by Dave Parkhill, Assistant Collection Manager
Clear as Glass?
Glass was used throughout the Roman world, with various applications and methods of manufacturing, and with colours ranging from an almost clear, pale green to vivid blues or other bright colours.
A Dubious Origin Story
Glass objects, mainly in the form of simple glass beads have been dated to approximately the 3rd millenium BCE, but it was not until approximately a thousand years later that the first glass vessels emerged in Syria and Mesopotamia, an area that now makes up Iraq, Kuwait, Syria, Turkey and Iran. One historical, albeit fanciful, explanation as to the origin of glass was recounted by Pliny the Elder, a Roman historian from the first century CE. In book 36 of his Natural History, Pliny gives a somewhat serendipitous account of how a Phoenician trading vessel, loaded with nitre, or potassium nitrate, was moored on the banks of the Belus River. The merchants were preparing their evening meal on the beach, and, not having any rocks to support their cooking pots, they turned to using large pieces of their cargo. The nitre melted and fused with the beach sand and as the entire concoction cooled and hardened, glass was formed. This theory has been generally discounted as the high temperatures needed to create glass would not be supplied by a simple cooking fire.
Three Methods of Manufacture
Prior to the 1st century BCE discovery of glass blowing, there were basically two methods of manufacturing glass vessels: slumping and core formed.
Slumping involved heating a piece of glass, roughly cut to size, over a mould and as the glass softened it would “slump” or drop into the mould. This was used to produce plates or bowls or other open containers. The resulting product had both a rough exterior and interior, which was then polished smooth; it was at this point that a design could be cut or etched into the glass.
Core formed glass requires forming a core of clay, or sometimes even animal manure and sand, around a steel rod and then dipping it into molten glass. Strings of glass could be laid onto, and wrapped around the core. The outer surface of the vessel was smoothed against a stone while still soft, and if needed, handles or feet could be added at this point. Once cool, the object, such as this core formed alabastron (image a), was snapped free of the rod and the core removed.
The third approach to making glass vessels was glass-blowing. The development of glass blowing is generally considered to have occurred sometime towards the end of the first century BCE in Syria, which at that time was a province of Rome. The Romans came to excel at glass blowing, a technology that allowed them to create objects with far thinner walls, thereby making the product more translucent and allowing for a greater range of designs. This method was also quicker and less expensive, which in turn gave rise to an increase in productivity, and glassware became more common as everyday items. To create blown glass, molten glass, known as a “gob” is attached to the end of a blow pipe and air is blown through the pipe by the artisan. As the gob inflates it is shaped and formed by rolling or swinging the blowpipe. While the glass is still soft, a rod, or pontil, is attached to the base of the object so that it can be held as the blowpipe is removed, and the mouth of the vessel is smoothed and shaped. Items such as this double balsarium (image b) were formed by glass blowing then folding the tube in half and pinching the fold to close each side. In this example, a glass trail was then added to the upper half and a handle attached. The chalky white encrustations on the balsarium are the results of the glass deteriorating due to the environment of the soil in which it was buried prior to its archaeological excavation.
Beauty and Function
Glass was not only functional but could also be decorative, as in the case of tesserae for use in mosaics for pavements as well as for walls or ceilings, but also for personal adornment such as bangles (image e) and pendants (image f). . With the discovery of new techniques, combined with greater skills of the glass makers, Roman glass became more accessible and served a wider range of uses. The advent of glass blowing, made for a thinner walled vessel, increasing its beauty and decreasing its cost.
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