by Marco Giamello

Scientific investigations into the facade of the Palazzo Pubblico of Siena

The preliminary investigations for the restoration, carried out by the Institute of Minerology and Petrography of the University of Siena, began in 1995 with the studies of the decoration of the “stone tower”. Consequently the working studio of the Central Institute of Restoration in Rome became involved for the examination of other elements including the marble columns, marble crests and cornices.

Supporting analyses also took place during the intervention on the whole facade and archeometric investigations of the mortar on various sectors of the facade.

Results

The most relevant data to emerge from the scientific investigations (conducted according to the methodology already established for the Duomo of Siena) were those relating to the presence of thin films of different types of calcium on the surface that derived from previous treatments; the distribution of which was linked to the material of the underlying layers and the architectural collocations.

The discovery of treatments in the area of the three-mullioned window hidden by the elevation in the 1400s of the Capella di Piazza indicated that the treatments had taken place some time during the 1300s.

The treatments on the first floor were carried out using red, orange and yellow ochres that were present in differing proportions and in some cases blackened with carbon; all of which were created using organic substances (linseed oil, animal glue and others). A final coat, usually of linseed oil had then been applied to all of the surfaces.

The purpose of this was to create a chromatic uniformity to the decoration of the brickwork and to accentuate, with diverse tones of red, all of the different decorative and structural elements of the three-mullioned window and its arches (fig1-3), the cornices of the same arches, the trilobite arches and the other architectural elements of the facade such as the corbel feet.

The crests present in the timpanies of the three-mullioned windows, made of white veined marble and dark green serpentine had also been applied with substances to create uniformity and maximization of the colour: residue of white lead was found on the marble and traces of linseed oil were discovered on the serpentine.

Even the stone decoration showed signs of previous treatments with ochre and the black of carbon that had been used to create uniformity where there could have been an unevenness of colour on the surfaces.