Archaeological site of Persepolis (Iran): study of the finishing technique of the bas-reliefs and architectural surfaces

One of the aims of the 5-year Iranian/Italian project for Persepolis, called ‘From Palace to Town’ was to contribute to the conservation of the stone monuments of the imperial site. As part of the activities dedicated to this purpose, a diagnostic study was carried out. Various aspects were considered: petrographic characterization of the stone, forms and factors of decay, and in situ testing of suitable conservation treatments. The present paper reports on the unexpected results of the study on the finishing of architectural surfaces. The results obtained on a limited, but nevertheless significant, number of samples collected from the monuments of the imperial Terrace, allow us to state that the dark grey limestone used for several (or many?) monuments was covered on purpose with a thin, fine whitish layer containing fluorapatite, as major component, and calcite. It is highly probable that the fluorapatite was obtained from calcined animal bones and that slaked lime was used as a binder. Further evidence for this is the discovery of a kiln with the remains of calcined bones and, nearby, a waste pit with animal bones containing fluorapatite. A second white layer, obtained with barium sulphate, was detected in one of the samples beneath the external, earthy encrustation. It could be perhaps interpreted as the remnants of a polychrome finishing.