The Distribution of Phosphorus in Romano-British Ironwork
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THE DISTRIBUTION OF PHOSPHORUS IN ROMANO-BRITISH IRONWORK Alain E.
Kaloyeros* and Robert M. Ehrenreich** The University at Albany-SUNY, Albany, NY 12222 Materials Advisory Board, National Academy of Sciences,
*Physics Department,
**National
Constitution Avenue,
NW, Washington,
DC
2101
20418
ABSTRACT Phosphorus is found to be a common impurity in many of the iron tools and weapons produced during the pre-Roman and Roman Iron Ages of Britain (600 BC - 300 AD). The effects of this impurity on the properties and performance of antiquarian materials is not well understood, however. This paper presents the initial findings of an in-depth study of the distribution, chemistry, and effects of phosphorus in Romano-British ironwork. For this purpose, two Romano-British iron artifacts from the site of Ircheoter, Northamptonshire, were examined using powerful techniques for archeological materials analysis that include electron microprobe, secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM) with energydispersive x-ray spectroscopy capabilities (EDXS), and Auger electron spectroscopy (AES). It was found that phosphorous was indeed present in the artifacts. The phosphorus atoms were predominantly segregated at grain boundaries and thus should have led to a lowering of grain boundary cohesion and a degradation in the performance of the tools. INTRODUCTION Archaeologists are in a quandary concerning the ancient use of phosphorus-rich iron. The modern metallurgical community regards phosphorus 1 content. ,2,3,4 as a detrimental impurity of iron and strives to limit its Phosphorus has a deleterious effect on the ductility of ferritic steels when it segregates to grain boundaries. The grain boundary segregation of phosphorus in low alloy steels is a primary cause of temper and creep 5 6 7 8 embrittlement, grain boundary corrosion and stress corrosion cracking. , , , Also, phosphorus concentrations exceeding 0.080% in iron restricts the diffusion of carbon and causes the metal to be "cold-short."'9, 10 However, a previous comprehensive metallurgical study of 884 Iron Age and Romano-British iron artifacts has shown that: (1) phosphorus was a relatively common impurity of archaeological ironwork, i.e., 324 of the 779 artifacts that could be analyzed had phosphorus concentrations in excess of 0.080%; (2) phosphorus seemed to act under certain conditions as a hardening agent of the metal, i.e., the average hardness of low-carbon, highphosphorous artifacts (172.5HV/Sg) was approximately midway between the average hardness of low-carbon, low-phosphorus iron (135.9 HV/5g) and that of high-carbon, low-phosphorus iron (198.1 HV/5g); and (3) phosphoric iron may have been preferentially used by early blacksmiths to produce harder, more 11 durable tools and weapons. The assemblage from the Late Iron Age hillfort of Bigbury, Kent, in the clearest example of the possible deliberate selection of phosphoric iron for its enhanced hardness. The edged tools recovered from Bigbury (i.e., blades, axes, spearheads, picks,
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