It would be a masterpiece of ancient art of whatever material. “It is indescribable, really – a tour de force. “It is immensely important,” agrees Martine Newby, a Fellow of the Society of Antiquaries of London who, at the invitation of Bonhams, was the first person to inspect the vase in 2009, reporting her findings in the Glass News journal the following year. “It’s as if the matching pair to Michelangelo’s David suddenly turned up out of nowhere – only this time, it’s Goliath. “It is the single most important piece of glass to appear since the Portland Vase,” explains the glassmaker David Hill, one of seven people invited to the British Museum to inspect the object alongside the Portland Vase and the Auldjo Jug. The lower frieze, which presents a battle scene containing 18 figures including five on horseback and five corpses, provides a clue to the original shape of the Portland Vase, which most likely was not flat-bottomed in antiquity, but tapered like an amphora. These are divided between two dense friezes including a scene that depicts the punishment of the mythological character Dirce, who was killed by being tied to the horns of a bull. The new vase, which stands 35.5cm (14in) tall and weighs 2.85kg (6lb), and is also made in dark blue glass with an opaque white overlay, boasts 38.

The Portland Vase, which is 24.5cm (10in) high and weighs 1.3kg (3lb), is decorated with seven figures. So when Bonhams auction house in London released photographs of a privately owned Roman cameo glass vessel four years ago, it was a big deal.

Unearthing a vessel as fine as the Portland Vase, then, is exceptionally rare. This may be partly ascribed to the time-consuming difficulty of producing cameo glass: it took John Northwood three years to produce the first glass replica of the Portland Vase in 1876. Very few examples of imperial Roman cameo glass survive – in 1990 the scholar David Whitehouse counted 15 major extant vessels and objects, including the British Museum’s Auldjo Jug and the Blue Vase from Pompeii that is now in the Naples Archaeological Museum. The Portland Vase is the most famous example of ancient cameo glass, a type of luxurious vessel inspired by intricate relief-cut gems that thrived in the early Roman Empire up to around AD 50 or 60. Solving its perplexity would diminish its mystique. Does each side represent a different mythological wedding – of Peleus and Thetis, and Achilles and Helen? Perhaps the figures represent historical personages such as Mark Antony and Cleopatra? Or maybe both scenes celebrate the first emperor of Rome Augustus, who is sometimes suggested as the vase’s original owner? The truth is that scholars will probably never agree on the meaning of the Portland Vase, which is now enshrined as a conundrum of the classical world more fiendish than the riddle of the Sphinx. The identity of the rest of the group is up for grabs. Holding a bow and a torch, and fluttering in mid-air, he turns back as though to entice a heroic young man towards a seated, semi-naked woman embracing a slithery monster, possibly a sea serpent, rearing up between her legs. Of the seven figures carved out of opaque white glass overlaying a deep and translucent cobalt-blue background, the identity of only one character is certain – that of Cupid. Over the centuries, scholars have suggested up to 50 different interpretations explaining the imagery on the vase. Since it was first recorded at the turn of the 17th century in Rome, where it was supposedly discovered in a sarcophagus in the tomb of the emperor Alexander Severus, this exquisitely worked example of imperial Roman cameo glass has resisted the ingenious efforts of generations of art historians to understand the mysterious scenes carved upon its sides. Of all the strange and surprising objects to have survived from antiquity, one of the most puzzling is the fragile Portland Vase in the British Museum.

Vidi glass vase