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Kent Archaeological Review extract

Science in Archaeology.
by Dr Tony Cox.

In recent years the crystalline form of metals has received some publicity and the fact that a lump of metal is made up of thousands of small crystals is widely appreciated. We owe much of this familiarity to the author Nevil Shute, who, in his novel "No Highway" utilised the idea that metals exist in the form of crystals. However, there is a common misconception that when metals are repeatedly stressed the final rupture is associated with the metal "crystallising." Credence for this idea is given by the highly reflective fractured surfaces which give the impression of being made up of many small crystals. Some fracture surfaces do show crystal faces but these are only visible under high power microscopes and are the exception rather than the rule and indicate that the metal has been incorrectly treated. Under normal circumstances broken surfaces show no obvious signs of the grains which may seem to contradict the idea that metals are composed of grains, but this is because the grains are torn in half during the fracture process.

PHOTO: Grains of metal in a steel.

Grains of metal in a steel
Figure 1 X700 Mag.

The crystals in a lump of metal, whether in a crudely formed implement or a cunningly devised brooch, vary in size from 1mm to 1/1000mm. An idea of what the crystals look like is obtained by breaking a piece of ceiling tile (expanded polystyrene) and looking at the new surfaces produced. The surfaces will be found to be irregular and made up of hundreds of tiny pea-shaped pieces with facets on them. These faceted peas (crystals) abut with their neighbours and produce a dense mass with no air space between the separate grains, as the crystals are called. Similar shaped grains are found in metals, and typical grains found in a piece of steel are shown in Figure 1.

In metals like copper and iron the shape of the grains is a record of what has happened to the metal. When pieces of metal are melted in a crucible and the molton metal subsequently poured in a mould, the metal solidifies in a characteristic manner. Initially, because the mould is cold, the liquid metal which first comes into contact with the mould walls is rapidly Chilled to below its freezing point. The metal solidifies and a thin skin of small crystals forms at the outside of the ingot or casting. The liquid metal then continues to solidify starting from the bottom and proceeds towards the filler head at the top of the mould. If the casting solidifies like this, with no bridging across the mould by premature freezing across the top, then as the metal solidifies and contracts more liquid flows into the lower portions of the mould and prevents cavities forming. After the small grains have solidified on the mould walls, long rod shaped grains (termed columnar grains) grow from the walls towards the centre. Depending on the actual casting conditions, these columnar grains may grow right to the centre and meet the grains growing in from the other side.

PHOTO: Section through a small metal casting showing the long columnar grains.

Section through a small metal casting showing the long columnar grains.
Figure 2. Magnification x 5.

As casting produces such a characteristic grain shape it is a simple matter to polish and etch a section of metal to determine whether it was cast. An example of typical grains produced by casting is shown in Figure 2. In some castings regions of small round grains may be found in the centre, but this is only an indication of changes in the cooling conditions and there will still be ample evidence in the other areas to indicate whether the metal was cast into its present shape. In the event of the final shape being obtained by forging at high temperatures, the columnar grains are absent and in their place are numerous equisized equiaxed crystals. So in metals like copper and iron it is a relatively easy task to differentiate between a cast or wrought artefact, merely by polishing and etching a small area of the surface and looking at it under a low power microscope. Coins will often show typical columnar crystals, though the owner may need to be convinced of the utility of a metallurgical examination, and the rounded grains present in the swords found in the Anglo Saxon graves on Polhill showed that they were hot forged into shape.

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