Drawing of the blocks which make up the four panels of the relief on the façade.
The marble blocks that make up the panels are of many different sizes and shapes. They are largely reused, some of the shapes reflecting their previous function as columns and pilasters, for example. The irregularity of shapes and the extensive recycling may be due to supply issues, especially as it is noticeable that the blocks are larger and more regular higher up. This is supported by documentary evidence which records the a donation of stone to the Cathedral from a house outside of Rome. The analysis of the carving suggests that the blocks were worked on the ground with the finishing done in place.