Caligula

Summary

On the portait of the Roman emperor Caligula well-preserved paint enhances the striking appearance. Mixtures of different pigments have been used to achieve refined tonal and realistic effects using carbon-based black together with Egyptian blue and red and yellow orchre on hair, black on eyebrows and eyelashes, red ochre on the skin mixed with Egyptian blue and the use of madder lake between the lips and in the rims and inner corners of the eyes. A cross-section taken from the skin only shows one layer of a red ochre directly on top of the marble. The VIL-imaging revealed a strong luminescence of Egyptian blue in the right eye and on the surface of the face and hair where it appears as scattered particles as part of a mixture of different colours used to obtain the right shade. On the UV-FL-image a strong pink fluorescense is observed on the lower rims and inner corners of the eyes. The same colour is seen between the lips due to the use of madder lake. The GC-MS analysis used to detect a probable binding medium revealed the presence of egg as the most likely candidate, which indicates the use of a polychrome egg-tempera treatment for the painting of the portrait bust.

Description of object

The head was originally set into a statue and has been broken off at the neck. The back part of the head is missing.

Choice of methods

Visual examination

• Macroscopic
• Microscopic in situ

Technical imaging

• UV
• VIL

Sampling

• Cross section
• Microscopy
• SEM/EDX
• XRF
• GC-MS

Visual examination

Left eye, eyebrows and eyelashes are painted with black in short strokes with a very fine brush: this is also the case with the right pupil and traces of a few eyelashes above the right eye. Two small grains of blue are observed in the area of the right iris. On the hair the use of red, yellow and black is visually apparent, most significantly next to the left ear, where delicate black brushstrokes highlight the carving of the hair of the sideburns, clearly visible on the left side of the head. A pale pink is observed on the rims and inner corners of the eyes. The same colour is seen between the lips. Areas with a red/pinkish colour are visible on the skin especially on the left side of the neck where a large concentration is present. Minute grains of blue are also found on the skin and hair.

Technical imaging

UV-FL: A pink fluorescense in the inner corners of the eyes and lower eye rims. The same fluorescense is also observed between the lips suggesting the use of an organic lake – probably madder lake. A strong yellowish fluorescense is observed in serveral places on the hair, the exposed skin of the face, the mouth, the left ear and the neck: probably due to a preparatory treatment of the surface using linseed oil.

VIL: On the hair, a large number of particles shining bright white are observed. Small particles also occur on the the skin and between the lips. A large concentration is observed in the right eye. It is still uncertain whether the luminesence in this area is due to Egyptian blue. The characteristic glow of the particles on the rest of the portrait confirms the use of Egyptian blue.

Other types of investigation

Sampling: Three samples were taken for binding medium and pigment analysis. These were taken in the following order; from the pink paint layer in the inner part of the lips, the black hair contours and from the reddish skin at the neck. One other sample was taken from the left side of the neck, embedded in polyester resin and prepared for paint cross-section.

Cross-section Sample 4 shows a one-layer structure with a red/orange layer applied directly on top of the marble.

SEM/EDX Sample 1: The pink colour from between the lips confirmed the presence of an organic, red lacquer fixed on an aluminum- and silicon-rich substrate. Elements detected were O, Ca, P, Si, S, Fe, Al, Cl, K, Mg, Pb. Sample 2: The black pigment was identified as a finely-ground bone black. Elements detected were O, Ca, P, Si, S, Fe, Al, Cl, K, Ba, Na, Mg. Sample 3: The red skin colouring contains red ochre. Elements detected were O, Ca, P, Si, S, Fe, Al, Cl, K, Na, Mg, Cu, Mn.

GC-MS The sample was prepared for binding medium analysis by treating the sample twice with a mixture of chloroform and methanol, followed by a second extraction with methanol and oxalic acid. Animal glue was not detected in the sample. Due to the relatively high content of a specific amino acid, serine, as well as the presence of other typical decay by-products, it could be concluded that egg is the binding medium. Whether the yolk of egg, the egg white or the entire egg was used cannot be determined.

Bibliography

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