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Physicists shed light on rarely seen 16th-century metal-working technique

20 November 2017

Imperial researchers have tested a 'blued' gauntlet from a 16th-century suit of armour with a method usually used to study solar panels.

Metalworkers have used various techniques to prevent steel from rusting, some of which turn the metal black-blue. This ‘blueing’ effect can be created in several different ways, including by applying heat or (in later years) chemicals.  However, it can be difficult to know which method was used in older metalwork like armour or weapons, because the blueing is oxidised over time, and because many armourers kept their methods protected as trade secrets.

Previously, they had teamed up with Dr Alex Mellor, from the Department of Physics at Imperial College London, to investigate the blueing of a sword blade from 1803, so they knew the method might work. But the gauntlet was a much trickier subject, thanks to its complex shape and the very small area of the surviving blued surface.

When conservators from The Wallace Collection discovered some blueing preserved beneath the overlapping finger plates of a 16th Century gauntlet, they decided to investigate.  The method used is called Spectroscopic Ellipsometry, and involves studying the reflections of light from the surface of a material. A monochromatic light beam is bounced off the material, and then analysed to see what changes occurred to the beam.

As Dr Mellor explained: “When the linearly polarised monochromatic light beam interacts with a very thin film of material, the direction of the vibration of the light may be altered. This same effect is produced in rainbows in soap bubbles, when the light interacts with very thin films of water.

“Usually, we use Spectroscopic Ellipsometry in our lab to look at the effect of different films applied to the surface of solar panels. If a film can help panels reflect fewer wavelengths of light, then more light, and more energy, can be collected.

“In this case, we were looking for the effect on the light that the thin film of blue produces. By comparing the signature from this experiment to those where the production method is known – such as 19th century guns, or sample strips of metal – we can determine how the gauntlet was blued.”  Read more

The J A Woollam VASE Spectrosopic Ellipsometer used in this research was supplied by Quantum Design UK and Ireland.

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