How Archaeologists Determine the Age of Artifacts and Historical Sites

No single instrument tells an archaeologist how old a bone, a pot shard or a buried wall is. Instead, researchers rely on a toolkit of dating methods, broadly split into relative dating, which orders objects from older to younger without giving a year, and absolute dating, which assigns a numerical age range. The method chosen depends on what material survives and how far back the site is expected to date.

Relative Dating: Reading the Order of Deposits

The most common relative method is stratigraphy. According to Yale University's Human Relations Area Files (HRAF), it assumes that soil layers in a deposit accumulate one on top of another, so that the bottom layers are older than the layers above them. By mapping which layer an object was found in, archaeologists can place artifacts in sequence relative to one another even before any absolute date is known.

HRAF also lists two other relative techniques: seriation, which tracks stylistic changes in artifacts such as pottery decoration over time, and fluorine dating, which measures the chemical absorption of fluorine in buried bone. Neither method produces a calendar date on its own; they show which finds came earlier or later within a sequence.

Radiocarbon Dating: Counting a Radioactive Clock

Radiocarbon dating is the best-known absolute method for organic remains. The Oxford Radiocarbon Accelerator Unit explains that radiocarbon (carbon-14) is formed in the atmosphere and absorbed by living organisms throughout their lives; once an organism dies, it stops replacing the carbon, and the radiocarbon it contains begins to decay at a known rate. By measuring the ratio of radiocarbon to stable carbon in a sample, researchers calculate how long ago the organism died. The unit notes this raw figure is "not a true calendar age" and must be calibrated against material of already-known age to correct for past variation in atmospheric radiocarbon levels. Because the method depends on organic carbon, it cannot directly date stone, metal or pottery unless organic residue is preserved on them.

The technique was developed by University of Chicago chemist Willard Libby, who proposed it in 1946, work for which he received the 1960 Nobel Prize in Chemistry, according to National Geographic. Sources describe slightly different outer limits for the method: Oxford's unit states the technique covers material from a few hundred years old to about 50,000 years old, while National Geographic reports that accuracy declines for samples older than 40,000 years and that the method cannot be used at all beyond roughly 60,000 years. Both sources note that contamination — from soil, conservation chemicals or, in some cases, nuclear-testing fallout — can distort results and must be removed before measurement.

Dendrochronology: Tree Rings as a Natural Calendar

Dendrochronology assigns calendar dates to the annual growth rings of trees. The U.S. National Park Service describes the method as matching the ring pattern of an unknown wood sample against an established regional chronology built from trees of known age. Accuracy depends on climate: trees in regions with inconsistent rainfall produce rings that vary noticeably in width from year to year, which makes their patterns easier to match precisely, while trees in consistently wet, temperate settings grow more uniform rings that are harder to date. The Park Service also cautions that a dated tree ring only shows when that wood grew, not when the tree was cut down or used in construction, since timber was sometimes reused years after a tree died.

Luminescence Dating: Pottery, Fired Clay and Buried Sediment

Thermoluminescence (TL) dating measures the time elapsed since a crystalline material, such as pottery or lava, was last heated, or since buried sediment was last exposed to sunlight. Radiation from the surrounding environment causes electrons to become trapped in imperfections within a crystal over time; heating the sample in the laboratory releases these electrons as a measurable flash of light, and the intensity of that light reflects the accumulated radiation dose. Dividing that dose by the known annual radiation level at the find site yields an age estimate. The method works on ceramics, burnt flint, lava flows and sediments, but it destroys part of the sample, is sensitive to variation in local background radiation, and may only distinguish ancient from modern material rather than produce a precise year.

HRAF notes that archaeologists also use potassium-argon and argon-argon dating, which measure isotope ratios in volcanic rock, and archaeomagnetic dating, which tracks shifts in Earth's magnetic field recorded in fired materials. Because every method carries its own margin of error and material requirements, sites are rarely dated with a single technique; researchers typically combine stratigraphic sequencing with one or more absolute methods to narrow an age range and check the results against each other.