The Lycurgus Cup – (Accidental?) Roman Nanotechnology

Here we present selected parts of the very interesting paper “The Lycurgus Cup – A Roman Nanotechnology“, by Ian Freestone, Nigel Meeks, Margaret Sax and Catherine Higgitt.

The Lycurgus Cup represents one of the outstanding achievements of the ancient glass industry. This late Roman cut glass vessel is extraordinary in several respects, firstly in
the method of fabrication and the exceptional workmanship involved and secondly in terms of the unusual optical effects displayed by the glass.

(…) The openwork decoration of the Lycurgus Cup comprises a mythological frieze depicting the legend of King Lycurgus from the sixth book of Homer’s Iliad. The figures, carved in deep relief, show the triumph of Dionysus over Lycurgus. However it is not only the cut-work design of the Cup that shows the high levels of skill involved in its production. The glass of the cup is dichroic; in direct light it resembles jade with an opaque greenish-yellow tone, but when light shines through the glass (transmitted light) it turns to a translucent ruby colour.

(…) The mythological scenes on the cup depict the death of Lycurgus, King of the Edoni in
Thrace at the hands of Dionysus and his followers. A man of violent temper, Lycurgus attacked Dionysus and one of his maenads, Ambrosia. Ambrosia called out to Mother Earth, who transformed her into a vine. She then coiled herself about the king, and held him captive. The cup shows this moment when Lycurgus is enmeshed in vines by the
metamorphosing nymph Ambrosia, while Dionysus with his thyrsos and panther, a Pan and a satyr torment him for his evil behaviour.

lycurgus cup

(…) Although now lost, due to breakage at some point in the past, the cup must originally have had an openwork base and may have had a taller rim. The current silver-gilt foot with open-work vine leaves and the rim mount of leaf ornament are thought to date to the eighteenth or nineteenth centuries. On stylistic grounds, and also from the dates of comparative pieces (some of which are associated with more easily dated objects), the Cup has been dated to the 4th century AD. Harden and Toynbee suggested that it is probably of Italian manufacture, although they considered an Alexandrian origin also possible.

(…) The most remarkable aspect of the Cup is its colour. Only a handful of other ancient glasses, all of them Roman, change colour this way; several of these are diatreta, with the
more typical geometric decoration, but tend to show a less spectacular colour change. It is therefore likely that the Lycurgus Cup was a special commission produced by a workshop which already made highly specialised and expensive glass products.

(…) Chemical analysis (in 1959) at GEC (General Electric Company Ltd) showed the glass to be of the soda-lime-silica type, similar to most other Roman glass (and to modern window and bottle glass), containing in addition about 0.5% of manganese. In addition, a
number of trace elements including silver and gold make up the final 1%. It was further suggested that the unique optical characteristics of the glass might be connected with the
presence in the glass of colloidal gold. It was also noted that “to obtain the colouring constituents in the state necessary to give the remarkable glass its special qualities a critical combination of conditions was required during manufacture. These would be associated with the composition, including the presence of minor constituents, time and temperature of founding, chemical conditions during founding, and subsequent heat treatment. It is perhaps not altogether surprising that no other example of a glass having such unusual properties has come to light”.

(…) in 1962 a sample was sent to Dr Robert Brill of the Corning Museum of Glass (…) Work carried out by Brill, latterly in collaboration with GEC, on the Lycurgus Cup and diatretum samples (and on another example of dichroic glass) as well as on experimental glass melts confirmed that the dichroism was linked to the presence of minute amounts
of gold (about 40 ppm) and silver (about 300 ppm) in the glass. However, simply adding traces of gold and silver to glass would not produce these unique optical properties and the critical factor was believed to be the formation of minute submicroscopic crystals or colloids of the metals. Colloidal systems can give rise to light scattering phenomena that result in dichroic effects. It was suggested that both the gold and silver contributed to the colour, the gold component being mainly responsible for the reddish transmission and the silver for the greenish reflection.

(…) The work of Brill and GEC suggested that glass containing minute amounts of gold and silver had been heat treated, using suitable reducing agents, to produce colloidal metallic particles within the glass which resulted in the green-red dichroic effects. The colours produced in such a process would have depended upon the precise colloidal concentration and the particle diameter and are highly dependent on the proportions and oxidation states of certain elements, the time and temperature of heating and probably the atmosphere during heating.

(…) in the late 1980s, a further small fragment of the Cup was subjected to examination by Barber and Freestone. Analytical transmission electron microscopy revealed the presence of minute particles of metal, typically 50-100 nm in diameter. X-ray analysis showed that these nanoparticles are silver-gold alloy, with a ratio of silver to gold of about 7:3, containing in addition about 10% copper. The identification of silver-gold alloy particles confirms the earlier inference that the dichroic effect is caused by colloidal metal. In addition to these metallic particles, the glass was shown to contain numerous small particles (15-100 nm) that were shown to be particles of sodium chloride; the chlorine probably derived from the mineral salts used to supply the alkali during the glass manufacture.

Of interest is the high gold to silver ratio of the alloy particles in the glass (c. 3:7) relative to the gold:silver (Au:Ag) ratio in the glass as a whole (c. 1:7). This is a reflection of the relative reduction potentials of Ag+ and Au+ and indicates that a substantial proportion of the silver remained dissolved in the silicate matrix after precipitation of the alloy particles. Recent work by Wagner and co-workers indicates that gold dissolves in glass in the monovalent form. The reduction of previously dissolved silver and gold, during heat-treatment of the glass, will have caused the fine dispersion of silver-gold nanoparticles responsible for the colour. A key agent likely to have been involved in the redox reaction that reduced the silver and gold is the polyvalent element antimony, which is present in the glass at around 0.3%. Antimony was commonly added to glass in the Roman period, as both an oxidising agent (decolourant) and as an opacifier.

lycurgus cup2

The fine particles of sodium chloride observed are likely to have exsolved from the glass during the heattreatment that caused the crystallisation of the alloy particles, but as they are colourless and their refractive index close to that of soda-lime-silica glass, their direct contribution to the colour of the glass is likely to have been minimal. However, halide additions have been found to promote the development of colour in gold ruby glasses so it is possible that the sodium chloride in the glass indirectly contributed to its colour.

(…) following a detailed examination of the surface of the Cup using low power microscopy, Scott suggested in 1995 that the Lycurgus Cup had been cut and polished using rotary wheels ranging from 6 to 12 mm in diameter. However, more recently Lierke has suggested that many current assumptions about early glass working techniques are incorrect. In particular she has suggested that diatreta such as the Lycurgus Cup were not formed by cold cutting of glass blanks but by moulding.

(…) The fragment of openwork (vine stem) found when the base of the Cup was removed was examined for traces of tool marks with a binocular microscope and a scanning electron microscope.

(…) Examination of the open-work glass fragment showed that faint tool marks remain on most of the surfaces. The tool marks provide extensive evidence for mechanical abrasion and polishing not only on the outer surface but also on the sides and underneath the fragment. The sides of crescentshaped cuts through the glass suggest the use of rotary abrasion and polishing. In contrast, the front and the back of the open work appear to have been worked with non-rotary files and abrasives. The evidence for the mechanical removal of glass from the undercut back area of the fragment suggests that cutting and grinding rather than moulding of soft glass was the method of producing the lattice design. The very highly polished surfaces of the fragment, once thought to have been fire polished, seems to have been produced purely by mechanical means as groups of regular fine parallel striations can be seen.

nanoalloy

The skill of the craftsman consisted not only in the cutting of such an intricate design in such a fragile material, but also in the design and layout of the figures, and the advantage taken of the colour effects.

(…) the glass inside the Cup and behind the bodies of the figures, which are not completely undercut, has been hollowed or bored out.

(…) The Lycurgus Cup is therefore made of a very rare glass, and this glass seems to have been saved for a very rare type of vessel – a figurative cage cup. The execution of the openwork was carried out in a very skilful manner and must surely have been the work of a master lapidary. Even using modern powerdriven tools, this type of vessel takes a great deal of time to complete. Unlike the majority of glass of its time, the Cup, with its unique colour and decoration, must have been highly valued and intended for some special purpose.

(…) It is clear that the colouring of glass using gold and silver was far from routine and something of a hit and miss affair. There were a large number of factors to control, including the overall concentration of the metals, their distribution and the time and temperature at which the glass was heat-treated. It seems that not even the absolute and relative concentrations of gold and silver were easily controlled, let alone the distribution and growth of particles. Gold and silver concentrations vary widely between the few examples known, and even the colour of the Lycurgus blank was not homogeneous.

(…) It is quite likely that the glassmakers were unaware that gold was the critical colourant, as most of these glasses are richer in silver. To introduce gold as a component of a gold-silver alloy (electrum) would make sense, as it would have allowed a more even distribution of the gold in solution. The addition of metals or metal oxides to colour glass was familiar to Roman glassmakers; for example, opaque red and brown glasses were produced by the addition of copper. Freestone et al. have speculated that the oxidised by-products of metallurgical processes (“dross”, “slag” etc) were sometimes acquired to colour glass, and that this might explain how the “Lycurgus effect” was discovered. It would also explain the relatively high levels of copper and lead oxides which are also present in the glass.

(…) it appears that replicating gold ruby was a challenge to the Roman glassmaker; the technology was very restricted and does not appear to have outlasted the fourth century.

(…) The Lycurgus Cup demonstrates a short-lived technology developed in the fourth century A.D. by Roman glass-workers. They discovered that glass could be coloured red and unusual colour change effects generated by the addition of a precious metal bearing material when the glass was molten. We now understand that these effects are due to the development of nanoparticles in the glass. However, the inability to control the colourant process meant that relatively few glasses of this type were produced, and even fewer survive. The Cup is the outstanding example of this technology in every respect – its outstanding cut work and red-green dichroism render it a unique record.

Research-Selection: Anastasius Philoponus

 

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