Radiocarbon

What is measured

Living tissue takes in carbon, including a small share of radioactive carbon-14. After death, that share declines in a regular way. A laboratory measures what remains and reports an uncalibrated radiocarbon age, often written as “BP” (before 1950) with a laboratory uncertainty.

The measurement dates the death of the organism in the sample. It does not, by itself, date the day a wall was raised, a pillar was carved, or a room was abandoned.

Why the number is then moved

The amount of carbon-14 in the atmosphere has not been constant. Calibration curves, built from independently dated archives such as tree rings, convert the laboratory number into a calendar range. A single laboratory result usually becomes a span of years, wider when the curve is flat.

A quoted calibration such as “95.4 percent” means: under the curve and the model used, the true calendar age is estimated to fall in that window about 95 times out of 100. It is not a promise that the building was founded in one year inside the window.

The sample has to belong to the event

Charcoal in a fill can be older than the fill (old wood reused or washed in) or can date the burial of a building rather than its construction. Plaster that trapped short-lived plant matter is closer to a construction or replastering event, if the plaster is truly sealed and uncontaminated. Bone needs surviving collagen; poor preservation makes the result fragile.

A date from a later pit, a disturbed layer, or a carbonate crust is evidence about that material, not automatically about the monument beside it.

What would revise a result

A new calibration curve, a better pretreatment of the sample, a demonstration that the sample was intrusive, or a set of sealed samples that cluster somewhere else. One radiocarbon result is a constraint. A set of results from the right contexts is an argument.