In 836CE, Abbasid caliph al-Mu’tasim ordered the construction of a new capital city on the east bank of the Tigris River, in modern-day Iraq. Since 762, al-Mu’tasim’s predecessors had ruled from Baghdad, but the presence of the caliph’s newly formed Turkish regiments had stirred unrest in the city, so he wanted to pack up his troops and move to a new capital. This also allowed him to build his own grand palace complex with walls of inlaid glass and intricate mosaics.

Archaeologist Nadine Schibille of the University of Orleans and her colleagues recently analyzed fragments of the glass that once decorated the gleaming walls of Samarra’s palaces and mosques, and their chemical composition offers some hints about the gritty reality of the industry and trade that built the glass palaces.

Fit for a caliph

Glass inlays of clear and purple geometric shapes and multicolored millefiori tiles, along with elaborate mosaics, decorated the walls of the audience chamber at Dar al-Kilafa palace. That glittering opulence may have been an allusion to the story of King Solomon’s glass palace, but the appearance of al-Mu’tasim’s audience chamber also created a physical manifestation of the caliph’s power. The awe and wonder that visitors experienced when they stepped into the glass-walled audience chamber were meant to transfer to the ruler himself.

The Abbasid rulers left Samarra to move back to Baghdad in 892CE; today pieces of the glass that once decorated its palaces and mosques are on display in museums around the world, mostly thanks to archaeological digs in the early 20th century. Schibille took tiny samples from 256 of those items and analyzed their chemical composition using a method called laser inductively coupled plasma mass spectrometry. (That's a scientific way of saying they mounted tiny fragments of glass in epoxy, then shot them with lasers and measured the chemical makeup of the vaporized material.)

Proportions of different elements in a piece of glass can tell scientists where the material for that glass came from, which impurities or additives it contained, and how it was manufactured. And that, in turn, can reveal a lot about the construction of a 9th century palace. It turns out that the caliph’s construction project supported a fine glassmaking industry near the banks of the Tigris. It also reused relics of the caliphate’s past and the products of ancient trade networks.

Advanced local industry

Nearly all of the glass for the audience chamber’s inlays and the palace windows had been produced locally, out of very high-quality materials, using a process adopted in the region just a few decades earlier. For centuries, glass makers in the Levant and Egypt combined silica with natron, but around the late 8th century or early 9th century, glass makers in the region started substituting plant ash for natron. The proportion of magnesia in inlays and window glass from Samarra’s palaces suggests plant ash, rather than natron, as the source of sodium carbonate in the glassmaking process. That means the inlays and windows were probably made around the time of the city’s founding.

And the chemical signatures in those glass pieces matched silica from the region around Samarra. That lines up well with historical descriptions of glass production in the village of al-Qadisiyya, about 25km (16 miles) away from the city and its glass palaces. A local industry could have expanded to fulfill royal commissions, and it seems that those ancient glass makers knew their work well. They appear to have used the purest silica sources for clear inlays and windowpanes, because impurities would discolor the glass. In Schibille and her colleagues' analysis, the chemical composition of those samples suggested very pure silica, such as clean silica sand or quartz pebbles.

For the colored glass inlays, however, the glass makers didn’t have to worry so much about impurities, since they would be adding colorants to the glass anyway. So, based on the chemical analysis, it seems that they used less expensive silica sources—very high quality but a bit less pure than what went into the clear glass. That suggests that the glass makers wanted to balance cost and quality, and the same pattern is reflected in glass containers and dishes from Samarra.

Scavenged mosaics and Byzantine trade

Despite the thriving local industry, tesserae for the elaborate mosaics of Samarra seemed to have been scavenged and recycled from several older palaces and mosques in Syria and Egypt. The tesserae have different concentrations of the elements aluminum and calcium, as well as a few heavier elements. That suggests that they came from different silica sources and so probably from different geographical areas. Some match glass from 4th century to 8th century CE sites in Israel, others match glass from 6th and 7th century CE sites across Europe and the Mediterranean, and yet another set has a chemical makeup that looks more like Egyptian glass from the 8th century CE. All of them predate the 9th century CE foundation of Samarra.

“They might instead have been scavenged from buildings no longer in use and imported to Samarra from the western regions of the Abbasid Caliphate, from Syria-Palestine and/or Egypt,” suggested Schibille and her colleagues. Some of those buildings might have been the work of rulers from the Umayyad dynasty, which al-Mu’tasim’s ancestors had overthrown in 750CE. Umayyad mosques were known for their wall mosaics, and historical accounts claim that an early 8th century Byzantine emperor sent tiles and skilled workmen to the Umayyad caliph for the mosques of Damascus in Syria and Medina in Saudi Arabia.

The fact that the silica that went into making the Samarra tesserae seems to have come from Byzantine territory and that it matches styles and techniques used during the Umayyad period suggest that some of the tiles for the Umayyad mosques might, in fact, have come from the Byzantine Empire in trade. It also suggests that later Abbasid rulers took them east to Samarra to decorate their own fabulous glass-walled palaces.

But that, too, was destined not to last. Caliph al-Mu’tasim poured money and resources into Samarra, as did several of his successors, and that constant influx of royal gold and royal attention kept the city thriving even in the desert. But when the Abbasids shifted their seat back to Baghdad in 892CE, the once-grand city of Samarra pretty much dried up without imported water and food, and most of the old city was abandoned within a few decades.

PLOS One, 2018. DOI: 10.1371/journal.pone.0201749.g001 (About DOIs).