The presence of small amounts of C-14 in practically all carbon-bearing earth materials rises an additional concern for the radiocarbon dating method. In dating experiments, the standard practice is to count C-14 atoms in the sample and also in a "procedural blank" or calibration standard. Many laboratories use Precambrian graphite, purified natural gas, or optical grade calcite for the counting standard. This "blank" is thought to hold no C-14 whatsoever, so any counts recorded with it in place... read more
