Re: Problems with the radio carbon dating of the Newport Tower

On Wed, 05 Dec 2007 20:05:50 GMT, "David B." <tronospamchos@xxxxxxxxx>
wrote:

JerryT wrote:
On 5 Dec, 16:04, "Steve Marcus" <smarcus_spamo...@xxxxxxx> wrote:

snip


My prior posts have stated that there is difficulty in trying to adjust for
the problems of the reservoir effect, and the use of shell lime, because
such adjustments will require knowledge of the conditions that existed when
the mortar was created, which may not be the same as the conditions that
exist today, and also it matters where the shells were obtained from and
what percentage of the lime used in the mortar came from shells.


Perhaps some may be interested...an update and indepth of mortar
dating.

From; http://www.kyrkor.aland.fi/Lindroos-JH-AR-MB-AES-2007.pdf (650
KB)

MORTAR DATING USING AMS 14C AND SEQUENTIAL DISSOLUTION: EXAMPLES FROM
MEDIEVAL, NON-HYDRAULIC LIME MORTARS
FROM THE ÅLAND ISLANDS, SW FINLAND

Alf Lindroos1 * Jan Heinemeier2 * Åsa Ringbom3 * Mats Braskén4 * Árny
Sveinbjörnsdóttir5

ABSTRACT. Non-hydraulic mortars contain datable binder carbonate with
a direct relation to the time when it was used in a building, but they
also
contain contaminants that disturb radiocarbon dating attempts. The
most
relevant contaminants either have a geological provenance and age or
they
can be related to delayed carbonate formation or devitrification and
recrystallization of the mortar. We studied the mortars using
cathodoluminescence (CL), mass spectrometry (MS), and accelerator
mass spectrometry (AMS) in order to identify, characterize, and date
different
generations of carbonates. The parameters-- dissolution rate, 13C/12C
and
18O/16O ratios, and 14C age--were measured or calculated from
experiments
where the mortars were dissolved in phosphoric acid and each
successive CO2
increment was collected, analyzed, and dated. Consequently, mortar
dating
comprises a CL characterization of the sample and a CO2 evolution
pressure
curve, a 14C age, and stable isotope profiles from at least 5
successive
dissolution increments representing nearly total dissolution. The data
is used
for modeling the interfering effects of the different carbonates on
the binder
carbonate age. The models help us to interpret the 14C age profiles
and identify
CO2 increments that are as uncontaminated as possible. The dating
method was
implemented on medieval and younger mortars from churches in the Åland
Archipelago between Finland and Sweden. The results are used to
develop
the method for a more general and international use.

Interestingly, there's not a single mention of shells in that paper.

I've now found more details of the 19th century study of the Newport
Tower mortar, to which I published a rather iffy link a few days ago.
The following summary combines information from Edward Peterson "History
of Rhode Island" (1853); Harper's New Monthly Magazine article "Newport:
Historical and Social" (August 1854); and William H. Boyd "The Newport
Directory" (1856).

In 1848 the Rev. Dr. Jackson of Newport collected samples from:
"the mill, Arnold's tomb-vault ["the tombs of Governor Arnold and his
wife"], the Bull-house of 1640-1, built for a garrison ["the old stone
house in Spring-street, which was built by Henry Bull, one of the first
purchasers of the Island, and immediately after the first settlement of
the town"], the Easton-house, the Atkinson-house, the Vernon-house, and
the Easton-house, (now Southwick), built in 1642-3" "and analyzed and
compared them, and found them of the same quality, and composed of shell
lime, sand, and gravel; and considered it very strong evidence that they
were built not far from the same time- all probably within a period of
thirty or forty years from each other."

Does the reference to "shell lime" mean that in that area, seashells
were used very extensively as a substitute for limestone in the making
of lime?

Yes.

I also wonder whether it would be possible to separate out shell
fragments from mortar samples and C14 date them on their own, perhaps
sacrificing a few 100-year-old shells from the Rhode Island Natural
History Museum to help generate a rough calibration scale.

The most useful thing to do would be to date a whole heap of new
shells to get a handle on the magnitude of the local reservoir effect.



Eric Stevens
.

Relevant Pages

  • Re: Problems with the radio carbon dating of the Newport Tower
    ... the problems of the reservoir effect, and the use of shell lime, because ... what percentage of the lime used in the mortar came from shells. ... Non-hydraulic mortars contain datable binder carbonate with ...
    (sci.archaeology)
  • Re: Problems with the radio carbon dating of the Newport Tower
    ... techniques they used to separate old -CO3 from new -CO3. ... Or that they can know for a fact the source of the shells used ... in the mortar, and to what extent the original mortar was mixed ... percentages of the carbonate and hence the radiocarbon dating. ...
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