Dating of Old Lime Based Mixtures with the "Pure Lime Lumps" Technique

A number of studies carried out over the last forty years describe the application of radiocarbon dating of lime mixtures such as mortars, plasters and renders. Despite the fact that this method is very simple in principle, several studies have highlighted various practical challenges and factors that must be considered. These arise mainly from the contamination of samples with carbonaceous substances such as incompletely burnt limestone and aggregates of fossil origin including limestone sand. However, recently studies have shown that accurate sample processing allow a significant reduction of these error sources and moreover adoption of a special sampling procedure based on the careful selection of lumps of incompletely mixed lime, provides an interesting alternative that avoids problems associated with contamination. The founding principle underlying this technique is the use of the pure lime lumps. These are thought to originate from imperfect mixing and are most prevalent in mortars, renders and plasters predating mechanical mixing. Previous sampling methods for radiocarbon dating did not discriminate between pure and contaminated lime lumps. As pure lumps contain the same lime as that used in other parts of the mixtures but importantly are free of contaminants such as sand grains or under burned pieces of limestone, they can dramatically reduce the errors in the radiocarbon dating.