Technicians and master builders for restoration of the dome of St. Peter’s in vatican in the 18th century: the contribution of Nicola Zabaglia (1664-1750)

In the early 18th Century, the empirical practice and oral transmission of the traditional operational knowledge are called into question by the pressing progress of science as well as by institutions. In this context, the works of Nicola Zabaglia (1664-1750), master mason of St. Peter’s Fabbrica, and of his talented students may be considered an influential model of cohesion among architecture, building yard and applied mechanics. Skilled conceiver of many scaffolds and ingenious mechanical devices for building, Zabaglia significantly improved the building technology, by making of adaptability, flexibility of use, safety and cheapness the main features of the scaffolds he designed for restoration and maintenance of St. Peter’s. Significant pictures of the most important scaffolds designed by Zabaglia are collected in the compendium titled Castelli e Ponti, published in Rome in 1743, and later reprinted in 1824 to include seven new engravings’ illustrations. The mechanical devices, scaffolds and working tools here described are those which will be used in the Roman building yard till the first decade of the 20th Century. Thus, Zabaglia’s work represents a basic milestone for the development of Roman building technology, as proved by his being deeply involved in the works for hooping of the St. Peter’s dome, directed by Luigi Vanvitelli (1700-1773) between 1743 and 1744. Nevertheless, most of the specialized studies relevant to these works have been more focused on engineering than practical aspects, by analysing the scientific theories involved more than the building techniques, and the role of Nicola Zabaglia has remained not well known even today. In fact, several technical boards were commissioned by pope Benedict XIV during St. Peter’s restoration works to give their scientific opinion on the problem of the cupola’s stability, which seemed to be near to collapsing. Results of these scientific assessments can be found enclosed to Memorie istoriche della Gran Cupola del Tempio Vaticano, written by Giovanni Poleni (1683-1761) and published in Padova in 1748. Thanks to his great experience in similar activities undertaken in St. Peter’s Fabbrica, like transportation of the Sallustian obelisk (height 14m) from Villa Ludovisi to St. John Lateran square, or the tribune’s restoration in the Basilica of St. Paul Outside the Walls in 1735, Zabaglia was charged of building the scaffolds necessary for the dome’s inspection by Poleni and Vanvitelli, as well as fitting the first five iron rings required to strenghten the dome. Later, in 1748, Zabaglia erected scaffolding also for the sixth ring. Zabaglia is also remembered not only for the project of scaffolding, but also to have sensibly improved the procedures used for assembly of the rings’ components, their transportation to the different heights of the dome, and fitting of the complete rings to the dome’s structure. Zabaglia carried out these delicate activities together with a team of seventeen labourers, he assigned to the construction of the scaffolds, as well as to lifting and installation operations. This team was composed by his better skilled apprentices, who were called again later to erect scaffolding also for other hard restoration works, even outside the Vatican’s border, like those carried out in 1758 on the Pantheon’s dome. Thanks to the information also provided by very interesting unpublished documents, this contribution intends to highlight the working procedures, phases, technologies and organisational models adopted by Zabaglia and Vanvitelli for the restoration works of the Cupola. These works represent one of the most extraordinary examples of the capability achieved by the technicians of St. Peter’s Fabbrica in the middle of the eighteenth century. It’s not a case that just in this period, with the foundation of the papal institute for building technicians and craftsmen, the Studio Pontificio delle Arti, the Fabbrica becomes an influential alter ego of the prestigious Academy of Saint Luke, as an attempt to defend the traditional operating practice from the pressing progress of theoretical science