In:
Revue d'Archéométrie, PERSEE Program, Vol. 20, No. 1 ( 1996), p. 47-56
Abstract:
The amphorae produced in the workshop of Canneto (Lazio) and found in the archaeological site of the Madrague de Giens, a ship sunk, in front of the coast of Hyères (France) the 1st century BC, have been weathered by the sea water resulting in chemical and mineralogical changes. Part of the freight was covered by marine sediments and part rested in contact with the sea water in movement, and as a consequence of this fact, the amphorae of both parts exhibit different visual, chemical and mineralogical alterations. The ceramics found in the uncovered zone show an important enrichment in Mg as hydrotalcite and the loss of K in their outer parts, as well as a crust on the surface formed by aragonite and Mg-rich calcite. The ceramics found in the covered zone became black by the transformation of hematite in to pyrite and gypsum has been determined on their surface, but from the chemical point of view no important changes have been reported. They also present fissures full up with crystallisations which are found to be needles of calcite an aragonite (of 1 mm of size) and spheroidal growing of Mg, K and Al carbonates (of 20 microns of size) coating the walls. Rarely some barite crystals are also found. Therefore, two main mechanisms of alteration are found : the formation of carbonates and the precipitation of sulfates and sulfides. Ca is supplied by incrustating organisms, Al and K come from the ceramic body and Mg and S are elements from the water sea. The low crystalline phases of the ceramic have been transformed into complex and basic carbonates of Ca, K, Al and Mg by either an inorganic process (by high alkaline conditions produced by the dissolution of calcite) in the uncovered zone, or an inorganic process (by the action of desulfuvio and streptococcus bacteria) in the covered zone. K-carbonates are highly soluble and in the uncovered zone are readily dissolved resulting in the lixiviation of K. On the contrary, Mg, Al-carbonates (hydrotalcite) are insoluble resulting in the fixation of Mg from the sea water. The dissolution of calcite did not affect the chemical composition of the ceramic as Ca-carbonates come from the calcareous crust. In the covered zone the process is localized inside the fissures where bacteria transformed the ceramic body in to carbonates and after oversaturation of the water contained in the fissure, the precipitation is done. The presence of sulfates can be explained from the alkaline conditions of the sea but in the covered zone (where water is not renewed) both (SO4)2' and S+ can coexist by the disproportion of the stable ion thiosulfate, then the respiration of anaerobic bacteria produces the reduction of Fe ?+ to Fe2+ and, thus, the formation of pyrite, leading to the simultaneous presence of sulfates and sulfides.
Type of Medium:
Online Resource
ISSN:
0399-1237
DOI:
10.3406/arsci.1996.936
Language:
French
Publisher:
PERSEE Program
Publication Date:
1996
SSG:
6,14
SSG:
8
SSG:
6,12
SSG:
6,11
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