A method for roughly determining the age of human or animal bones, and for comparing the relative ages of two bones recovered from the same context. The technique works on the basis that fluorine percolating through deposits in groundwater slowly replaces the calcium in buried bones. The exact rate of replacement depends on the amount of fluorine present and is thus neither a universal standard nor even the same across a single site. The replacement is, however, irreversible, so that bones of very different date from the same general area can be distinguished. This technique led to confirmation that Piltdown man was a modern forgery.