Colourful Marble Statues in Istanbul

The touring exhibition on the use of colour in Greek and Roman sculpture is currently showing at the Istanbul Archaeological Museum. It was shown in Copenhagen in 2004 under the name ‘ClassiColor‘, and has also been shown in Germany and the Vatican. It is always interesting though to be reminded of just colourful the ‘sculptural landscapes’ of the Mediterranean would have appeared. Plus in Istanbul you could get the added bonus of the famed Alexander Sarcophagus from Sidon which preserves parts of its original colours. The exhibtion is displayed until 16 July.

Colourful interpretations of the Peplos Kore alongside an Archaic kouros, Istanbul Archaeological Museum. Photo: TMK, May 2006.

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  2. copying Caligula for photos

    Detail of left eye, showing traces of original polychromy

    Seen here is a marble head of the Roman Emperor Caligula, probably carved between 39 and 41 AD. Originally such sculptures were painted (polychrome) and the piece has traces of the original polychromy remaining. The painted areas are on the neck below the left ear, around the left eye, in the mouth, on the hairline and in the sideburns. Roman marble sculptures retaining their original polychromy are exceedingly rare.

    The object now belongs to the Ny Carlsberg Glyptotek in Copenhagen, Denmark. The curators and conservators in Denmark wished to study the pigments used on the marble head of Caligula to determine their exact composition and then reconstruct a possible colour scheme on a replica object. Their intention was to display the original and a painted replica side by side.

    The problem was that traditional replication techniques, such as taking a mould from the original and then producing a plaster or synthetic marble replica, were not suitable in this case. The process of taking a mould would have damaged the delicate and important painted surface of the original.

    To overcome these problems, it was decided to use non-contact 3D laser scanning to create an accurate 3D computer model of the original. This was done without having to touch the original at any stage in the process. The data captured during the laser scanning was then used to produce a replica marble head using highly accurate machining.

    The head of Caligula was brought from Copenhagen to the National Conservation Centre in Liverpool. It was scanned using a hand held Modelmaker 3D laser scanner mounted on a 6 axis precision Faro arm. The scanner head emits a thin strip of low-power laser light (about the same strength as a laser pointer) onto the surface of the object. The reflected light from the surface was recorded by an off axis digital camera, also located in the scanner head. The data, which represents the 3D surface, was collected as a set of points. This data set, known as a point cloud, consists of many millions of points, is highly accurate (∼0.1-0.2mm) and is non-light subjective.

    Detail of the point cloud of the scanned head.

    The 3D image can be then be rendered with a smooth surface.

    The scanning of the marble head of Caligula took three and a half hours to complete. Once the entire surface of the object had been scanned, the point cloud was transformed into a wire-frame mesh, consisting of triangles manipulated by the computer to produce a variety of rendered surfaces. This preparation step, called “post-processing”, took about a week to complete. Post-processing ensures that the data is an accurate representation of the original surface, and that there are no “mathematically abnormal” triangles in the mesh. Such errors in the data would cause problems in the machining stage.

    The marble being carved by the CNC machine.

    Once post-processing was complete, the file was exported in a machine-readable format and burnt to CD. The final file size for the Caligula head data was around 70 MB. The CD was then used to drive a five-axis computer numerical controlled machine (CNC machine), controlling the cutting path of a drill as it moved across the surface of a new block of Carrara marble. In this way, a replica was produced in marble without the original object being touched at any stage during the process. The machining of the replica of Caligula took six days. A 10mm drill bit was used for “roughing out” and a 6mm drill bit was used for the fine detail.

    When machining was complete, the replica came back to the National Conservation Centre for the final production stage. This final stage is called hand finishing, and was undertaken by a skilled sculpture conservator. Hand-finishing in the case of Caligula included sharpening details such as deepening the mouth and ears. This was achieved using a variety of tools. A point chisel and a flat bladed chisel were used to sharpen facets in the hair, a drill was used to deepen the mouth, and a dremmel (small drill) was used to deepen the ears. Finally, a fine abrasive paper was used to remove tool markings from machining from the nose and face. To help the sculpture conservator during this process, a thin watercolour wash was applied to the surface of the replica, as it is difficult to see the details on a new “clean white” marble statue clearly. The hand finishing took 12 hours to complete.

    The copy (left) and original (right) sculptures.

    The pigments preserved on the head were analysed at the Doerner Institute, Munich, and the replica head painted in the light of their findings. It was then displayed at the Staatliche Antikensammlungen und Glyptothek in Munich as part of an exhibition entitled ‘Bunte Götter’ (‘Painted Gods’) and at the Glyptotek in Copenhagen as a part of an exhibition entitled ‘ClassiColor’ (‘Colour in Antique Sculpture’). The final reconstruction is shown here.

    watch a video
    Watch a video of the main stages of the creative process (Approximately 1 min 30 sec),

    The copied head painted as it may have looked originally. Image copyright of Ny Carlsberg Glypotek.

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    The video clip shows the main stages in the creation of the replica. First, we see the head, with detail of the polychromy. Then the scanning of the object (note the red laser light moving down the surface of the marble). The point cloud gathered by the scanning process is shown in purple, whilst the wire-mesh is shown in white. Then we see the CNC machining of the head from a new block of marble, and then to a comparison of the copy and the original, following hand finishing, the copy standing on a wooden base.


    Sent by Joe Geranio

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