The beginning of the first truly commercial artisanship of glass making was in Roman Italy, arising from the combined talents of Levantine and Roman workers in workshops such as Aquileia. However, the Levantine coast continued to be at the forefront of glass innovation for the next thousand years.
One technique invented by the Levantine glassmakers was a recipe for opaque glass. The earliest forms of glass were transparent and colored various shades of blue green. The recipe for clear glass was created in the Roman/Levantine workshops. Opaque glasses, which allow for a greater range of color, were achieved by the Levantines. Although long believed to have been invented within the workshops of the Venetian lagoon, recent investigations at the site of Torcello suggest that the opaque glasses used in the mosaics of the Santa Maria Assunta Basilica illustrated in the photograph were not created in Torcello, but rather imported as raw glass and reworked in the workshop there.
It wasn't until about the 12th and 13th centuries AD when glass makers in Venice learned the secret and transformed their recipes from the natron-based Roman clear techniques to opaque techniques invented in the Levant, based on soda-ash.
Sources and Further Information
Stern EM. 1999. Roman Glassblowing in a Cultural Context. American Journal of Archaeology 103(3):441-484.
Verità M, Renier A, and Zecchin S. 2002. Chemical analyses of ancient glass findings excavated in the Venetian lagoon. Journal of Cultural Heritage 3:261–271.
