ORGANIC SUBSTANCES AT METAL SURFACES: ARCHAEOLOGICAL EVIDENCE AND THE ELDER PLINY's ACCOUNT*

Abstract

This paper discusses the implications of X-ray photoelectron spectroscopy (XPS) and scanning Auger microscopy (SAM) results that revealed the presence of sizable amounts of carbon-bearing species at the joint of a Roman lead pipe (fistula) and at the surface of a Roman bronze statue. The detailed description in Pliny's Naturalis Historia on the use of oil, pitch and bitumen in metallurgical contexts offers convincing grounds for an interpretation that the experimental findings arise from deliberate addition of these substances to lead and bronze. The surface of ancient metals is in itself a source of archaeological evidence, and XPS/SAM techniques are ideally suited to bringing them to light.

A multitechnique study of archeological bronzes

“…We propose that such an excess arises from intentional use of organic substances, such as oil, wax etc., and that these materials are also responsible for the lower copper oxidation extent in bronze A. [19] One could argue that the high copper 'metallization' and the high carbon abundance are not necessarily related to one another because even adventitious, ambient contaminants themselves produce a C1s component at BE ∼285 eV. In response to this objection, we call attention to the following two considerations: (i) As is well known, [3,4,12] tin minimizes the surface oxidation of copper in bronze (and also that of Pb in tin-lead alloys), [19] and the force of this beneficial effect scales with the tin abundance at the surface.…”

“…[19] One could argue that the high copper 'metallization' and the high carbon abundance are not necessarily related to one another because even adventitious, ambient contaminants themselves produce a C1s component at BE ∼285 eV. In response to this objection, we call attention to the following two considerations: (i) As is well known, [3,4,12] tin minimizes the surface oxidation of copper in bronze (and also that of Pb in tin-lead alloys), [19] and the force of this beneficial effect scales with the tin abundance at the surface. In fact, the tin content in bronze A is about 10% lower than in bronze B (Table 1), and this conflicts with the finding that the former bronze features the higher copper metallization.…”

“…(ii) In a somewhat related case, i.e the XPS/AES analysis of the joint of a Roman 'fistula', [19] we found that lead could be soldered to itself if, and only if, both a tin-bearing ingredient and a (-CH-CH-) -bearing carbonaceous ingredient (oil or wax) were used, and the synergic action of the two ingredients kept lead oxidation to a sufficiently low level for lead-lead metal sealing to occur. This finding clearly shows that (-CH-CH-) carbonaceous species have a direct influence on the metal oxidation induced by the surroundings, and that surface science techniques are capable of experimentally verifying such an influence.…”

“…It should be borne in mind that the as-inserted bronze A produced a C 1s spectrum peaked at 285.0 eV, which is characteristic of hydrocarbon species (-CH-CH-), whereas it was virtually free of any component at BE ∼288-290 eV, characteristic of carbonate species. [4,12,19] Conversely, although they possessed a low intensity, such components contributed to the C1s spectrum of sample B, whose maximum again lay at BE = 285.0 eV. The lower panel of Fig.…”

“…Especially noteworthy are the quantitative results involving oxygen and carbon because these two 'light' elements are of strategic importance in surface studies of ancient metalwork. [1,3,4,12,19] Again, the main emphasis is on the results relevant to the samples analyzed in the as-inserted condition, which are not affected by Ar + -ion induced effects which preferentially remove some chemical elements over others. [3,4,12] The…”