Among the artist’s iron-based natural earth pigments, the so-called terra di Siena (raw sienna), terra di Siena bruciata (burnt sienna) and terra d’ombra (umber) have been among the yellow-brown and reddish-brown earth pigments most widely used by Italian and European painters since the Renaissance. We present the history of discovery, designation, and production of these famous pigments, their geological, lithological, and geochemical characterization, and the recognition of their genesis and places of origin, based on new geological field surveys, and on the critical analysis of textual documents and rock sample collections assembled during the eighteenth and nineteenth centuries. In particular, the terra di Siena earth pigment exclusively originated at, and was extracted from, late Pleistocene paleo-lake basins surrounding the Monte Amiata volcano. This earth pigment consists of primarily lacustrine sediments composed of hydrated iron oxide (limonite/goethite) produced by biochemical authigenic precipitation from fresh waters rich in metal solutes.

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