Entering St. Peter’s in Vatican, after having recovered from the shock caused by the sight of so much magnificence and exaggerated dimensions, when you begin to look around, you notice that each altar is surmounted by a painting.
Painting? No way.
In all of St. Peter’s there is only one painting and it is found in the chapel of the Blessed Sacrament.
All the other altarpieces are mosaics.
Yes, mosaics. For those who don’t know what a mosaic is, we’ll explain. A mosaic is an image made of small regular or irregular pieces of colored stone, glass or ceramic. These are held in place generally by plaster, when a mosaic is used for decorating a wall, or mortar when the mosaic covers a surface, i.e., a floor.
Each one of the pieces that form the mosaic is called tessera (plural tesserae) and the mosaic artist can make his tesserae from colored stones, which he has to cut to obtain the size he needs it to be, or other materials.
When they started building St. Peter’s they decided that all the altarpieces had to be mosaics and not paintings.