One of the prize possessions of the British Museum, the Portland Vase is a work of art so stunning it can stop even the most casual museum browser like a brick wall. But how exactly was this intricately detailed vase made? Associate Professor Richard Whiteley from the Australian National University School of Art and Design says that previous theories about how 2000 year old vase was created are wrong, and that he has evidence of a more likely explanation.
The Portland Vase isn't very big (only around 9.8 in (25 cm) high and 22 in (56 cm) around at its widest) but the artistry and craftsmanship that went into it are difficult to surpass. It couldn't be duplicated for over 1,700 years because the secret of its manufacture was a lost art.
The Portland Vase was made around 25 AD, give or take 50 years, during a half-century long period of intense experimentation in cameo glass by Roman glass blowers. It consists of a very blue, almost cobalt, glass body with a finely carved layer of white glass on top that covers the lower half of the vase.
The outer layer is carved in fine detail to show six people, a cherubim, and a snake lounging about in an idyllic garden landscape. Though many theories have been advanced, just who the subjects are and what they are doing remains a secret lost in time.
No one is entirely sure of the origins or purpose of the Portland Vase or what its history was before it was discovered in Italy in the 16th century. The most likely theory is that it was a Roman funerary amphora urn that was later cut down into a vase with a glass plate depicting King Priam added to make a new base.
In the 17th century, it passed to the Barberini family before being sold to the Duchess of Portland in 1784. It was later loaned in 1810 by the later Duke of Portland to the British Museum, which purchased the vase in 1945.
The Portland Vase came to particular notoriety, not only through association with Keats's poem "Ode on a Grecian Urn" and Josiah Wedgwood's attempts to duplicate it in Jasperware, but also from the bizarre episode in 1845 when a drunken Irish university student named William Lloyd went on a rampage in the British Museum and smashed the vase to bits. Due to a misspelling on the charge sheet, Lloyd could only be charged with breaking the glass case the vase was in and was fined three pounds.
This vandalism resulted in herculean effort to glue the vase back together, which was not entirely successful, as was learned when a box was discovered in 1948 that contained 37 fragments left over from the Victorian repair effort. That same year, another restoration effort was made, but by the 1980s the adhesive used deteriorated to the point where the vase was in danger of falling apart. So in 1988 the latest restoration project began using a newly developed polymer adhesive that only sets under ultraviolet light and can be dissolved at any later date if needed.
For centuries, the Portland Vase was considered the premiere example of Roman cameo glass, which is made using a technique so advanced that no one could duplicate it. This was such a challenge that in the 19th century a £1,000 prize was offered to anyone who could crack the secret. The contest was won in three years by glass maker Philip Pargeter, and one other duplicate was made at about the same time.
The very general method used to create the Portland Vase was to blow an elongated bubble of blue glass, dip it while still glowing hot into a vat of molten white glass, then blow them together. However, that simple explanation is like saying you play the flute by blowing on one end while running your fingers up and down on the holes. The process is actually much more complex due to the difficulties of getting the glass layers to adhere without cracking, bubbling, or simply sloughing off.
The question is, is the Portland Vase actually a work of blown cameo glass or is it something else? Whiteley says that his research indicates that Roman cameo glass may not be blown, but made using a cold-pressing method called pate de verre. In this, finely ground glass is mixed with a binding agent like gum arabic and water, then pressed into a mold. The mold is then heated into a kiln until the glass melts and conforms to the shape of the mold as it hardens.
By examining a piece of Roman cameo glass from the ANU Classics Department under a Computed Tomography scanner at the ANU Research School of Physics and Engineering, Whiteley says he could see the shape, direction and composition of air bubbles trapped between a blue and white layers of glass. The shape of these were inconsistent from what one would expect in blown glass.
"We saw a bubble configuration within the glass that results from a pressing and turning motion, says Whiteley. "I believe that cold granulated glass has been packed into a mold and then a blob of molten blue glass introduced and pressed against mold heating the white granules from behind.
"You just would not get a bubble that size and flat-shaped from blowing. The most striking thing about it, is not its size and its flatness, but we found a section where the blue glass has mixed with the granulated white specks of glass."
The theory that pate de verre was used to make the Portland Vase isn't new. It was first put forward by Rosemarie Lierke in the 1990s, but Whiteley says this is the first time that there is physical evidence to back the claim.
"It's not about proving people wrong," says Whiteley. "It's about correcting the historical record and reviving and restoring a technique lost for over 2,000 years."
Whiteley is expected to present his findings at a historical glassworks conference at the British Museum this week. He hopes one day to see an international research team recreate the Portland Vase using pate de verre.