Radiocarbon dates things that were alive. Pottery, the commonest find on any Egyptian site, was never alive. Luminescence dating can date it directly.
Key idea
Crystals in clay and sediment, such as quartz and feldspar, slowly store energy from natural radiation in the ground. Heating to about 500 °C empties that store, and so does sunlight for some kinds of signal. Thermoluminescence (TL) dates the last time pottery was fired (or a flint was burned); optically stimulated luminescence (OSL) dates the last time sediment saw daylight.
Why it works
After the clock is reset, energy builds up again at a steady rate set by the radiation around the object. In the laboratory the sample is heated (TL) or lit (OSL), and the stored energy comes out as a faint glow. Measuring the glow gives the total dose absorbed since the reset; measuring the radioactivity of the sample and its surroundings gives the dose per year. Age = total dose ÷ dose per year.
| Radiocarbon | TL | OSL | |
|---|---|---|---|
| Dates | death of a plant or animal | last heating | last exposure to light |
| Materials | wood, seeds, linen, bone… | pottery, burnt flint, fired brick | sand and silt |
| Typical uncertainty | decades for Egyptian samples, after calibration | often about 5–10% of the age | often about 5–10% of the age |
Worked example: Computing an age
A potsherd has absorbed 12 grays (Gy) of radiation since it was fired. The dose rate in its sample and soil is 3 Gy per thousand years. How old is it?
- Age = total dose ÷ dose rate.
- 12 Gy ÷ 3 Gy per thousand years = 4 thousand years.
- So the pot was fired about 4,000 years ago, c. 2000 BC.
Answer: About 4,000 years.
Worked example: Reading the margin
The lab reports the same sherd as 4,000 ± 8%. What range of years does that give?
- 8% of 4,000 = 320 years.
- 4,000 − 320 = 3,680 and 4,000 + 320 = 4,320.
- So about 3,700 to 4,300 years ago: too wide to choose between the late Old Kingdom and the Middle Kingdom. Pottery style would date it more closely.
Answer: About 3,680 to 4,320 years ago.
Common mistake
Expecting luminescence to be as precise as radiocarbon. Its margins are usually wider, often several centuries for Egyptian material. It is most useful where nothing else can be dated, and for testing whether a "pottery antiquity" was fired recently: forgeries fail a TL test.