Iridescent glass flask, Palestine, 40 BC-30 AD

Iridescent glass flask, Palestine, 40 BC-30 AD
Dating:40 BC–30 AD
Origin:Roman World, Eastern Roman World
Material:Glass (all types)
Physical:9.7cm. (3.8 in.) - 24 g. (.8 oz.)

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  Although it is now cloaked in a lovely understated display of diaphanous iridescence, this flask was originally blown from transparent pale green glass. The concave bottom is unusually smooth and regular. The spherical body yields seamlessly to a gently tapered neck, topped by a narrow infolded rim. The glass is thin and uniform. Eastern Mediterranean World, 40 BC-30 AD.

“Unguentarium. Height 15.5 cm; greatest diameter 8.85 cm; Weight 71 gr. Translucent dark blue glass...Bottle. Everted rim, rounded and thickened in flame; tubular neck, tapering upward; spherical body; flattened base, slightly concave. Provenance not recorded. Eastern Mediterranean, probably Palestinian. Late first century BCE or early first century CE” (Stern 2001:64 #8, 65).

“Unguentarium. Ht. 9.5 cm. Pale blue glass. Unguentarium with a rounded conical body extending into a tubular neck with a splayed and infolded rim. Roman. 1st.-2nd. century AD” (Fortuna 1991:83 #153).

Glass Iridescence
The iridescent effect that so often enhances immeasurably the beauty of ancient glass was not planned by ancient glass artisans. Instead, it is the combined result of weathering processes and the properties of light. The rainbow effect you commonly experience in daily life, such as on soap bubbles or drops of oil spread on water, stem from the same action: light bouncing on a extremely thin transparent film.

When a glass bottle is new, there is no such thin film. The wall of the bottle is homogenous. But “as glass is exposed to water in its burial environment, some of its [chemical] components can be dissolved by the water and carried away (leached out). This generates a thin surface layer of glass that has a different composition that the undegraded bulk of glass. Often, there is a think layer of air between the corroded surface and the bulk” (Bezúr 1999).

When ordinary white light strikes the bottle, some of the rays bounce off the top surface of the thin film, and some go through the thin film and then bounce off the glass-air interface between the thin film and the underlying glass. When the rays coming back from the bottom of the thin film reemerge into open air, they combine with those that simply bounced off the surface. But since they have been delayed by their additional travel, their waves are no longer in phase (in synch). When these two streams of out-of-phase white light combine, some of the wavelengths cancel out (and therefore those colors disappear), and other wavelengths are reinforced (and therefore those colors become very intense), thus turning white light into vivid random colors.

Glass artists of the late 19th Century, such as Louis Comfort Tiffany, admired the iridescence of Roman glass, and devised ways to produce it deliberately by placing the glass piece while still very hot in an oven filled with vapors (tin and iron chlorides) that would alter the surface and create a thin film of different composition, yielding an iridescent effect that did not require a thousand years to develop.

A more thorough technical discussion of the phenomenon by Aniko Bezúr of the University of Arizona Department of Materials Science and Engineering is available from

Glass Blowing
The invention of glass blowing at the end of the first century BC was one of a handful of momentous technological breakthroughs that transformed civilization.

We believe that Mesopotamians started making glass around 3000 BC (Merril 1989), and we can accurately date some glass artifacts to the mid-third millennium BC (Newman 1977:10). These were small items, such as beads and ornamental accents on larger pieces. At that time, glass was used as a “synthetic stone.” Although it could not have been cheaper than stone, it was probably an exciting novelty which may have offered colors not available in stone.

It would take another thousand years of continuous craftsmanship for the first glass vessels to appear in the Kingdom of Mittani, and then in Egypt, where the art of glass forming in Dynasty 18 workshops was raised to a level of refinement never surpassed. However, the extraordinarily time consuming methods they utilized limited the enjoyment of these sumptuous creations to the extremely wealthy.

Astoundingly, it would take another 1500 years for someone to discover that a lump of softened glass mounted at the end of a pipe could, like a soap bubble, be inflated by blowing. Glass blowing was born. “The skill of Syrian workers… was so remarkable that within 20 or 30 years, they proved capable of developing almost all of the many inflation techniques still present nearly 2000 years later” (Harden 1987). Suddenly, vessel shapes that took days to make could be fashioned in minutes. Glass would soon be available to wide segments of the population throughout the known world. For reasons unknown, but which may have had to do with the considerable amount of firewood required by glassworking (Newman (1977) notes that in American Colonial times you could build a two-story house with the wood required to melt a batch of glass), glassworkers were a highly mobile group. Developed on the Syrian coast, glass blowing know-how spread very rapidly throughout the Roman empire. No longer solely a high luxury item, glass packaging could foster commerce and improve hygiene everywhere. After three thousand years of slow germination, mankind had entered the glass age in one generation.

The initial invention of glass blowing, or inflation, was followed by numerous improvements and subsidiary technical developments.

The first blowpipes were probably made of clay. Within a hundred years, they were replaced by iron pipes, which were stronger and could be made longer, allowing the craftsman to work at a more comfortable distance.

Today so familiar, the workflow consisting of inflating the body of the vessel, working the foot, then transferring the piece to a pontil rod attached to the foot, so one could conveniently work the mouth of the vessel, “did not become common until the end of the first century AD… The wad of glass that serves to attach the punty [pontil rod] to the bottom of the vessel leaves a pontil scar… Annular scars on ancient glass vessels suggest that many glassworkers used the blowpipe as a punty. . .” (Stern 2001:20).

Before the invention of glass blowing, several established glass forming techniques used molds. So, blowing the glass into a two-part mold that would constrain and shape the bubble as well as provide surface texture, was a very natural evolution of known skills. This full-size mold-blowing technique was “invented on the Syro-Palestinian coast, presumably in Sidon or vicinity, in the early first century CE” (Stern 2001:26)

Conceptually much more novel--because it took full advantage of the plastic properties of molten glass, but also much faster was the pattern blowing technique. This consisted in inserting the blob of molten glass at the end of the blowpipe into a tapered one part mold that would give it surface texture, pulling it out, and then inflating it. It was a particularly effective way to create ribbed patterns, which could be swirled if desired. It “was probably developed in the late third or early fourth century by Syrian glassblowers who began to expand geometric patterns made in full-size, multipart molds” (Stern 2001:27).

Pinching, folding, cold-cutting, grinding, applying blobs, threads, and coils (a coil is a thick thread), either imbedded or in relief, were glassblowing techniques derived from earlier skills. One seemingly obvious operation not available, however, to early glass blowers was cutting soft glass. Stern (2001:21) explains that pivoted shears were not available to glassblowers during Roman times, and that the “U” shaped shears common at the time could not be used for that task. “Many idiosyncrasies of Roman glass vessels, e.g. the way handles are drawn thin, folded back and forth at the point of attachment, and snapped off, rather than being clean cut, can be ascribed to the lack of pivoted shears. One operation that requires pivoted shears is that of trimming the rim after separating the vessel from the blowpipe. There is very little evidence that ancient glassblowers did this… Countless Roman glass vessels are proof that their makers were not perfectionists”

If most glassworking techniques in use today had already been worked out by the middle of the Roman empire, glassmaking (which was carried out by a completely different group of craftsmen, not necessarily in the same location or even the same country) still suffered from a tragic misunderstanding of glass chemistry.

What makes glass glass is melted silica. But melting silica in its pure form requires high temperatures unattainable in antiquity. A mixture of silica (river sand) and an alkali such as sodium carbonate (natron or plant ashes) melts at much lower temperatures, resulting in ‘water glass’ or sodium silicate. But sodium silicate is water soluble. The adjunction of lime fixes the glass so it won’t dissolve in water. But that was not fully understood in antiquity, and “we can only guess how large is the proportion of ancient glass that has completely disintegrated, leaving only those objects which happened accidentally to contain sufficient lime. Ancient glassmaking recipes indicate that the lime came either with the silica, for example in the form of crushed shells mixed with sea sand, or with the alkali [sodium carbonate], because some plants contain lime” (Stern 1994:19). As pointed by Vandaveer (2002), favorite plants for the production of suitable ashes were the halophytes (saltworts), also known today as glassworts for obvious reasons. She further notes that Gerard's Herbal(1633) gives clues to both the etymology of the word ‘alkali’ and the production method: "Saltwoort is called by the Arabians Kali, and Alkali...the...ashes hereof are named...Soda: of most Sal Alkali: diverse call it Alumen catinum. Stones are beaten to powder, & mixed with ashes, which being melted together become the matter whereof glasses are made.”

In the three thousand years of glassworking before the invention of glass blowing, craftsmen had been able to produce a wide palette of colors, generally using metallic oxides (copper for blues, greens and reds, manganese for pinks and purplish reds and cobalt for deep blues), as well as a broad range of opacities. But in these early Roman times, the new ideal standard pursued was fully transparent and colorless glass which, “according to Pliny (Nat. Hist. 36.198 and 37.204) was the most valuable product of the earth surface” (Stern 2001:41). By the fourth century, with clear glass no longer a novelty, colored glass came back in fashion.

As splendid as they may have been, the original colors can hardly be more spectacular than the iridescence developed since, as a product of the aging process of glass completely unforeseen by the original artists.

Bibliography (for this item)

Fortuna Fine Arts, Ltd.,
1991 Shining Vessels: Ancient Glass from Greek, Roman, and Islamic Times. Fortuna Fine Arts, Ltd., New York, NY. (83 # 153)

Stern, E. Marianne
2001 Roman, Byzantine, and Early Medieval Glass; 10 BCE-700 CE; Ernesto Wolf Collection. Hatje Cantz Publishers, Ostfildern-Ruit, Germany. (64 #8, 65)

Bibliography (on Glass Iridescence)

Bezur, Aniko
1999 Online Notes on Iridescence ( University of Arizona, Department of Materials Science and Engineering, Tucson, AZ.

Bibliography (on Glass Blowing)

Harden, Donald B.
1987 Glass of the Caesars. Olivetti, Milan, Italy.

Merrill, Nancy O.
1989 A Concise History of Glass Represented in the Chrysler Museum Glass Collection. The Chrysler Museum, Norfolk, VA.

Newman, Harold
1977 An Illustrated Dictionary of Glass. Thames and Hudson, London, UK.

Stern, E. Marianne, and Birgit Schlick-Nolte
1994 Early Glass of the Ancient World 1600 BC - AD 50 Ernesto Wolf Collection. Gerd Hatje, Ostfildern, Germany.

Stern, E. Marianne
2001 Roman, Byzantine, and Early Medieval Glass; 10 BCE-700 CE; Ernesto Wolf Collection. Hatje Cantz Publishers, Ostfildern-Ruit, Germany.

Vandaveer, Chelsie
2002 How Were Plants Used to Make Glass?., World Wide Web.

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