Tuesday, October 10, 2006

Silver-in-the-pottery update I took a (fairly quick) look at the original paper.


This is the first study on the differential distribution and concentrations of silver in ceramics recovered from archaeological excavations. The chemical compositions of 1174 pottery vessels from 38 Roman-period sites in Israel have been determined. Unusually high and variable abundances of silver were discovered in pottery samples of all vessel types and chemical compositions from four distinct archaeological contexts dating to late first century bce to 70 ce Jerusalem. The large majority of the Jerusalem vessels could be distinguished by their silver abundances from all analysed pottery pieces recovered at rural sites outside Jerusalem, even when the pottery types and chemical compositions, except for silver, of pottery found within and outside Jerusalem were indistinguishable. The evidence is suggestive of a human origin for the high and variable silver abundances, and dispersion of the silver by aqueous transport. The differential silver concentrations found in excavated pottery from Jerusalem and other urban and rural sites suggest that attention to the distribution of silver in pottery from excavated contexts may be helpful for evaluating the nature and function of archaeological remains and patterns of urban contamination.

No page numbers for the quotes, as I lifted them from an HTML version.

Relating to the sample prep:
The pottery pieces were carefully cleaned and the surface layer removed before grinding and preparation of samples for analysis. The detected major Ag anomalies, therefore, were in the fabric of the pottery, not just on the surface. The only feasible way for the Ag to enter the pottery fabric, therefore, would most likely have been by aqueous transport.

From the above arguments, it appears that a likely source of the Ag anomalies in the Jerusalem samples is in situ contamination by aqueous transport after deposition, although a use-related source for the Ag in some samples is also possible.

That occurred to me initially -- whether they were getting surficial contamination or if the silver was getting deep into the fabric. So it seems as they sampled deep into the wall, indicating whatever put the Ag there, it had to penetrate deep into the structure.

One other question was whether there was any evidence of silver nearby that could serve as a source for the aqueous transport:

After six years of excavation in the Jewish Quarter during which no silver coins were recovered, a hoard of 13 silver coins, of the first, second, third and fourth years of the First Jewish Revolt (see below), was found in an excavation area about 50 m south-east of Area E
. . .
Although only the above-mentioned hoard of 13 silver coins has been reported from the Jewish Quarter excavations, over the past century a relatively large number of silver coins has been recovered in the many excavations that have been conducted in Jerusalem and its vicinity, and in chance finds in the area.

There seems to be at least some silver material present in the surrounding sediments. It should also be noted that the samples came from a variety of types -- "bowls, cooking pots, storage jars, jugs, juglets, flasks and lamps" -- and one group came from a mikveh which is described as a "ritual immersion bath". According to Figure 6(c), there is no significant difference between the various types, suggesting that it probably wasn't a function of use.

They did deal somewhat with the possibility of vessels containing silver objects by testing a vessel that did contain a mass of silver:
In order to learn more about the possible patterns of Ag corrosion and dispersion, we measured by HPXRF the Ag concentrations within the ceramic of a large Iron Age jug (c. late 11th to early 10th centuries bce) from Dor (Fig. 1) that contained a hoard of about 8.5 kg of silver (disc-shaped tokens, small fragments, and scraps of jewellery). The jug, discovered intact, was found standing in an upright position, with its mouth covered by a bowl (Stern et al. 2000; Stern 2001). Two samples were taken from the jug, one from the upper shoulder, above the present level of the mass of metal, and one from the base. Routine procedures for sample preparation were followed, including removal prior to analysis of the surfaces from both sampled pottery fragments. The Ag concentrations in the shoulder and base were 29.8 ± 0.6 and 101 ± 1 ppm, respectively. These data indicate that Ag entered the ceramic fabric.

Those are much higher concentrations than the ones they were getting from the other samples.

I'm tending to lean toward something other than the coins-in-the-vessel idea, mainly because the high concentrations are in a variety of vessel types in a variety of contexts. But I'm still not convinced that it was aqueous transport, since not a lot of information was provided on the likely sources of the silver in solution. This isn't really a criticism of the paper per se since that's a whole other can o' worms, and this largely dealt with the pottery itself. So, eh, interesting hypothesis.