Turning a carbon ratio into a timeline
A laboratory does not read a bone’s age directly. It measures the amount of carbon-14 relative to stable carbon in suitable organic material—usually collagen, the protein preserved in bone. The smaller the carbon-14 share, the longer the material has been isolated from the living carbon cycle, all else being equal.
Carbon-14 decays at a known rate: its amount halves about every 5,730 years. Scientists use the measured ratio to calculate a radiocarbon age, which is first expressed in “years before present,” with “present” conventionally set at 1950. That result is not yet necessarily the same as a calendar date.
Why the result needs calibration
The method’s clock depends on how much carbon-14 entered the atmosphere when an organism was alive. That amount has varied over time, so equal spans of decay do not always translate neatly into equal spans on the calendar. Researchers compare radiocarbon measurements with records from materials whose calendar ages can be established independently, such as tree rings. This calibration turns a radiocarbon estimate into a calendar-age range, often with uncertainty rather than a single exact year.
The sample also has to represent the bone’s original carbon. Soil chemicals, conservation materials, or other later contaminants can shift the measured ratio; laboratories clean and assess samples to reduce that risk. The method dates the organism’s death, not the moment a bone was buried, discovered, or used in a tool.
Where the clock is useful
Radiocarbon dating works best for once-living material within a limited span of the past. As time passes, so little carbon-14 remains that measurement becomes difficult; very old samples may fall beyond the method’s practical range. Its strength is that it can turn a trace of surviving organic matter into a testable timeline—while the result still depends on careful sampling and calibration.
