Most are Christian, but many pagan and a few Jewish; their iconography has been much studied, although artistically they are relatively unsophisticated. In contrast, a much smaller group of 3rd century portrait levels are superbly executed, with pigment painted on top of the gold. The same technique began to be used for gold tesserae for mosaics in the mid-1st century in Rome, and by the 5th century these had become the standard background for religious mosaics.[23]