The following text comes from the Youtube description of this wonderful restoration project:
Here come all those who wish to see admirable things” is the English translation of the words set in the marble inlay of the floor of Florence’s baptistery, as visitors enter through the Gates of Paradise. These worthy items include the fourteenth-century mosaics depicting prophets, bishops and cherubs, which are enjoying renewed vigour after the restoration of four of the eight sides of Florence’s oldest monument.
The internal walls of the baptistery began to be restored towards the end of 2017 following a restoration campaign on the external walls and roof. Many discoveries emerged from the diagnostics, the first of their kind to be conducted on the monument, including the original technique used in the parietal mosaics; the presence of a pigmented wax on the green Prato marble, used to cover the white limestone that had formed due to water coming in through the roof, now removed to reveal the stone’s natural hue; and traces of gold leaf on one of the capitals of the matroneum, which could form evidence that the capitals were all originally covered in gold leaf.
In the first couple of decades of the fourteenth century, having completed the colossal feat of the mosaics inside the baptistery’s dome, the decision was made to extend the technique to the parietal sides, something that wasn’t part of the original plans. It was a solution that allowed the mosaics to be superimposed over the marble covering and solve the issue of the monument’s static nature. Made-to-measure hollow terracotta tiles were used, cut and fixed to the marble on the baptistery’s walls with central iron linchpins driven back and welded in a straight line.
“A hurried sinopia was then conducted on the tiles and later the mosaic with a direct method and over days, which can still be identified and interpreted today,” explained Beatrice Agostini, planner and head of the restoration campaign of Opera di Santa Maria del Fiore. “Even the mixture used to apply the mosaic tiles is absolutely unique. Ordinary mortar wasn’t used. Instead it was more of a glue, and it’s the decline of this compound that has caused the most problems in this restoration.”
See also this great article: