The Vatican Museums holds an extraordinary group of seven models realised by Giovan Lorenzo Bernini and his collaborators for the bronze castings of the Altar of the Chair and of the Blessed Sacrament Chapel of St Peter. The models came to the Vatican Museums in 1981, after the ‘Bernini in Vaticano’ exhibition. Although they officially belong to the Fabric of St Peter, the models are on permanent deposit at the Vatican Museums. These life-size models served the dual purpose of verifying the in situ composition and were used as forms for casting. They are rare and fragile, made of a wrought iron skeleton attached to a wooden base and covered with vegetable fibres that reinforce the successive layers of raw clay and straw. Revealing new documents and technical findings, this paper shows how crucial the modernity of Bernini’s oeuvre was to the development of XVIIth century working practices and reconsiders Bernini’s creative process on vast construction sites. This Group of clay models has been accurately restored during a 10 years period. The models have been consolidated and cleaned up of previous, non-conservative, restoration works. In that occasion specialized analysis regarding both the preservation status and the realization techniques, had taken place.