Luminescence Dating

Luminescence dating (including thermoluminescence and optically stimulated luminescence) is a type of dating methodology that measures the amount of light emitted from energy stored in certain rock types and derived soils to obtain an absolute date for a specific event that occurred in the past. The method is a direct dating technique, meaning that the amount of energy emitted is a direct result of the event being measured. Better still, unlike radiocarbon dating, the effect luminescence dating measures increases with time. As a result, there is no upper date limit set by the sensitivity of the method itself, although other factors may limit the method's feasibility.

How Luminescence Dating Works

Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight.

To put it simply, certain minerals (quartz, feldspar, and calcite), store energy from the sun at a known rate. This energy is lodged in the imperfect lattices of the mineral's crystals. Heating these crystals (such as when a pottery vessel is fired or when rocks are heated) empties the stored energy, after which time the mineral begins absorbing energy again.

TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated. In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight. Luminescence dating is good for between a few hundred to (at least) several hundred thousand years, making it much more useful than carbon dating.

The Meaning of Luminescence

The term luminescence refers to the energy emitted as light from minerals such as quartz and feldspar after they've been exposed to an ionizing radiation of some sort. Minerals—and, in fact, everything on our planet—are exposed to cosmic radiation: luminescence dating takes advantage of the fact that certain minerals both collect and release energy from that radiation under specific conditions.

Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight.

Crystalline rock types and soils collect energy from the radioactive decay of cosmic uranium, thorium, and potassium-40. Electrons from these substances get trapped in the mineral's crystalline structure, and continuing exposure of the rocks to these elements over time leads to predictable increases in the number of electrons caught in the matrices. But when the rock is exposed to high enough levels of heat or light, that exposure causes vibrations in the mineral lattices and the trapped electrons are freed. The exposure to radioactive elements continues, and the minerals begin again storing free electrons in their structures. If you can measure the rate of acquisition of the stored energy, you can figure out how long it has been since the exposure happened.

Materials of geological origin will have absorbed considerable quantities of radiation since their formation, so any human-caused exposure to heat or light will reset the luminescence clock considerably more recently than that since only the energy stored since the event will be recorded.

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Archaeological Dating: Stratigraphy and Seriation

By K. Kris Hirst

Measuring Stored Energy

The way you measure energy stored in an object that you expect has been exposed to heat or light in the past is to stimulate that object again and measure the amount of energy released. The energy released by stimulating the crystals is expressed in light (luminescence). The intensity of blue, green or infrared light that is created when an object is stimulated is proportional to the number of electrons stored in the mineral's structure and, in turn, those light units are converted to dose units.

The equations used by scholars to determine the date when the last exposure happened are typically:

• Age = total luminescence/annual rate of luminescence acquisition, or
• Age = paleodose (De)/annual dose(DT)

Where De is the laboratory beta dose that induces the same luminescence intensity in the sample emitted by the natural sample, and DT is the annual dose rate comprised of several components of radiation that arise in the decay of natural radioactive elements.

Datable Events and Objects

Artifacts which can be dated using these methods include ceramics, burned lithics, burned bricks and soil from hearths (TL), and unburned stone surfaces that were exposed to light and then buried (OSL).

• Pottery: The most recent heating measured in pottery sherds is assumed to represent the manufacturing event; the signal arises from quartz or feldspar in the clay or other tempering additives. Although pottery vessels can be exposed to heat during cooking, cooking is never at sufficient levels to reset the luminescence clock. TL dating was used to determine the age of Indus Valley civilization occupations, which had proved resistant to radiocarbon dating, because of the local climate. Luminescence can also be used to determine the original firing temperature.
• Lithics: Raw material such as flints and cherts have been dated by TL; fire-cracked rock from hearths can also be dated by TL as long as they were fired to sufficiently high temperatures. The resetting mechanism is primarily heated and works on the assumption that the raw stone material was heat-treated during stone tool manufacture. However, heat treatment normally involves temperatures between 300 and 400°C, not always sufficiently high enough. The best success from TL dates on chipped stone artifacts likely are from events when they were deposited into a hearth and accidentally fired.
• Surfaces of buildings and walls: The buried elements of standing walls of archaeological ruins have been dated using optically stimulated luminescence; the derived date provides the age of burial of the surface. In other words, the OSL date on a foundation wall of a building is the last time that foundation was exposed to light before being used as the initial layers in a building, and hence when the building was first built.
• Others: Some success has been found dating objects such as bone tools, bricks, mortar, mounds, and agricultural terraces. Ancient slag left from early metal production have also been dated using TL, as well as absolute dating of kiln fragments or vitrified linings of furnaces and crucibles.

Geologists have used OSL and TL to establish long, log chronologies of landscapes; luminescence dating is a powerful tool to help date sentiments dated to the Quaternary and much earlier periods.

History of the Science

Thermoluminescence was first clearly described in a paper presented to the Royal Society (of Britain) in 1663, by Robert Boyle, who described the effect in a diamond which had been warmed to body temperature. The possibility of making use of TL stored in a mineral or pottery sample was first proposed by chemist Farrington Daniels in the 1950s. During the 1960s and 70s, the Oxford University Research Laboratory for Archaeology and History of Art led in the development of TL as a method of dating archaeological materials.

Sources

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Forman SL, Jackson ME, McCalpin J, and Maat P. 1988. The potential of using thermoluminescence to date buried soils developed on colluvial and fluvial sediments from Utah and Colorado, U.S.A.: Preliminary results. Quaternary Science Reviews 7(3-4):287-293.

Fraser JA, and Price DM. 2013. A thermoluminescence (TL) analysis of ceramics from Applied Clay Science 82:24-30.cairns in Jordan: Using TL to integrate off-site features into regional chronologies. 

Liritzis I, Singhvi AK, Feathers JK, Wagner GA, Kadereit A, Zacharais N, and Li S-H. 2013. .Luminescence Dating in Archaeology, Anthropology, and Geoarchaeology: An Overview Cham: Springer.

Seeley M-A. 1975. Thermoluminescent dating in its application to archaeology: A review. Journal of Archaeological Science 2(1):17-43.

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