In the altar of the Chapel of Theodolinda is kept the Iron Crown, one of the most important and dense products of goldsmithery of all the history of the West.
Miraculously preserved to the present day, the Crown is composed of six gold plates – adorned with relief rosettes, castons of gems and enamels – bearing inside a metal circle, from which it takes the name of “ferrea”, already reported by Saint Ambrose at the end of the fourth century, identifies with one of the nails used for the crucifixion of Christ: a relic, then, that Saint Helena would have found in 326 during a trip to Palestine and inserted into the diadem of his son, the emperor Constantine.
The tradition, which links the Crown to the passion of Christ and to the first Christian emperor, explains the symbolic value attributed to it by the kings of Italy (or by aspirants such as the Visconti), that they would use it in coronations to attest the divine origin of their power and their link with Roman emperors.
Recent scientific investigations suggest that the Crown, which as it appears derives from interventions made between the IV-V and the IX century, may be a royal emblem of late antiquity, perhaps Ostrogoth, passed on to the Lombard kings and finally reached the Carolingian sovereigns, that they would have it restored and donated to the Cathedral of Monza.
From then on, the history of the diadem was inextricably linked to that of the cathedral and the city.
In 1354, for example, Pope Innocent VI sanctioned as an undisputed right – even if then disregarded – of the Cathedral of Monza to be able to host the coronations of the kings of Italy, while in 1576 San Carlo Borromeo established the cult of the Sacred Nail, in order to make official the recognition of the diadem as a relic, or to tie it to another Sacred Nail, kept in the Cathedral of Milan, which according to the same ancient tradition Saint Helena would have forged in the form of a bite for the horse of Constantine, as a further metaphor of divine inspiration in the command of the Empire.
Because of its sacred value, the Iron Crown is preserved in an altar dedicated to it, erected by Luca Beltrami in 1895-96.
