Contrary to general belief, only a few glassmakers who worked in Rome in the sixteenth and seventeenth centuries came from Venice or Murano. Many of them came from elsewhere, as Piegaro, near Perugia, or from Altare, Northern Italy. Important technical questions arising between the late 16th century and the early 17th century concerned the preparation of the wall and the use of oil stucco together with the marble tesserae, as well as the use of painted terracottas (cocci dipinti) instead of the tesserae, and the glass composition.Because of the high cost of materials, in the mosaic history there were several cases of adulteration and cheating, by using “cocci dipinti” or painted tesserae.From archive documents have emerged stucco recipes used in some phases of the mosaic decoration of the New Saint Peter's Basilica. By comparing them with old technical treatises and the outcomes of conservative interventions, we maintain that in Rome various stucco compositions were tested, not always identical to that used in Venice.On account of the extortionate price of materials and manufacture, the Fabrica di San Pietro, availing itself of the big competition among glassmakers, applied for “smalti” supply to workmen of various provenance, who could have lower prices.