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If you get a chance, check out the Shroud because a photographic negative actually showed the image on the Shroud for the first time in the late 1800's.
For older items, isotopes of potassium have been used.
Geologists have a bit of awkwardness in their language in talking about the deep past: distinguishing dates in the past from durations or ages. 200 happened 2216 years ago, and that an object made back then is 2216 years old today.
Ordinary people don't have a problem with the weirdness of historical time—in 2017; we can easily say that an event in B. (Remember, there was no year 0.) But geologists have a need to separate out the two types of time with different abbreviations or symbols, and there is a debate about establishing a standard way of expressing it.
Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! This dating method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
Another fascinating example is the Shroud of Turin (you might check this out on the Internet).
Mass spectrometers detect atoms of specific elements according to their atomic weights.
They, however, do not have the sensitivity to distinguish atomic isobars (atoms of different elements that have the same atomic weight, such as in the case of carbon 14 and nitrogen 14—the most common isotope of nitrogen).
A widespread practice has arisen in the last few decades that gives dates (not ages) in the format " And instead of saying that a rock is "5 Ma old," geologists use a different abbreviation, such as m.y., mya, myr, or Myr (all of which stand for millions of years, in reference to age or duration).
This is a little awkward, but the context makes things clear.