A correction factor may be applied or the sample rejected on this basis. Recrystallisation refers to the modification of shell aragonite to calcite, often involving an exchange with modern calcite and a subsequent altering of isotopic ratios. In investigating the extent and effect of these types of sample contamination, the radiocarbon laboratory is faced with two major problems. First, it has to identify precisely the nature and size of contamination. A set of graphs were shown by the authors' to show the range of errors associated with samples contaminated by older and younger carbon.
More complex problems arise when the contaminating fraction changes temporally in size.
The other major issue in sample contamination concerns samples which contain small errors. Dates which are clearly too old or young are easily recognised and investigated, those that contain less significant errors are more difficult to identify. There are a number of ways to guage the effects of pretreatment upon samples. The most common is to analyse and date the different fractions that have been removed. This will reveal the magnitude of error.