During the Dark Ages, the Franks had some rudimentary glass-making skills. Of course, the Mediterranean regions continued to make glass as the Romans had done, and during the Islamic era new glass techniques came from the East. Eastern glass was most commonly found as small, thick vials for perfume (here’s another), cosmetics, or medicine.
The Franks used glass mainly for drinking cups in a style that we call beakers. The glass itself was not very clear and usually greenish. Because horns had been their traditional cups at feasts, many glassmakers imitated horns, adding feet so that the horns stood on the table. They also made wider cups with stands, perhaps used as bowls.
The Franks added decorations to the beakers by sticking small lumps of hot glass onto the outside of the vessel, then pulling them outward and down into claws. (Pinterest collection of images) By the close of what we call the Dark Ages, they were able to make red and blue glass, and sometimes mixed the colors in a streaked decorative way. However, there’s no denying that Northern European glassmaking was very primitive compared to what was being made at the same time in Persia and Egypt.
Fine glassmaking stayed in the Muslim countries and in Constantinople for several medieval centuries. They blew glass in molds, created colored layers to carve as cameos, and kiln-fired silver and gold enamel onto glass. These wares were rare in Europe until after Venice asked the Fourth Crusade to sack Constantinople in payment of shipping debts.
Venetians literally kidnapped glassmakers and other artisans, who had been kept from leaving Constantinople in order to maintain trade secrets. The glassmakers were installed on the island of Murano and provided with anything they needed. Boats brought firewood daily, as well as shipments of broken glass to be melted down, sand from the Ticino River, and imported barilla soda ash from Syria. This ash came from burning plants that grew in a salty environment, and it contained lime. Glass made with Syrian barilla was superior to all other. Now that Venice controlled Constantinople and its former trade routes, Murano glass became the standard.
Medieval standards did not at first match ours. We value glass for complete clarity and delicacy; they liked thick, colored glass with bright designs. But by the late Middle Ages, fine drinking glasses could be perfectly transparent. Venetian glassmakers then added crushed quartz to create milky-white glass.
Venetian secrets, stolen from Constantinople, gradually leaked out; beautiful glass could be made in Northern Europe as well. Bohemia had recently discovered a vein of silver, which made it newly affluent. Now they started to make fine glass, too.
But right when it looked as though glassmaking would spread the way papermaking and tin-glazed pottery had done, the plague struck. Many industries were disrupted, and the more dependent they were on very high-level training, the more they were disrupted. Glassmaking did not recover until the Renaissance years, about a century later. In fact, the close of the medieval period saw Northern Europeans making greenish beakers with claws, just as they had done before.
Glass never became an ordinary piece of tableware until much later. By the end of the Middle Ages, drinking glasses could be used by wealthy merchants in towns, but through most of the period, it was found only in castles and wealthy monasteries.