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Carbon dating, or radiocarbon dating, like any other laboratory testing technique, can be extremely reliable, so long as all of the variables involved are controlled and understood. Several factors affect radiocarbon test results, not all of which are easy to control objectively. For this reason, it’s preferable to date objects using multiple methods, rather than relying on one single test. Carbon dating is reliable within certain parameters but certainly not infallible.
When testing an object using radiocarbon dating, several factors have to be considered:
First, carbon dating only works on matter that was once alive, and it only determines the approximate date of death for that sample. For example, a steel spearhead cannot be carbon dated, so archaeologists might perform testing on the wooden shaft it was attached to. This provides good information, but it only indicates how long ago that piece of wood was cut from a living tree. Radiocarbon dating can’t tell the difference between wood that was cut and immediately used for the spear, and wood that was cut years before being re-used for that purpose. Nor can it tell if a much older spearhead was attached to a brand-new shaft.
Most archaeological items can’t be directly carbon dated, so their dating is based on testing done on nearby objects or materials. This makes the results subject to the researchers’ assumptions about those objects. If the spear head is dated using animal bones nearby, the accuracy of the results is entirely dependent on the assumed link between the spear head and the animal. This is perhaps the greatest point of potential error, as assumptions about dating can lead to circular reasoning, or choosing confirming results, rather than accepting a “wrong” date.
Second, radiocarbon dating becomes more difficult, and less accurate, as the sample gets older. The bodies of living things generally have concentrations of the isotope carbon-14, also known as radiocarbon, identical to concentrations in the atmosphere. When an organism dies, it stops taking in new carbon-14, and whatever is inside gradually decays into other elements. Carbon-14 normally makes up about 1 trillionth (1/1,000,000,000,000) of the earth’s atmosphere. So even brand-new samples contain incredibly tiny quantities of radiocarbon.
In nature, carbon exists as two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C), and a radioactive isotope, carbon-14 (14C), also known as "radiocarbon". The half-life of 14C (the time it takes for half of a given amount of 14C to decay) is about 5,730 years, so its concentration in the atmosphere might be expected to reduce over thousands of years, but 14C is constantly being produced in the lower stratosphere and upper troposphere, primarily by galactic cosmic rays, and to a lesser degree by solar cosmic rays.