Walk n. 1 - Looking for the differences between the Pietra Serena and Pietraforte sandstones
During the walk we can see several buildings made of Pietraforte and Pietra Serena Sandstones; in particular, in Piazza della Santissima Annunziata (stop 1), were both of the sandstones were used, it is possible to look at the Basilica della Santissima Annunziata, the Loggiato dei Servi di Maria, the Spedale degli Innocenti, and the Palazzo Grifoni.
In detail, the columns, the arches, the tympanums and the cornice of the Basilica della Santissima Annunziata are made of Pietra Serena Sandstone. The façade is plastered with traces of frescoes (now replaced by copies). The floor of the loggia is in Apuan Marble, Alberese Limestone and Colonnata Black Limestone. The emblem of the Pucci Family, which financed the loggia construction, is in Apuan Marble and Colonnata Black Limestone.
In the Loggiato dei Servi di Maria, the columns, the external arches, the corbels, the window frames and doors under the loggia are in Pietra Serena Sandstone; the tympanums and window frames, the decorations and the string courses above the loggia are in Pietra Serena Sandstone. The entire structure is finished with Plaster. The steps of the entrance stairway are made of Pietra Serena Sandstone from Firenzuola (Firenze). In the external loggia, there are rounds realized with painted Plaster (marked by the S of the Servi di Maria order) with a finishing in Pietra Serena Sandstone.
The Spedale degli Innocenti is characterized by columns, external arches, capitals, buttresses, string course, tympanums and frames of the windows and corbels, under the arcade, in Pietra Serena Sandstone. The entire structure is in white Plaster; under the arcade, there are frescoes and busts in Apuan Marble. The roundels by Della Robbia are in Glazed Terracotta. The staircase is in Pietraforte Sandstone.
The façade of the Palazzo Grifoni is clad in red and ochre Brick; the ending cornices, the string courses, the corner ashlars, the windows (kneeling, gable and plate), and the arched portals, framed by rays, are in Pietra Serena Sandstone (Pietra Bigia variety). Many decorated ornaments and friezes, as well as the bench, are made of artificial stone (Mortar).
The main façade, in Via dei Calzaiuoli, of the Chiesa di Orsanmichele, whose entrance is in Via dell’Arte della Lana, is made of Pietraforte Sandstone and Plaster, the mullioned windows are in Apuan Marble, the statues are made of Apuan Marble and Bronze, the coats of arms in Polychrome Glazed Terracotta. In some cases, the interior of the niches is in Apuan Marble like the external decorations, in other cases is in Pietraforte Sandstone or in other stone materials. The façade of the Chiesa di San Carlo is realized in Pietraforte Sandstone, the steps and the pediment of the door are in Pietra Serena Sandstone.
The walk ends in the Piazza della Signoria, entirely surrounded by Pietraforte Sandstone buildings. For the city of Florence, the Piazza della Signoria can be considered the “Pietraforte Square”: here we can admire the Palazzo della Signoria, seat of the municipality and several other monuments such as the Loggia dei Lanzi, the Palazzo delle Assicurazioni Generali, the Palazzo del Tribunale di Mercatanzia, and the Palazzo Uguccioni.
The Palazzo della Signoria and the tower are entirely made of Pietraforte Sandstone. The mullioned windows are in Apuan Marble. The radiant trigram of Cristo Re above the main door is in Apuan Marble; behind it there is a painted Plaster (blue background with golden lilies). The coats of arms with the symbols of the Florentine Republic are painted on Plaster.
The Loggia dei Lanzi is entirely made of Pietraforte Sandstone, the balustrade of the terrace is in Pietra Serena Sandstone, the allegorical figures on the façade are in Apuan Marble. The sculptures under the arches are in Apuan Marble, except for the Perseus statue, made of Bronze. The lions at the sides of the steps are in Apuan Marble with a Travertine base.
The main and secondary façades of the Palazzo delle Assicurazioni Generali are made in Pietraforte Sandstone. The mullioned windows are in Apuan Marble.
The façade of the Palazzo del Tribunale di Mercatanzia is realized by a lining made of ashlars in Pietraforte Sandstone. The top floor of the building is plastered. Above the windows of the first floor there are decorations in graffiti Plaster. The sprandels are in Pietra Serena Sandstone.
The façade of the Palazzo Uguccioni is made of Pietraforte Sandstone. The bust of Francesco I, above the entrance door, is made of Apuan Marble.
The Pietraforte Sandstone was used mainly as cladding material in most of the Florentine palaces due to its high resistance while Pietra Serena Sandstone has been widely used for decorative purposes due to its high ductility but low durability. The main differences between the two sandstones consist in a very fine grain size crossed by veins of calcite and presence of convoluted laminations in the Pietraforte Sandstone while the Pietra Serena Sandstone, characterized by a medium coarse grain size, is more compact.
The colour of these sandstones is not diagnostic; when cut, the unaltered sandstones display a gray-bluish colour that can become ochraceous by alteration.