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Rosemarie Lierke

Antike Glastechnologie / Ancient Glass Technology

English summary of
R. Lierke: Zur Herstellung der antiken Kameogläser
On the manufacture of ancient cameo glasses.
Restaurierung und Archäologie 4, 2011, 75-105

According to the recently published BM-cameo glass catalogue [1], ancient cameo glasses were cut from blown overlay blanks, and they were manufactured during a very short period of time, between 15 BC and 25 AD. However, there exist several reasons to disagree with these statements. Most considerations concerning this topic have been published fifteen years ago , but a few have been added in the meantime [2]. They are summarized in this article.

1. Cameo glasses were not blown

Glass blowing was invented around the middle of the first century BC. According to recent investigations, the technique did not spread very fast, and blown vessels remained to be rather small, lightweight, and thin-walled till the second half of the first century AD. However, some cameo glasses are tall and heavy, some are shaped like earlier non-blown vessels, and the Auldjo Jug seems to have been sagged. Several technological preconditions for blowing an overlay blank suitable for cutting had yet to be developed, such as the use of metal blowpipes, and the pontil technique.

2. Cameo glasses were not made from overlay blanks

Blown overlay blanks with two glass layers necessarily have a smooth boundary between the layers. It is shown that this is not always the case with cameo glass. The use of crucibles with low viscous fluid glass is not attested yet for the assumed period of cameo glass production. The Roman glass blowing furnace was very small, it had room for just one crucible. Two crucibles or even more would be required for the assumed dip-overlay with up to six layers. The compatibility of the differently coloured glass layers seems to have been less of a problem because of the common basic raw glass. Therefore, true overlay glass does exist in antiquity, but with a very thin internal layer only, and not before at least 30 AD – that is after the assumed cameo manufacturing time.

3. The cameo decor was not cut

Cameo objects made from stone may be cut and ground. Cameo objects made from glass are related in their appearance, but this does not apply to their manufacturing technique. In crystalline stone, stress is scattered and sometimes neutralized at the boundaries of the crystals. In amorphous glass, the stress potential increases with the thickness of the wall. However, glass needs to be free from stress for cutting. To free glass from stress after the hot manufacturing process, an exact time-temperature cooling cycle is required. This cooling cycle depends on the composition of the glass, the manufacturing process, the shape, and the object’s wall-thickness. A perfect cooling was hardly possible in antiquity because a proper temperature measurement and regulation was not possible. Therefore, caution is advised when interpreting ancient relief glass as having been cut. Cameo glass in particular was not cut.

This conclusion is justified by several features: The background of the cameo decor is generally smooth which today is hardly achieved after cutting without using hydro-fluoric acid. The typical and frequent cutting flaws of the much younger and simpler intaglio-cut glass do not exist in cameo glass. The surface of the (white) cameo decoration is smooth and almost bubblefree instead of porous, as one would expect if the glass, said to contain »a myriad bubbles«, had been cut. The generally smooth and rounded features of the cameo decor suggest it was hot formed with a mould. Several ancient cameo glasses show additional features indicating they were made by hot forming, or features that can not be reconciled with a cold abrasive finishing.

4. The internal scratches of cameo glasses are no grinding marks

Many ancient non-blown glass vessels show typical horizontal scratches.These scratches are distinct and sharply incised in a shiny surface (unless the vessel is too deteriorated). They are not continuous, and they are usually not entirely parallel. The scratches usually occur on the inside of vessels, but glasses without relief on the outside may feature them on both sides. The scratches have been noted also on the inside of cameo glass vessels, as for example on the Portland Vase. It has been suggested that the scratches on the interior of the Portland Vase are grinding marks. However, it is shown that the reasons given for an internal grinding of this and other cameo glass vessels are not valid. The typical scratches appear also on the interior of small cameo glass vessels with long narrow necks precluding the possibility of grinding the inside. Moreover, true grinding marks look very different! The typical scratches obviously are generated by sharply pointed tiny protrusions of the ancient mould material during the hot forming.Scientific investigations identified them as »hot scratches«.

5. Cameo glass vessels and plaques were made by hot forming

Glass cameos were made since Ptolemaic times by melting glass powder for the cameo decoration in moulds. The same method was used to create glass cameo plaques of larger sizes. The subjects and motifs seen on cameo glass vessels and mould made relief ceramic vessels are often similar, or almost identical. Their manufacturing technique also is related. However, a glass relief vessel can not be pressed in a reusable mould. The artisan prepared a wax model and made a mould for instance of plaster or plaster mixed with quartz powder. He filled the cavities of the mould with glass powder, with or without an anorganic binder. Often, lead was added to lower the melting temperature. The addition of lead was restricted to vessel shapes (as is confirmed by chemical analysis), because plaques did not need such an addition. Very hot glass was pressed into the mould. The heat of the hot glass melted the glass powder. The crystal water evaporating from the plaster mould generated temporarily a thin layer of steam which prevented the glass from sticking to the mould and created a smooth glass surface. The mould became weak and brittle from the loss of the crystal water. It could be broken off and released a cameo glass vessel with a smooth surface. Vase and bottle shapes had to be sagged in a subsequent manufacturing step. Since the wax model and the plaster mould were both destroyed in the process, every cameo glass vessel is a unique objet d’art.

The hot forming of cameo glass is a logical step in the history of glass technology. It is unrealistic to assume the existence of a perfect high relief cameo cutting technology more than a hundred years before the first primitive figural intaglio glass cutting appeared with its numerous typical cutting flaws. The cameo manufacturing steps which are here proposed were successfully tried at several occasions with improvised experiments. Modern equipment and glass material was used as is customary today (see BM cameo catalogue p. 26). It would be desirable to perform more scientific experiments, using close to original preconditions, especially a glass material molten as in antiquity at low temperatures over several days which definitely must have influenced the working properties. More scientific experiments may result in a variation of the proposed manufacturing process, but no doubt seems to remain about the hot forming of ancient cameo glass.

The author acknowledges with great gratitude especially the fairness and valuable help of Veronica Tatton-Brown and later Paul Roberts during repeated investigations of the original cameo glasses in the British Museum.

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Notes

[1] P. Roberts/ W. Gudenrath/ V. Tatton-Brown/ D. Whitehouse, Roman Cameo Glass in the British Museum (London 2010)
[2] R. Lierke, Die Portlandvase – ein Produkt der Töpferscheibe? Antike Welt 27/3, 1996, 191-207; R. Lierke, Glass Vessels made on a Turning Wheel: Cameo Glass. Annales du 13e congrès de l’association internationale pour l’histoire du verre, Pays Bas 28. 8. -1. 9. 1995 (Lochem 1997) 63-76; R. Lierke/ M.R. Linding, Recent Investigations of Early Roman Cameo Glass 1. The Cameo Manufacturing Technique and Rotary Scratches of Ancient Glass. Glastechnische Berichte 70/6, 1997, 189-197; H. Mommsen et al., Recent Investigations of Early Roman Cameo Glass 2. X-ray Fluorescence Analyses Induced by Synchotron Radiation. Glastechnische Berichte 70/7, 1997, 211-219; R. Lierke et al., Antike Glastöpferei – ein vergessenes Kapitel der Glasgeschichte. Mainz 1999; R. Lierke, Die nicht-geblasenen antiken Glasgefäße/ The non-blown Ancient Glass Vessels. Offenbach 2009.