What types of artifact are best dated with carbon-14 - a method called radiocarbon dating?
Very recent artifacts that are of human origin containing carbon.
Carbon 14 has a very short half life of about 5,700 years. Consider that every half life cuts the percentage of carbon 14 in half. In six half lives thee is a little more than 1% of the original carbon 14 left in the sample. six half lives is only 34,000 years. Small loses of Carbon 14 can make large changes in the estimate age of the sample.
Also the sample must contain carbon. Most fossils do not contain carbon, the original organic material has been replaced by minerals and cannot be dated using Carbon 14.
One junior high textbook I taught from stated that dinosaur bones were dated at 65 million years old using Carbon 14. This was laughable because the bones would have been turned to rock if they were that old. ( There have been some reports that dinosaur bones containing carbon blood tissue) At 65 million years there would be no carbon 14 left.
Radiocarbon dating is best used for biological artifacts that originally got their carbon from atmospheric carbon dioxide and are less than 50 000 years old.
The age limitation
It is difficult to measure the relative concentration of an isotope when it has decayed to less than about 0.3 % of its original value.
This corresponds to about 8.5 half-lives.
The half-life of carbon-14 is 5730 years.
Thus, the maximum age of an artefact that can be fixed by radiocarbon dating is 8.5 × 5730 years ≈ 50 000 years.
We cannot use the technique on fossils that are millions of years old.
Their original carbon has disappeared, and they are often contaminated with carbon from the environment.
The limitation to atmospheric carbon
We cannot use carbon dating for most aquatic organisms or animals that consume these organisms, because they often obtain at least some of their carbon from dissolved carbonates in rock.
The age of the carbon in the rock is different from that of the carbon in the air, and this upsets the calculations.