Fluoride (or fluorine) dating is a relative dating method that can be used to date archaeological bone. As a relative dating method, it can determine the relative age of specimens, but cannot provide a calendrical date unless the fluoride chronology is calibrated with an absolute dating method.
Bones are primarily composed of the mineral calcium hydroxy
apatite. When exposed to water that contains fluoride, a fluoride ion
(F-) can replace a hydroxyl ion (OH-) in the bone mineral. The resulting
fluor-apatite is more stable than the original form, thus the fluoride content
of a bone will increase over time if it is exposed to a solution containing
fluoride ions. Fluoride ions are present in trace amounts in most soils
and groundwaters. Over time, buried bones pick up fluoride ions from soil
moisture or exposure to groundwater. Older specimens have higher fluoride
contents than younger ones when burial conditions are identical. The
requirement of identical burial conditions means that fluoride dating works
best when it is applied within a single site with little variation in soil
Many different techniques can be used to measure bone fluoride content, but measurement by ion selective electrode is the easiest and simplest method available today. The fluoride selective electrode uses the same principle as the familiar pH electrode. When the electrode is placed in a solution that contains fluoride, it produces a voltage that is proportional to the amount of fluoride in the solution. A calibration curve can be produced by measuring standard solutions of known fluoride concentration. The calibration curve is then used to determine the fluoride content of unknowns. A desciption of the method is given in:
Schurr, Mark R.1989. Fluoride dating of prehistoric bones by ion selective electrode. Journal of Archaeological Science 16: 265-270.
It can be used to determine the relative dates of faunal materials. The relative dates can be used to date contexts that did not contain diagnostic artifacts or that did not provide carbon suitable for radiocarbon dating.
The Service Center charges $16.50 per sample (please contact the Center for the most current rates). This covers cleaning the sample, preparing it for analysis, making three replicate measurements on each specimen, calculation of the fluoride content, and entry of data into a spreadsheet. Statistical evaluation of the data and consulting reports are available on request.
Fluoride dating has been successfully used in a wide
variety of settings, ranging from extremely arid Sudanese Nubia, to the humid
eastern USA. It has been applied to sites ranging from very old to very
recent (for example, the famous Piltdown specimens contained both), and with
bones that range from partially fossilized to less than a thousand years
old. A bibliography
of fluoride dating applications is available.
Fluoride dating of control samples and burial treatment clusters was used to establish the contemporaneity of the burial treatments at the Middle Mississippian Angel site in southwestern Indiana. Each of the clusters represents a different type of burial treatment. The error bars on the fluoride measurements are one standard deviation limits, so all of the treatments represent contempory burial modes, with one cluster (Cluster 6, flexed burials with artifacts, lowest fluoride content) possibly being among the latest.
At the Los Pozos site in Arizona, fluoride dates from intersecting features
(such as those shown here) excavated by Desert Archaeology, Inc. were used to
establish a relative chronology for house basins at the site.
Where more older features were cut into by more recent ones, the more recent features tend to produce bones with lower fluoride contents. The results from these control features show that fluoride dating can be used to date house basins at Los Pozos that have not been cross-cut by other features (the majority of house basins at the site). The complete text of the poster presented at the 2000 Society for American Archeology Annual Meeting is now on-line at http://www.nd.edu/~alfac/schurr/ .
Fluoride is transmitted through ground water, so optimum performance is obtained on sites where the soil chemistry is consistent, the are not dramatic differences in soil moisture, and where the sediments are relatively fine grained.
In most cases, we recommend a pilot project using samples of known chronology to evaluate the effectiveness of fluoride dating on your samples before starting a large-scale project. We will be happy to consult with you to develop a project custom tailored to your needs.
Fluoride dating works best when bones of similar density are compared. Therefore, it is rarely possible to mix results from skeletal elements with widely differing densities (i.e., femur cortical bone versus vertebral fragments), or to compare juveniles with adults, or to compare bones from very large or very small mammals. For faunal specimens, we recommend long bone fragments, which are usually very abundant. For humans, either ribs or cortical bone fragments both give good results, as long as one type of bone is consistently used.
We require at least 0.1 gram of bone simply for handling
convenience. The actual measurements will only consume about 15 mg.
The remainder will be returned to you in a labelled vial.
Dr. Mark R. Schurr
Director, Fluoride Dating Service Center
Department of Anthropology
University of Notre Dame
Notre Dame, IN 46556 USA
Phone: (219) 631-7638
FAX: (219) 631-8209
This page's WebCounter count says you are visitor number