Why were medieval open saucer lamps less effective than closed Roman lamps?

While historical records regarding medieval lighting are limited, it is widely recognized that the open, saucer-style lamps used during the Middle Ages were significantly less effective than the closed lamps developed by the Romans.1

The Roman design represented a technological advancement because it utilized a closed vessel, often manufactured using two-part molds, which provided a more controlled and efficient environment for combustion. In contrast, medieval lamps—such as the cruse lamp—typically featured an open, shallow bowl design. These open vessels lacked the structural refinements of Roman lamps, which often included specific nozzles or spouts to manage the wick and a covered top to contain the fuel.12

The transition to more effective lighting did not occur until the 18th century, when the development of the central burner and the use of glass chimneys allowed for better aeration and control of the flame.1