The Medusa Parade Shield by Caravaggio: Making Its Structural Replica, Laboratory Testing, and Numerically Modelling Their Hygro-Mechanical Distortion Behaviour

The parade shields are artefacts present in the 15th century, up to the 17th. Even if their exact function is not clear, it is known they were not used in battle; according to most scholars, they might have been used as display objects, carried by soldiers and retainers of princes and noblemen in military and religious parades, or as tournament prizes. Both the shape and the materials are distinctive aspects of these parade shields and depend on the historic period they were manufactured. Among them, is the Medusa shield painted in Italy by Caravaggio around 1598, and presently exhibited in the Uffizi Gallery (Florence). During the restoration campaign it underwent in 1998–2002, several information were collected about its wooden structure, geometry and materials used. Computed Tomography showed that the wooden structure consists of wooden lamellae forming two adjacent almost spherical shell layers glued together and having the grain directions approximately perpendicular to each other. Anatomic examination confirmed that in Medusa’s and in other cases the lamellae were made of Poplar wood (Populus alba L.). Based on such information a hypothesis was developed about the construction technique originally used, and by applying this technique (which included soaking the Poplar wood lamellae in water and adapting them on a spherical counter form) an exact structural replica was manufactured. Several experimental tests were then carried out on such replica to interpret its mechanical behaviour, including some load tests to study its elastic behaviour, and some hygroscopic ones (i.e. variations in ambient humidity) to observe the induced distortions. The experimental results thus obtained were key to understand the mechanical and hygro-mechanical behaviour of this type of structure, and to explain its tortoise-shell shape, which is typical for the shields consisting of two (or even more, provided it is an even number) wooden layers; a numerical model was also developed, to explain the observed hygro-mechanical behaviour.