St Michael Church, Pavia, dated 1130 AD.
Part of the incredible heritage in the church is a bas relief set above one to the two entrance to the Crypt. It is carved in the local sandstone, the same of facades and interiors. It represents two angels which support a central round frame, inside which it is carved a Lamb, representing Christian Faith (Agnus Dei). The whole scene is inserted into a frame whose section is a “S” with insertion of flower decorations.
The restoration was offered for free to the Church, it was supposed to be an easy restoration work, a present to the priest, who had a particular devotion for this masterpiece. Diagnostics was offered by Università di Pavia and Laboratorio Arkedos s.r.l., under supervision of Dr Pia Riccardi. Researchers planned both cross sections and diffraction.
Once we got results, we had the most surprising revelation: surveys clearly demonstrated the presence, under the dust, of a painted layer, which has been impossible to see before. It was decided to take further samples of extremely small dimensions (<2mmq) on what was supposed to be the red color, to be included in a cross section for being observed on Stereomicroscope and Electronic Microscope SEM.
Characterization of materials were done with spectrophotometry EDS for non organic materials, and FTIR spectrophotometry for organic.
Surveys confirmed there were not organic materials in the layer of color, which was actually a coating with calcium-magnesium binder, painted directly on the sandstone.
Later, during restoration, other samples will be taken on different colors founded; results will be approximately the same.
Due to this results, the project had to change completely, it was clear we had different colors on the carved stone, under a hard layer of candle smoke and biological dust. At the same time we knew colors were extremely fragile and easy to be damaged, so we needed an appropriate consolidation and cleaning not to lose them.
We decide to use laser cleaning technique, involving a professional in this kind of specific cleaning: Dr. Ignazio Tombini. He planned to use two different machines.
The first laser SQS was used with fluence values included between 0,7-1 J/cmq to remove the dust on the part of bas relief both with and without polychromy. In the parts with no polychromy the stone appeared clean, while on the others, once removed the dust, the color resulted generally dark white. The removal of the dark white layer to get the final color was completed with the second laser machine, the SFR, with different characteristics which allowed to remove with a more precise selection the thin layer above the color, to obtain the clean final color without any risk for its safety. In some point the process was helped with support of medical lancet.
After the restoration, the bas relief had all the parts of the original polychromy perfectly visible and in good conditions. (figure 13) All parts of colors which laid under the dust were saved and no materials lost, anyway the most of polychromy has gone many years before we decided to restore it. But what the portions we found out changed completely the idea of the inner decoration of the church, not anymore a grey stone decoration, but colorful refined artwork.
What is interesting is that what was supposed to be an easy conservative work revealed to be a milestone in the knowledge of the Basilica history. And this story involves several different professionals and institutions, which have worked together with a high standard of professionalism.
There was a rare synchrony between the hosts (the church), the academic and researcher (university) and the professionals (private restorers).
At the end a symposium was hosted in the Basilica, with a good participation of researchers and public. Articles were published on papers and conservation magazines. The work was presented at Stirling Stone Conservation "Stone in the Stone", year 2019
