Some of the finest examples of ancient Roman glass are represented in cameo glass, a style of glassware that saw only two brief periods of popularity. The majority of vessels and fragments have been dated to the Augustan and Julio-Claudian periods, from 27 B.C. to 68 A.D., when the Romans made a variety of vessels, large wall plaques, and small jewelry items in cameo glass. While there was a brief revival in the fourth century A.D., examples from the later Roman period are extremely rare. In the West, cameo glass was not produced again until the eighteenth century, inspired by the discovery of ancient masterpieces such as the Portland Vase, but in the East, Islamic cameo glass vessels were produced in the ninth and tenth centuries.
The popularity of cameo glass in early imperial times was clearly inspired by the gems and vessels carved out of sardonyx (42.11.30) that were highly prized in the royal courts of the Hellenistic East. A highly skilled craftsman could cut down layers of overlay glass to such a degree that the background color would come through successfully duplicating the effects of sardonyx and other naturally veined stones. However, glass had a distinct advantage over semi-precious stones because craftsmen were not constrained by the random patterns of the veins of natural stone but could create layers wherever they needed for their intended subject.
It remains uncertain exactly how Roman glassworkers created large cameo vessels, though modern experimentation has suggested two possible methods of manufacture: “casing” and “flashing.” Casing involves placing a globular blank of the background color into a hollow, outer blank of the overlay color, allowing the two to fuse and then blowing them together to form the final shape of the vessel. Flashing, on the other hand, requires that the inner, background blank be shaped to the desired size and form and then dipped into a vat of molten glass of the overlay color, much like a chef would dip a strawberry into melted chocolate.
The preferred color scheme for cameo glass was an opaque white layer over a dark translucent blue background, though other color combinations were used and, on very rare occasions, multiple layers were applied to give a stunning polychrome effect. Perhaps the most famous Roman cameo glass vessel is the Portland Vase, now in the British Museum, which is rightly considered one of the crowning achievements of the entire Roman glass industry. Roman cameo glass was difficult to produce; the creation of a multilayered matrix presented considerable technical challenges, and the carving of the finished glass required a great deal of skill. The process was therefore intricate, costly, and time-consuming, and it has proved extremely challenging for modern glass craftsmen to reproduce.
Although it owes much to Hellenistic gem and cameo cutting traditions, cameo glass may be seen as a purely Roman innovation. Indeed, the revitalized artistic culture of Augustus’ Golden Age fostered such creative ventures, and an exquisite vessel of cameo glass would have found a ready market among the imperial family and the elite senatorial families at Rome.