A clock in the clay
The ancient city of Beikthano, one of the Pyu city-states in central Myanmar, is a site of low brick mounds and fortifications. For archaeologists, determining the precise timeline of its construction and occupation posed a challenge. Traditional methods offered a broad picture, but the city's long history remained blurry. The answer came from the bricks themselves, through a technique called thermoluminescence (TL) dating. This method measures the accumulated radiation dose in a mineral since it was last heated to a high temperature.
The science works like this: crystalline minerals, such as quartz and feldspar found in clay, are constantly exposed to natural radiation from the soil and cosmic rays. This environmental radiation knocks electrons out of their normal positions within the crystal lattice. Most electrons fall back, but some become trapped in imperfections in the crystal structure. Over centuries, more and more electrons get trapped, storing energy. When a sample of this brick is taken to a laboratory and heated to over 500°C, the trapped electrons are freed. They release their stored energy as a faint pulse of light, or thermoluminescence. The intensity of this light is directly proportional to the total radiation dose absorbed since the brick was first fired. By measuring the light and calculating the annual radiation dose the brick received from its burial environment, scientists can determine how much time has passed since its "clock" was last reset by the heat of the kiln.
Reading Beikthano's timeline
The application of TL dating at Beikthano, supplemented by radiocarbon dates, provided a detailed chronology of the city's life. The results established a period of occupation that flourished for over 1,000 years, from approximately 200 BCE to 900 CE. This confirmed Beikthano as one of the earliest urban centers of the Pyu Kingdoms. The technique was able to distinguish between different construction periods with a precision that stratigraphy—the study of archaeological layers—could not. For instance, radiocarbon dating of the six-meter-thick fortified walls places their construction between 180 BCE and 610 CE.
Beikthano was a large, fortified settlement covering about 300 hectares, enclosed by rectangular walls measuring approximately 3 kilometers by 1 kilometer. Excavations have uncovered monastic buildings, stupas, and burial grounds containing urns with ash and iron objects. The city's name is a version of "Vishnu," showing early Hindu cultural connections, though it became a major center for the introduction of Buddhism into Southeast Asia. The dates derived from its fired bricks are evidence of its long duration as an economic and religious center. Along with the cities of Halin and Sri Ksetra, Beikthano is recognized as a UNESCO World Heritage Site, preserving the history of the Pyu civilization.