Comparison of Thermal Decomposition and Sequential Dissolution—two Sample Preparation Methods for Radiocarbon Dating of Lime Mortars

Abstract

Dating lime mortar samples using the radiocarbon (14C) method can be difficult. This is because the contamination is similar to the primary dating material (CaCO3) and consequently difficult to remove. Mortar can also have late-in-formation pyrogenic carbonate from interactions with the environment after the initial hardening phase, such as recrystallization, fire damage or delayed hardening. When 14C dating a system of primary dating material, contamination and late-in-formation pyrogenic carbonate, one approach is multi-fraction dating with conclusiveness criteria. If a sample has sufficient contamination or late-in-formation pyrogenic carbonate, the criteria evaluate the result inconclusive. To improve inconclusive results from such samples, this study investigates sample preparation by thermal decomposition. Here samples that were inconclusively dated by the authors’ traditional method, sequential dissolution with 85% phosphoric acid, are investigated further. This study finds that CO2 thermally decomposed at low temperatures contains some late-in-formation pyrogenic carbonate. By rejecting CO2 decomposed at low temperatures, Kastelholm castle and Kimito church in Finland are conclusively and accurately dated. Furthermore, a preheating method removes some late-in-formation carbonate, but not enough for a conclusive result. Finally, thermal decomposition finds difficulty in discerning binder carbonate from limestone and marble contamination.

A field guide to mortar sampling for radiocarbon dating*

“…Mortar 14 C preparation techniques and analysis are designed to handle the risk of open system condition by preserving only those fractions or lime binder components that are assumed to represent closed-system conditions. Research into new and more sophisticated 14 C mortar preparation techniques is an active research field; for example, experimental preparation methods based on thermal decomposition or preheating before sequential dissolution show good results in reducing problems with recrystallization (Barrett et al 2020b;Daugbjerg et al 2020).…”

“…Thermal decomposition is a chemical separation method (Labeyrie and Delibrias 1964;Daugbjerg et al 2020;Toffolo et al 2020). The literature reports thermal decomposition conclusively and accurately dating complication-free mortar samples and mortar samples complicated by recrystallization or alkalinity.…”

“…The literature reports thermal decomposition conclusively and accurately dating complication-free mortar samples and mortar samples complicated by recrystallization or alkalinity. Samples with considerable dead carbon can challenge the method (Barrett et al 2020b;Daugbjerg et al 2020).…”

The Need for a New Approach to the Radiocarbon Dating of Historic Mortars

“…The importance of characterizing the binding fraction for a correct interpretation of the radiometric dating of mortar samples Most (if not all) papers dealing with the 14 C dating of historic lime mortars assume that the only chemical reaction leading to the formation of 14 C-bearing minerals is the carbonation of calcium hydroxide (Ca(OH) 2 ) represented in Equation (1) (Lindroos et al 2007;Heinemeier et al 2010;Ringbom et al 2014;Daugbjerg et al 2021aDaugbjerg et al , 2021b.…”