurements because dating other grain-size fraction is unnecessary. In order to prove the effectiveness of binder refining, the extraction method has been applied to 5 lime mortars of different archaeological periods from the perimeter walls of Santa María la Real parish church (Zarautz, northern Spain).

Hydrotalcite and Hydrocalumite in Mortar Binders from the Medieval Castle of Portilla (Álava, North Spain): Accurate Mineralogical Control to Achieve More Reliable Chronological Ages

“…In order to assess the potential of the binder as a time record, the extracted fractions have been characterized mineralogically. Unlike in Ortega et al [28], the mortars from Portilla Castle show a more complex mineralogy in the finest fraction (<2 µm). The occurrence of potential contaminant phases such as LDHs has led to an adjustment of the extraction procedure to refine the fraction suitable for dating.…”

“…The mineralogical composition of carbonate phases of the mortar binder and the granulometric distribution has determined the particle size to be extracted for radiocarbon dating. The selection of fine-grain size fraction (<2 µm) is only useful when the mortar binder is formed solely by calcite, e.g., Ortega et al [28]. In contrast, when the binder includes hydrotalcite and hydrocalumite besides neo-formation calcite, as in Portilla Castle, which determines the granulometric distribution of carbonate minerals is essential in selecting effective thresholds.…”

“…A settling extraction procedure has been performed to obtain different grain size fractions of the binder based on the procedure described by Ortega et al [28]. In order to assess the potential of the binder as a time record, the extracted fractions have been characterized mineralogically.…”

“…Since the proposed methods do not seem to provide consistent and reproducible results, it is imperative to develop new procedures to assess the contaminant nature and the granulometric distribution within the binder. Since particle size is the main issue to obtain a suitable fraction for dating, the extraction and monitoring of the adequate grain size particle of the binder becomes vital [28].…”

“…Several methodologies and preparation procedures have been developed to isolate a pure neo-formation calcite fraction without any other carbon contaminant [17][18][19][20][21][28][29][30][31]. The first procedures to date the binder consisted of the hydrolysis of lime binder particles of different grain size fractions by using diluted hydrochloric acid (HCl) or 85% ortophosphoric acid (H 3 PO 4 ) [12,13,16,17,[19][20][21][32][33][34][35].…”