Understanding the provenance and production process of historic mortars—a novel approach employing calcareous nannofossils

Abstract

Limestones (CaCO3) have been an important source for masonry and mortars throughout approx. 10,000 years of human history. They are often composed of calcitic shells of minute marine algae, known as calcareous nannofossils. The 0.25–30-µm large calcitic skeletons of these primary producers have been well documented from various archaeological materials including building stones of masonry. Surprisingly, these tiny microfossils were recently also observed in medieval mortars and mortar-based materials, even though the carbonate-based source rocks of the mortars have been heated in kilns for quicklime production. Burning experiments of carbonate-rich sedimentary rocks, containing well-preserved and abundant calcareous nannofossils, documented a deteriorating preservation and a decrease of diversity and relative abundance of the nannofossils with increasing temperatures.Alongside the lime-based binder historic mortars often contain under- and overburnt lime lumps and carbonate-rich aggregates; the latter were added after the heating process. Lime lumps and aggregates offer additional, so far not yet fully understood information on the burning process. Here, calcareous nannofossils were studied in ultra-thin sections of historic mortars, resulting in the separate analysis of the binder, lime lumps and aggregates. The findings allow (i) a more precise provenance of the limestones and (ii) a more accurate reconstruction of the temperatures reached during historic quicklime production. Our study thus improves the provenance analysis approach of limestones and sheds light on historic technology of mortar production by using calcareous nannofossils.