Iridescent Roman Glass:Iridescence is the rainbow-like effect that changes according to the angle from which glass is viewed or the angle of incidence of the source of light. It is this quality that makes many pieces of Roman glass so appealing to viewers. On ancient glass, iridescence is caused by interference effects of light reflected from several layers of weathering products. A similar effect can be achieved by introducing tin of lead oxides into melted glass or by spraying the same chemicals on the outside of already formed pieces and re-firing them. You might think that this would lead to widespread forgery of Roman glass vessels, but very little of that is done: the real thing is so common that it's not worth while to make forgeries.
Looking at the prices of "Roman" glass pieces on the Internet and at stores around the Mediterranean, you would think that Roman glass is extremely rare. In reality, there are tons of the stuff available, but the people who have it maintain control of the markets. I knew one dealer in Jerusalem who had more than 30,000 pieces in his warehouses, most of them intact -- I didn't actually count them, but he showed me the inventories and the very, very long rows of shelves. He was not even a major dealer. I also knew a private collector with about 5,000 pieces, mostly intact, which he had collected over thirty years as a guide in Israel and Lebanon.
Almost everything on the market today would be classified as "common" Roman glass, with little or no ornament -- perhaps a squiggle or two of appliqued lines or swirls in the same color as the base material. There are cups ("beakers"), goblets, bowls, jugs, vases, and many many unguentaria, those small thin bottles, some with bulbous bottoms, that oils, salves, perfumes and liquid cosmetics came in. (Try saying "bulbous bottomed bottles" three time fast.) A casual observer might think that Roman women spent a lot of time at the makeup mirror -- and be right! You might also conclude that most of the glassware that came out of Roman factories were small, but there you would be wrong -- it's just that the smaller pieces were more likely to survive to be marketed again two thousand years later. (That's, in fact, a common archeological and paleontological problem: we really study what survived rather than what might originally have been there.)
The Romans use more glass than any society until modern times and also more than many of the less developed societies use even today. We deride things made of "plastic" because plastic is so common -- and to some extent "low class." The Romans felt the same way about glass. The lower classes of Rome used common glass while the upper classes used silver and gold. The measured of success of a member of the middle class was his adoption of metal plates and cutlery and abandonment of glass. The most cutting insult you could offer if you were "high class" would be to serve wine to a guest in a common glass goblet -- or worse still in a glass beaker. It would be your way of commenting on the worth of your guest. (We'll get to "uncommon" glass later.)
Three paragraphs back, I put "Roman" in quotation marks, and there was a reason. It's unlikely that glass was ever made or formed by "real" Romans.
(We'll get to "made or formed" in a bit.) Glass work was hot, dirty, and dangerous and was always the work of foreigners and slaves. It's true that some master craftsmen might eventually have attained citizenship in "Greater Rome" but they still would have been considered foreigners by the denizens of the city itself. And glass, whether we call it "Roman" or not, wasn't likely to be made or formed in the city, or, for that matter, in cities in general -- the fire hazard was to great, and Roman cities had fire-prevention ordinances that kept the glass works beyond the fringe.
The technology, like so many "Roman" technologies, was borrowed from elsewhere. Nobody knows exactly where the first glass was made, and there are various theories. There are certainly naturally occurring glasses: volcanic obsidians and "petrified lightning" (what you get when lightning strikes the right kind of moistened sand) that could have inspired some primitive genius glassmaker. "Could have" but evidence that it actually happened is nor existent -- that whole theory smells like a red herring. Most commonly accepted is the theory that middle-eastern potters discovered glass by overlaying mineral glazes on terra cotta pots. In fact, the earliest known manmade material that fits the definition of glass is the outer layer of middle-eastern beads. When and where they figured out how to fire the glaze materials without the terra cotta underlay isn't know, but it was certainly done more than three thousand years ago.
And once it was figured out, glass products spread rapidly. Glass technology spread more slowly, because those who had it wanted to keep their processes secret. Glass making and forming centers sprang up in places where the knowledge was present. And now we get to that "made and formed" from above. It really is two completely separate technologies, and quickly the processes became geographically separated. First, somebody had to make the glass, and then somebody, somewhere else, had to take the raw glass, melt it again, and make a finished product.
For simple economic reasons, glass was most often made in places where the proper quartz sands are common. The sand had to be clean (or at least "cleanable", with impurities could be washed out), and there had to be some available calcium either in the natural sand mixture or from external sources. Usually the calcium was merely "lime" in the form of calcined limestone, and it probably was originally introduced fortuitously into the batch, either because there might be marine shell debris already mixed into the quartz sand or because the furnace that the batch was made in was made of limestone. It's safe to assume that glassmaking started out small, but eventually very large batches were made in big furnaces that were built on the spot. After the glass had cooled, the large furnaces were broken apart leaving a big block of glass that could be fragmented to be shipped to the places where glass objects were made. That's where the "economic reasons" came in. It was cheaper to ship chunks of glass than to ship huge quantities of glass sands. It also was usually cheaper to ship the fuel needed to fire the furnace than to ship the sand to where there was fuel.
How big were those big chunks of raw glass? Nobody knows exactly how big they could get. Because of the nature of the process, the furnace would be taken down and the cooled glass would then be taken away -- not much would be left to find and measure. But in one place, something went wrong, and we have the remains of a failed melt. It appears that part of the roof of the limestone and plaster furnace collapsed into the melting glass making the cooled product useless -- glass with too much calcium crystallizes and can't be used. The abandoned slab of spoiled glass, known as the Beth She'arim slab for the place in Israel where it was found, is the third biggest known piece of glass ever made by man until the huge blanks for telescope lenses made in the 20th century. Known is the important word, of course. There's no reason to suppose that this particular batch was the biggest made in those days. A few statistics: the slab is six feet by 11 feet and is 18 inches thick. It weighs about 18,000 pounds. It was melted at about 1100 degrees and had to be kept at that temperature for five to ten days. The furnaces on the periphery of the melt would have required about 20 tons of wood. Pottery found nearby (beer jugs for the overheated workers?) was made in the 4th century AD. If it hadn't been spoiled, this batch of raw glass would have made 50 to 60 thousand Roman glass vessels. (There's more info on the slab at http://www.cmog.org/page.cfm?page=277.) The "find spot" is also significant: this was Jewish glass, and there is strong evidence that glass making was, for a long time -- perhaps for centuries -- a Jewish monopoly. It's often called "Syro-Palestinian" glass, but the fact of the matter is that almost everyone in the Syro-Palestinian area was Jewish until the Moslems arrived in the 7th century -- draw your own conclusions about why one name is used rather than the other. More info on the Jewish glass-making tradition is at http://www.hebrewhistory.org/factpapers/glass6-IIa.html.
The chunks of raw glass were shipped to factories around the empire -- they made wonderful ballast for westbound merchant ships. The factories were usually in areas where there was abundant fuel to fire the many smaller furnaces needed to support the glass forming industry. Factories were initially located all over the "Roman area", but, toward the end of the Roman period, the biggest glass manufacturing sites were outside the Italian peninsula, in the north, which still had plenty of firewood. That's one reason why, when the empire fell, the Germans and other northerners still had glass and the Italian peninsula mainly shifted back to pottery -- pot-making also required fuel, but not as much.
When it arrived at the "glass factory" the bigger chunks were broken for easier and quicker melting in the furnaces. A furnace would have a lower or rear fire chamber and one or several receptacles to hold glasses of different colors. Colors were achieved by the addition of metals, usually in the form of oxides, because they were easier to mix into the re-melted raw glass. All of the processes used today in hand making glass items were known in Roman times -- only petrochemical fuels and electric blowers have been added. In some artisan shops (sometimes also known as tourist attractions) glass items are still made without these modernities.
Items could be made in the ancient shops by molding or by blowing. Molding was (and still is) accomplished by putting globs of softened glass into presses. An earlier technology, which was essentially a casting process -- liquefied glass poured into one-shot of reusable molds -- was quickly abandoned. Blowing could be "free" or into molds, terms that are pretty much self-explanatory, but if you need illustrations, go to http://www.hfmgv.org/village/crafts/glassweb/index/processj.htm.
There was a tremendous amount of common glass on people's tables, in architectural decoration (glass tesserae in mosaics), and in windows. Colored glass was everywhere, because, even though its manufacture required the addition of metallic additives, it was often cheaper to make colored items than clear glass -- color hid both cheaper raw materials and manufacturing flaws and made lower-grade work saleable. Besides, colored glass was more popular, because it could brighten up even the dullest upper floor insula apartment.
But there was also some very uncommon Roman glass -- glass fit for emperors and for the tables and mantels of the richest Roman families. Very few of these pieces would have been made -- an experienced slave or freed artisan may have spent a lifetime practicing to make and then finally making one or two masterworks. I will only mention two of these masterpieces.
In early days, during the republic, the rarest stuff was "cameo glass". Pieces or vessels would be made in two or more layers and then cut to remove parts of the outer layers and expose the underlying layer(s). Although modern glassmakers have mastered the layering process, we are not quite sure how the Romans did it. The hard part is getting the layers to fuse without mixing. Perfect temperature control is obviously vital, and, as far as we know, the Romans had no way of measuring the high temperatures involved. They may have just discarded or reused a lot of failed efforts, or perhaps, mirroring the claims of some modern glass fabricists, Roman master glassmakers somehow might have judged accurately the temperatures of the layers by sight. In surviving pieces, the base layer is usually very dark glass (black or dark blue) and the overlay layer was usually white. A few examples of colors overlaying clear glass do exist. Getting the top layer to adhere without re-melting the inner layer is still a problem, but modern technology makes it easier. Modern technology has also provided high-speed precision cutting tools, tools that Roman glass-cutters would have given their left -- well, never mind what they would have given. It's clear that, at least by late Roman times, glass was cut with rotating abrasive wheels, but we don't know much about the mechanics of their tools.
The Romans appear to have kept the technology for making and cutting cameo glass throughout their history, but it was lost shortly after the fall of the empire and wasn't really found again until the 19th century. The search for the lost technology was inspired by attempts in the previous century to reproduce one of the most famous pieces of this kind of glass. Made in the first century AD -- some experts say for Augustus himself -- the first of the two masterpieces was excavated near the southeastern corner of Rome from the mausoleum known as the Monte del Grano on Via Tuscolana. It is now known as the Portland Vase after one of its modern owners who loaned it to the British Museum. It's still displayed there and is listed as one of the museum's most important treasures, along with the Elgin Marbles and the Rosetta Stone. Josiah Wedgwood initiated a run of 43 fine china copies in the late 18th century, and they, along with the original, were immensely important in alerting western art circles to the beauty of Roman glass. Replicas of the Wedgwood vases are still made by Wedgwood from the original molds, and other companies also make their own versions, which are universally inferior. One of the Wedgwood copies was used as a model for the reconstruction of the original after a deranged Irish painter smashed it in the 19th century. A short article about this vase is at http://www.cmog.org/page.cfm?page=276, and much more information can be found using the Google Internet search engine, http://www.google.com. Search for either "portland vase" (all lower case, with the quotation marks) or for "barberini vase", its name before it was sold out of Italy. (Be aware, however, that there is another celebrated Barberini Vase, brass inlaid with silver medallions and mottoes in Arabic script -- but they are easy to tell apart.) If you want to know about the best modern cameo glass, go to http://www.cameo-glass.co.uk/.
The second great Roman glass masterpiece, the Lycurgus Cup, is a figured "cage cup." It was made in the fourth century AD, but the gilded bronze base and rim were more recent additions. There is no documentary evidence to show where or when this cup was made, or who found it. Based on analysis of the glass, it was probably made in the late fourth or fifth century. A frieze showing the myth of King Lycurgus surrounds the inner solid cup. Lycurgus is being dragged into the underworld by Ambrosia, who has been turned into a vine. The frieze stands out from the body of the Cup, connected to it only by small shanks or bridges, and it is this layering that make it a cage cup: an inner layer is "caged" by an outer layer. Both layers and the almost invisible supporting posts were cut from a single piece of glass. But the most unusual feature of the Lycurgus cup is its color -- or rather the fact that it appears to be different colors under different light conditions. When lit from the outside (reflected light) the inner cup and frieze are milky green, but when lit from within (transmitted light) it glows in shades of red. There are a few other pieces of ancient glass, all "Roman", that show a similar but less marked effect. Modern glassmakers know how to do this -- by adding a bit of metallic gold to the glass mix -- but it was only proved recently that this is also the way Roman fabricators did it. It is not known whether the Romans were working from a formulary or whether the gold was introduced fortuitously into the Lycurgus batch. Many experts favor the latter theory, because no other piece of Roman glass -- not even scrap -- has been found with the Lycurgus formulation. The other color-changing pieces also all have different formulations. The Lycurgus cup is also in the British Museum, and there is more information about the piece at http://www.thebritishmuseum.ac.uk/science/text/lycurgus/sr-lycurgus-p1-t.html. As might be expected, a search for "lycurgus cup" with the Google search engine will give you more than you ever wanted to know about this masterpiece. Because of its unique color characteristics, some authors, mostly fiction writers (some of whom breathlessly pretend to write non-fiction), have linked the Lycurgus Cup to the legend of the Holy Grail. For more information on cage cups in general, enter "cage cup" as your search parameter in the Google search engine, or enter "diatreton", "diatretum", "diatret", or "diatreto" which are variations of a more technical name for this kind of work.
There are other great Roman glass masterpieces documented on the Internet: several more cage cups, including a slightly damaged but still splendid symmetrically patterned cup (no floral or faunal figures) at the Corning Glass Museum (http://www.cmog.org/page.cfm?page=273) and another, which is both cameo glass and a cage cup (a dark glass cage surrounding a clear glass interior vessel -- picture at http://www.hebrewhistory.org/factpapers/glass6-IIa.html) immediately come to mind. Space prevents more detail here, and you will want the thrill of researching for yourself.
And there is a vast amount of information on the net and in print where you can start to do your research. For a good bibliography of currently available print materials, go to http://www.whitehouse-books.com/ancient.html. (Why is it no surprise that the company address is in Corning, New York?)
Go to http://www.mmdtkw.org/Veneto2002.html for articles on other subjects.
P.S.: You can and should see real Roman glass at Museums in Rome and elsewhere. There is no museum dedicated specifically to glass in Rome (there should be!) but several of the EUR museums have very good displays. The Vatican Museums also has a splendid collection of Roman glass. The Glass Museum on Murano Island in Venice is small, but it has a good collection, and on the island you can also see modern glassmakers at work.
P.S.2: James Bond is responsible for the modern display cabinets at that Murano museum. In the "Moonraker" movie (1979) all the old display cases and a lot of fake Roman and Venetian glass was destroyed in a big fight scene. When the filming was done, the production company bought the museum new display cases.
P.S.3: If you have shelled out your hard earned sesterces for pieces of Roman glass, do not, repeat do not, try to wash off particles of dirt. Water, and almost every other liquid will damage the glass irrevocably. Roman glass needs to be cleaned (not washed) only by expert preservationists.
Links to some Internet sites used in compiling this article:Ancient and Islamic Glass books
Beth She'arim Slab
Glass Links from the Glass Encyclopedia
The Glassmakers, a Judaic Tradition
History of Glass and Glassmaking
Illustrated Glass Dictionary
Looking Through Roman Glass
The Lycurgus Cup
Lycurgus formulation PDF
Making Cage Cups
McClung Museum - ROMAN GLASS
Museo Vetrario di Murano
The Portland Vase
Portland Vase - Corning
Römisches Glas (in English)