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Kent Archaeological Review extract

Science in Archaeology --
Metallurgy in Archaeology part 1.
by Dr Tony Cox.

Metallurgy, the scientific study of metals, provides the archaeologist with a chronicle of the technological status of past civilisations. In the structures of metals lies the complete processing history, like a book, waiting to be opened, and interpreted by the trained mind. One of the first steps in unravelling the early craftsman's skill is to polish a small part of the surface until it reflects like a silvered mirror. Such a high scheen can be obtained by abrading with progressively finer emery cloths, with a final polish using alumina powder. Even in this reflective state little is generally seen when viewed in a microscope magnifying at a hundred times, but after a short immersion in dilute acid the surface becomes etched and the crystal structures revealed. More sophisticated and expensive pieces of apparatus are used for looking at surfaces, but considerable information can be derived from this relatively simple and inexpensive technique.

A question to be answered about any artifact once its utility and composition are identified is whether it was cast into its final shape, or alternatively, cast into an ingot (pig) and then hammered (wrought) into its final form. If cast, the shape is obtained by pouring the hot liquid metal into moulds, normally of sand, though metal moulds may be used for low melting point alloys or if fine detail is required in the finished item. The principal difference between the two processing methods is in the quality of the finished product. Casting often contain foreign matter occluded during the pouring, sand washed from the mould and dross from oxidition. These reduce the integrity of the casting and in the most severe cases render it brittle. In contrast, the final shape in a forged product is obtained by hammering the pig at elevated temperatures during which the foreign particles are dispersed in a relatively inocuous manner throughout, rather than being concentrated in a few critical regions. Consequently, when the warrior looked for a trusty and reliable blade he preferred the wrought product. The patterned blades of the East are one illustration of this. Even today, with considerable advances in casting technology, there is still a tendency to place more reliance on forged than cast components.

A metallurgical examination will differentiate between a cast and forged artifact, but in some cases they may be distinguished visually. In the manufacture of a casting, liquid metal is fed along channels to the main body and these feeding channels are broken off after solidification, but the protuberances may still remain.

Another indication of a casting is the absence of re-entrant surfaces, for these hinder its extraction from a permanent mould, which is facilitated if the item tapers from top to bottom. Castings also show signs of porosity (holes) due to dissolved gas being present in the liquid metal, and because metal contracts as it freezes, a marked surface depression may be evident in the final regions to solidify.

Forgings are generally less intricate than castings and small hammer impressions may be seen on the surface together with imbedded oxide scale. If forged between an upper and lower mould (dies) a thin sliver of metal (flash) exists at the junction of the dies. In forged iron artifacts which have corroded longitudinal cracks form along stringers of foreign particles elongated by the hammering.

If visual examination is insufficient to determine whether the artifact was cast or forged, then the metallurgical examination is necessary. This will be discussed, together with the effects of heat treating in subsequent issues of the KAR. Heat treatment is a combination of heating and cooling and improves the metal's quality and hence the durability of the implement.

(to be continued)
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