Take a walk to the end of Room 73 at the British Museum and you will meet some of my favourite ladies. Made of terracotta and dressed in vivid shades of pink and blue, these Hellenistic figurines were once made all across the Mediterranean as offerings to be placed in tombs.
They are painted using some of the most precious and vibrant pigments available in the ancient world, such as Egyptian blue, cinnabar red and bright pinks and purples known as organic lake pigments. Over the more than 2,000 years since these figurines were made, many of these colours have faded or peeled away but their flowery coloured garments have hidden properties – under certain lighting conditions they ‘glow.’
This is because these pink and blue pigments are luminescent, which means that they are able to absorb light of a particular colour and then re-emit it at longer wavelengths. For pink organic lake pigments, particularly those made from the root of the madder plant, such as those in the ancient paint pot below, ultraviolet or visible light is absorbed and pink/orange light is produced.
For Egyptian blue (a bright blue inorganic pigment), red light is mostly absorbed and infrared light is produced. Infrared light has no colour, but using a specially modified camera we can record the light emitted as photographic images, which appears bright white. As these properties are specific to these two pigments, we can use this to identify them on objects, even when very small particles remain.
Usually these images are taken using two different experimental set-ups, but now thanks to new research carried out in the Museum’s Department of Scientific Research, together with colleagues in Greece, we can use the same set-up to record both of these images. The new technique uses different colours of visible light, blue and red from LEDs in turn, to make the pigments luminesce. It is much safer for both objects and users.
The images can also show us where each pigment is present much more precisely than our eyes can make out. So by looking closely at both images we can see that although areas of the lady’s dress appear pink (and luminesce pink/orange), the same area also shows some blue particles that are faintly luminescing white.
By allowing us to see that a mixture of blue and pink pigments were used to make the purple colour of the lady’s dress, the new set-up also allows us to begin to understand more about the ancient painting techniques used to create these beautiful objects.
You can read the full paper on the new research here.