Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.

That is, the first digit tells you how many ones you have; the second tells you how many fours you have; the third tells you how many sixteens (that is, how many four-times-fours) you have; the fourth tells you how many sixty-fours (that is, how many four-times-four-times-fours) you have; and so on.

These techniques are applied to igneous rocks, and are normally seen as giving the time since solidification.

The isotope concentrations can be measured very accurately, but isotope concentrations are not dates.

This also has to be corrected for.[2] Second, the ratio of C in the atmosphere at that time to be estimated, and so partial calibration of the “clock” is possible.

Accordingly, carbon dating carefully applied to items from historical times can be useful.