CURRENT STATUS
CF
THE FLUORIE METHOD OF AGE DEERMINATION
H. C. Ezra, S. F. Cook, and H. A. Leon
As a result of Oakley's extensive use of fluoride incorporation as a
dating index for archeological bone specimens, and with his unquestioned suc-
cess in obtaining convincing RI, and, on occasion, RII data, the determination
of this element in fossil bone is today a very popular tool. We felt that it
was appropriate and advisable to examine more profoundly the limitations involved
in the application of fluoride fixation to archeological work. In the course of
this fairly systematic investigation and review of the uses of the method several
interesting speculations presented themselves. Also, some pertinent questions
arose regarding the objective validity of the method as it is employed at
present.
Let us examine the chemistry of the procedureO in 1956 at Chicago,
Oakley mentioned that his 1953 fluoride analyses presented an improvement in pre-
cision of the chemical procedure over his 1949 determinations. We were unable to
find anywhere a published report by his chemists of this improvement in method,
nor does he refer to such in any bibliography. However, the procedure used in
1949 was fully described by,Hoskins and Fryd, Oakley's chemists, in the J. of
Appl. Chem., 1955. The method is the classical one of Willard and Winters; a
quantitative fluoride analysis which has been in use for the past 25 years.
Hoskins and Fryd have utilized the micro adaptation of the original method, but
even so, the lower limit exceeds quantitatively the range in which they found them-
selves working with archeologically recent bone specimens, so that they had to
extrapolate the concentration curve of fluoride to zero and compensate for the
introduction of interferimg salts by a so-called "internal blank" . This mani-
pulation of the method, though not in itself without justification, is not the
most satisfactory way of quantitatively determining trace amounts of chemicals or
elements such as fluoride. It would be interesting to learn what the English
laboratory has done to improve its procedures that have brought more precision to
the 1953 data, as Oakley stated .
Wholly apart from Oakley's work the advent of radio isotopes leads to the
possibility that new micro methods for the determination of traces of fluoride could
be developed. The standard isotope dilution method would very likely be appropriate
for use with such chemically hete ogenous materials as bones and soils. However,
the fact that the -half-life of F10 is only 112 minutes means that ready access to
the isotope as soon as it is formed would be necessary. Another method, activation
procedure, would probably prove most straightforward and accurate. Possibly mass
spectrography can offer a spectacular and simplified means of quantitating the smll
amounts of fluoride found in bones. However, the application of the mass spectro-
graph to bone analysis is hardly an hypothesis at this time. It might very well be
that the mechanics involved would prohibit the application of this instrument for
bone analysis.
Aside from isotope methods it has occurred to us, s'ince fluoride is a specific
enzyme inhibitor, that a biological assay technique could well be devised to quanti-
tatively detect such limited amounts of fluoride as .01 of 1% by weight, or, in
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absolute amounts, as little as 500 gamma0
Though not directly concerned with this pape we would like to state that when
there is any consideration of methods based on Fl, s uch as those just mentioned,
thoughts are directed, perhaps by association, to C1   dating. There is no particu-
lar tie-up between the two methods, nor is there any reason to assume that results of
the two methods would in any way be related. To our knowledge, no C14 dating on bone
material itself has ever been published, since it is obvious that both the inorganic
C and organic C will be radioactive. Of course, as Oakley suggested in 1956, it
would be relativelJy simple to remove the organic carbon from bone by hydrolysis and
count it separately. How to get rid of the foreign invasion organic carbon would
have to be kept in mind, since it has not yet been possible to remove the muccopoly-
saccharides, osseins and collagens intact from bone, fresh or fossil.
We may turn now to the mechanism of fluoride fixation in biological systems,
especially bone. Oakley has stated in Antbropology Today the theoretical maximum
fluoride content of fossil bones is 3.8T    We were unable to find his reference for
this figure, and this led us to wonder whether it was derived empirically by labora-
tory investigations of the saturation curve of fossil bones in different soil - water
concentrations of fluoride, or whether this figure -is based on theoretical calculation
assuming a one-for-one replacement of hydroxyl ion by fluoride ion in the apatite
molecule.  Other investigators in osseons systems, living and non-living, find a
more complex situation exists than simple replacement of one ion for another. The
relationship between the available apatite lattice surface area and the amount of
fluoride affixed is of primary importance to understanding the mechanism. Initially
it needs to be clearly and conclusively demonstrated by experimental investigation
whether the organic phase of bone is or is not at all involved in fluoride fixation.
P. C. W. Durbin in 1953 with in vivo studies on rats has shown by radioautographs
the areas of deposition of fluoride in bone. The paths of vascularization, the pern-
osteum and endosteum, marrow cavities and epiphysis are loci for concentration of
fluoride. She found that the areas of compact bone in the long bone shafts have a
meagre deposition of fluoride, and that this follows the Volkmann and Haversian
systems closely. We may raise the question as to what happens to fluoride deposition
in biologically inert material such as bone exposed to a foreign environment. We are
not aware of much data on this subject. However, Band in 1956 investigated the crystal
lography of the mineral phase of bone saturated with fluoride and found that the amount
of fluoride present is not a simple function of hydroxyl ion replacement, but that
there is some othe process going on simultaneously which could not be defined by
crystallographic methods. His work shows that saturated dead bone can contain more
than 3.% fluoride. In fact, what the real saturation limit of bone is, is not
known. All this would lead us to conclude, in contrast to the classical point of
view: firstly, the fluoride ion is not easily diffusible in bone, and secondly, the
fluoride concentration is not necessarily the same throughout the diameter of a speci-
men. Very possibly there is a considerable gradation in the amount of fluoride
fixed from the external surface progressively toward the interior of the bone.
Neuman in 1950 showed that no calcium fluoride is formed in bone, and that fluoride
does not compete with phosphate ion, but that it does compete with hydroxyl and
bicarbonate ions on the crystal surface. In some archeological sites the ratio
of environmental bicarbonate ion to hydroxyl ion on the crystal surface of the bone
would be considerably different from that found in other sites, How could the varia-
bility in exposure to available bicarbonate from one fossil to another be controlled?
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As far as RI dating is concerned, a relationship between the soil carbon dioxide
(or pH) ard the bone bicarbonate ion could be established0 In addition to the carbon
dioxide - bicarbonate ion relationships it is clear that numerous other chemical fac-
tors exist which will determine not only the mechanism, but the actual rate of fluoride
uptake. This, in turn, will seriously influence the apparent age of the bone,, provid-
ing that the latter is considered to depend upon quantity of fluoride found on
analysis. Clearly, we must attack the problem of fluoride dating from the standpoint
of the interaction between the soil environment and the fossil bone.
NOTES
1. This paper was delivered by Miss Ezra at the second Annual Kroeber Anthro-'
pological Society meetings in Berkeley, 1958.
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