Vitrum - Roman glass

Difficult beginnings of fragile material
Glass was probably discovered in the 3rd millennium BC as a by-product of pottery making. At the earliest glass-making sites, a suitable mixture of sand, soda and calcium was naturally occurring and needed only to be "merely" melted. At the same time, greenish glassy-transparent slags and deposits were probably also obtained when smelting copper. It took about 500 years before glass vessels appeared. It wasn't until another 1,500 years later that the breakthrough discovery was made in the first century BC that glass could be inflated with air. Although, according to Pliny the Elder, the cradle of glass art was said to be in Palestine, the oldest glassware we know of comes from Mesopotamia and Egypt. The first objects made of glass were rare and expensive. Raw glass was smelted in only a few places and transported long distances.

Apart from the high fragility of glass, also chemically it is not as stable as one would like. Many ancient glasses were, for lack of sufficient calcium, water-soluble and simply disappeared. But copies that have survived to our time also suffer from weathering. The surface of the glass discolors and can form a silvery-white to black outer layer. Even such attractive opalescence is in fact a flaking of tiny plates and refraction of light in bright colors.

From the first century A.D., glass production expanded so much that wealthy homes often had glass windows, and glass vessels became available to many people. By the time of the Migration of Peoples (the migration of tribes in the 4th to 6th centuries), the knowledge of making raw glass was lost in many provinces and recycled Roman glass was resorted to.

Antique glass

Until the beginning of the Roman Empire, raw glass was produced by a few specialists and sold over long distances. It is not known exactly what such trade looked like; finds usually involve already tinted material.

Ancient Roman glass is composed of soda, calcium and silica. The latter is actually the main ingredient in glass, but it has a high melting point, which was a major challenge for ancient technology. By adding alkali (soda), the temperature is lowered to an achievable 1000°-1100°C.

In contrast, the importance of the addition of calcium was probably not known. Without it, however, the glass mass is soluble in water. Many ancient glasses simply dissolved over the years, or found themselves in an unstable state. Where sand impurities (shell fragments, river stone) provided the needed calcium addition, a durable glass mass was formed.

Roman glass from the imperial era consists of 55-75% silicon dioxide (silica), 10-20% alkali, 5-15% calcium and trace elements.

Colored Roman glass

Even the oldest glass fragments found show that dyeing the glass mass was practiced from the beginning. Often it was a final treatment performed by the manufacturer of the raw glass. Nevertheless, coloring may also have been performed during processing. Such treatments have been confirmed in Egypt since around 1300 BC. Multicolored vessels from that period must have been made from raw glass subjected to several stains. Mixing different glass masses would have led to destructive stresses.

Glass has a natural color of transparent green with blue. By a small addition of antimony, glass can be discolored. This was known at least as early as the 8th century BC in Mesopotamia. Decolorized Greek glass has been known since the 5th century BC. In the 1st century BC, pure glass began to be made with the addition of manganese.

Roman glassworking techniques

Glass pottery of the Romans

Surprisingly, the technique of forming glass on the potter's wheel, which was often used in antiquity, has become quite a thing of the past. Glass has a plastic nature at the right temperature, which allows it to be deformed. Combined with the classical potter's wheel, it manages to produce intervening effects. The early history of antique glass was alongside ceramic pottery. Not surprisingly, many forms were introduced in parallel. It was not until the late Hellenistic period that artists in the field of glassmaking developed new and only appropriate forms of expression for this material. Vessels made on the potter's wheel usually have thick walls and rounded edges - ideal qualities for utility glass.

Roman blown glass

In the early period, the technique of blowing hot glass into complex three-dimensional forms was developed. In the refinement of complex multi-part models led initially Syria with Palestine. It was not until the Roman imperial period that the northern provinces joined them. Many decorated relief containers and vessels were made using this technique. Complex models made using the blowing technique often showed visible seams between segments. These unwanted blemishes were attempted to be masked by incorporating them into the overall design, giving the impression of intentionality.

Roman glass reliefs

First, a plaster model was made with a very accurate relief. The cavities were then filled with white glass powder and poured over the glass mass at the end. The liquid glass was hot enough to melt the white powder.... Subtle and precise reliefs on glass would not have been possible with the grinding technique of the time.

Miniature mosaic glass

During the period from the first century BC to the first century AD, particularly precise miniature tiles were produced using the mosaic technique. The spectrum of colors used and the accuracy of the workmanship will amaze you. Decorative tiles were often inserted into furniture. Alexandria in Egypt is mostly mentioned as the center of mosaic glass making.

Roman window glass

Window glass has been known since the beginning of our era. First came the glazed windows of baths and wealthy homes. Over time, window glass became more common, and was also found north of the Alps in palaces, public buildings and Roman villas.

For a long time it was thought that Roman window glass was poured and pressed. However, English researchers found that such a technique using ancient means would not have been possible. The following method of making window glass was established:

Molten glass was poured onto a moist surface and immediately rolled with a wooden roller. A circular disc about 5 mm thick was formed. This one was then transferred to a backing sprinkled with a release agent to prevent the glass from sticking. The disc on the backing was heated again. One edge was fixed with a metal bar, and the opposite edge was stretched with tongs until a near-right angle was formed. The other three angles were treated in a similar manner. Finally, the entire pane was heated and the edges were leveled by applying pressure with a metal bar. At the end of the second century AD, another technique was invented:

Hot glass was blown into a large cylindrical form, the ends were removed, and the cylinder was split along the forming and unrolled. The glass thus produced was smooth and usually less durable. The new technique did not supplant long-practiced methods, but may have been preferred for particularly valuable glazing.