REPUBLIC OF TURKEY MINISTRY OF CULTURE AND TOURISM

Glass Techniques

Ancient Glass of Asia Minor

Glass-making Techniques and Other Glass Terms

Virtually all the techniques of glass-making in use today were known in antiquity. The tools have not changed essentially since the discovery of glass-blowing, although on technical level furnaces, chemistry and mass-production methods have been improved. Most of the advances in glass manufacture are the result of the industrial revolution; mechanical pressing, acid-etching and sand-blasting are techniques that have evolved only in the last two hundred years.

Core-forming

the earliest method of making glass vessels is known as core-forming. Small containers were produced by trailing molten glass over a shaped, clay core fashioned on the end of a metal rod. Upon completion, the rod was removed, the vessel annealed (gradually cooled), and the clay core scraped out. The Erimtan Collection contains only one example of a core-formed vessel. It represents one of the last types made by using the technique.

Casting

This process involved the shaping of molten glass in a closed mould or over an open former. The earliest use of casting is found in the production of mosaic glass vessels during the Late Bronze Age. They were made by fusing together thin slices of coloured glass made from canes. In the Iron Age, when translucent monochrome or colourless glass became popular, a simpler method was used for making open-shaped vessels, whereby the hot glass was sagged over a former. Closed vessels, on the other hand, were probably cast using the lost-wax technique. A mould was made by creating a wax or wax-coated model of the object to be produced. The model was enveloped in clay or plaster and then baked, so that the wax melted, leaving a mould into which molten or, more probably, powdered glass could be poured. After casting, the vessels were allowed to cool, and then they were usually cut and ground into their final form.

Blowing

the discovery of glass-blowing occurred slightly before the middle of the first century BC in the Syro-Palestinian region of the Near East. It was, however, not until the use of a hollow metal blow-pipe became accepted practice (probably in the last quarter of the first century BC) that the invention was fully appreciated. The combination of the blow-pipe and the knowledge of inflation revolutionised the glass industry, enabling craftsmen to make vessels more quickly, at less expense and in a greater variety of shapes.

Mould-blowing

This technique developed from the invention of glass-blowing, probably in the first quarter of the first century AD. It allowed the glass-maker to replicate designs and shapes at will, using a pre-made clay, metal or wooden mould.

Pattern-moulding
This was a method in which a gather of glass was partially formed in a mould. The pattern thus created was then expanded and usually spiral-twisted during the re-inflation of the vessel.

Tralling
The application of glass threads, often in a contrasting colour, gave added interest to free-blown glass vessels.

Cutting

Cutting as a type of decoration was a separate operation carried out when the vessel was cold. The Romans distinguished clearly between the glass- maker and the glass-cutter, the former being called a vitrearius and the latter a diatretarius. While the glass-blower was principally involved in the manipulation of hot glass, the glass-cutter employed skills that were more closely related to those of the gem-cutter.

Fire-polishing

The re-introduction of a vessel into the furnace in order to soften and thus smooth out any irregularities on the surface of the glass.

Marvering

Rolling softened glass over a flat surface, usually in order to smooth an added trail decoration into the side of the vessel.

Pontil

A metal rod that enabled the glass-maker to hold and manipulate an object while working on it. After blowing, the pontil was attached to the bottom of a vessel while the rim, handle(s) and other decorative elements were added. The pontil often left a scar or mark where it had been attached to the vessel. Not all vessels, however, have a pontil mark. This may be because the glass-maker had taken the trouble to remove the scar by grinding and polishing the finished article. Alternatively, the vessel may have been held by a clamp or pair of pincers, instead at the end of a pontil. Many small perfume bottles show signs of such tooling around the neck, indicating that for mass-produced, cheap items this was the preferred way of holding a vessel.

Kick

A depression in the bottom of a vessel, usually caused by pressure exerted in the application of the pontil.

Weathering

The iridescence now visible on many ancient glasses is not original or intentional but the result of weathering. Over time the composition of glass can deteriorate, creating a chemical deposit that often flakes off to leave a pitted and iridescent surface. This not only obscures the original colour and transparency of the glass but can also destroy the decorative details and, in extreme cases, the vessel itself. Not all ancient glass, however, is weathered. The weathering is caused by contact with certain types of soil, by moisture and even by frequent changes in temperature and humidity.