Creation.com article cited previously - items 51-56:
Radiometric dating and the age of the earth:
Carbon-14 in coal suggests ages of thousands of years and clearly contradict ages of millions of years.
Carbon-14 in oil again suggests ages of thousands, not millions, of years.
Carbon-14 in fossil wood also indicates ages of thousands, not millions, of years.
Carbon-14 in diamonds suggests ages of thousands, not billions, of years.
Note that attempts to explain away carbon-14 in diamonds, coal, etc., such as by neutrons from uranium decay converting nitrogen to C-14 do not work.
Incongruent radioisotope dates using the same technique argue against trusting the dating methods that give millions of years.
Incongruent radioisotope dates using different techniques argue against trusting the dating methods that give millions of years (or billions of years for the age of the earth).
Radiocarbon dating is a radiometric dating method that uses (14C) to determine the age of carbonaceous materials up to about 60,000 years old. The technique was developed by Willard Libby and his colleagues in 1949[7] during his tenure as a professor at the University of Chicago. Libby estimated that the radioactivity of exchangeable carbon-14 would be about 14 disintegrations per minute (dpm) per gram of pure carbon, and this is still used as the activity of the modern radiocarbon standard.[8][9] In 1960, Libby was awarded the Nobel Prize in chemistry for this work.
One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants fix atmospheric carbon during photosynthesis, so the level of 14C in plants and animals when they die approximately equals the level of 14C in the atmosphere at that time. However, it decreases thereafter from radioactive decay, allowing the date of death or fixation to be estimated. The initial 14C level for the calculation can either be estimated, or else directly compared with known year-by-year data from tree-ring data (dendrochronology) up to 10,000 years ago (using overlapping data from live and dead trees in a given area), or else from cave deposits (speleothems), back to about 45,000 years before the present. A calculation or (more accurately) a direct comparison of carbon-14 levels in a sample, with tree ring or cave-deposit carbon-14 levels of a known age, then gives the wood or animal sample age-since-formation.
So Carbon 14 tests are referenced to tree rings, giving a way to calibrate results. Tests on coal or anything can be inaccurate, if contaminated. That is why, reference the Shroud, the testimony of the workers that the samples did not come from patches, procedures were used to avoide contamination.