Carbon 14 Dating Facts - Cloth Fibers, Bones, Wood . . .

 in the peer-reviewed scientific journal Thermochimica Acta See: Volume 425 pp. 189-194)

Carbon 14 dating is an accurate method for determining the age of things that have a biological origin such as bones, cloth made from natural fibers, artifacts made of wood, and charcoal.

All plants and animals are made up of chemicals that contain three types of carbon atoms or isotopes. The most common isotope is carbon 12. It generally accounts for roughly 99% of all the carbon atoms found in a plant or animal. Another isotope is carbon 13 that comprises about 1% of the total. The third isotope is carbon 14. It is found in only tiny amounts compared to carbon 12. In a living plant or animal there is only about one carbon 14 atoms for every trillion carbon 12 atoms.

Carbon (C12, C13 and C14) is acquired by plants from carbon dioxide (CO2) in the atmosphere. Animals get carbon by breathing in carbon dioxide and by ingesting plants and other animals. The result is that all living things have a specific ratio of carbon 12 and carbon 14 isotopes. Generally speaking, it is the same ratio found in the atmosphere. The ratio begins to change, however, when a plant or animal dies and it is no longer taking on carbon from nature.  Carbon 14, unlike carbon 12 and carbon 13, is radioactive. This means that over time the carbon 14 atoms will decay. When the isotope carbon 14 decays it gives off a beta particle and in doing so becomes nitrogen 14. The amount of carbon 12 and carbon 13, however, remains constant.

The half-life of different isotopes range from fractions of a second to millions of years. The half-life of carbon 14 is 5730 years. This means that in 5730 years, half of the carbon 14 atoms will have become nitrogen 14. In the next 5730 years, half of the remaining carbon 14 will have decayed.

The following table shows the approximate amount of carbon 14 that will remain years after a plant or animal has died.

Half-Life Years C14 Remaining
1 5,730 50.0%
2 11,460 25.0%
3 17,190 12.5%
4 22,920 6.3%
5 28,650 3.1%
6 34,380 1.6%
7 45,840 0.8%

Thus, if we are able to measure the percentage of carbon 14 that remains in a sample we can determine its age. The sample may be from a human bone, an animal antler, a piece of line made from flax fibers, a wooden tool or charcoal from an ancient fire. All of these things were once part of something that was alive.

Two Carbon 14 Methods

Two carbon 14 methods are commonly used for determining how much carbon 14 remains:

  • Radiometric: The radiation emitted by the carbon 14 in a sample is measured. The lower the radiation level, the lower the amount of carbon 14 that remains. Both gas proportional counters and liquid scintillation systems are used for radiometric carbon 14 dating.

  • Accelerator Mass Spectrometry (AMS): A particle accelerator is used to sort ions according to their mass so they may be counted with special detectors. The advantage of AMS is that only a small sample of a few milligrams is required carbon 14 dating.

Both methods produce accurate carbon 14 results. The radiometric method is simpler and less costly but requires larger samples. AMS only requires very small samples but it is costly because of the complex equipment used.

Both methods require that the sample be destroyed to convert the carbon into a usable form for measurement.

Carbon 14 testing is only accurate for material that is less than 50,000 years old.

What Can Go Wrong in Carbon 14 Dating?

Mistakes can and do happen. But modern carbon 14 dating laboratories are very careful to avoid mistakes. Equipment is carefully calibrated, test samples are used for comparisons and the samples to be carbon 14 dated are carefully cleaned in chemical baths to remove contamination.

Groundwater or carbonated rocks such as limestone can contaminate a carbon 14 sample. Insect and worm holes as well as cracks and fissures may allow different parts of a sample to be contaminated differently and such possibilities must be carefully accounted for in carbon 14 dating.

In recent years, some scientists have suggested that biological polymers might grow on some materials such as the fibers of ancient linen cloth or that fires near a sample might introduce significant newer carbon into an object.  These ideas have been floated to try and explain why the carbon 14 dating of the Shroud of Turin might be wrong. But most scientists discount these hypotheses, pointing out how much new carbon would be required to change the date significantly. In the case of a growing (and even still living) bioplastic polymer, the amount needed to alter the carbon 14 date significantly would be very large.

Justified in Questioning the Carbon 14 on the Shroud

Nonetheless, many researchers have felt justified in questioning the accuracy of the carbon 14 dating on the Shroud of Turin. There is, they point out, significant forensic and historical evidence that suggests that the cloth is much older than the range of dates determined by the carbon 14 dating: 1260 to 1390 CE.  Such questioning may be valid. But it is not because of contamination, not because a biological polymer or fire introduced carbon 14. And it is not because of any errors on the part of the carbon 14 laboratories.

Recently, as reported by National Geographic News and PBS in April 2004, and as discussed in several scientific papers, researchers have discovered that the carbon 14 dating samples cut from the cloth are not chemically representative of the whole cloth. Examination of threads from the vicinity where the samples were taken show clear evidence of a reweaving with newer thread. Hence the carbon 14 date would be the average of older and newer thread.



© 2004 Daniel R. Porter, Bronxville, New York