Rapid prototyping helps recreate ancient colours

Multicoloured Gods breaks new ground as the first large-scale effort to recreate the original appearance of ancient sculpture.

It was long believed that the statues and reliefs of Greek and Roman antiquity were left unpainted, unlike contemporary works from other advanced civilisations like the Egyptians.

Archaeologists have known for some time that classical marble carvings would have been painted originally, although it is rare for any of the polychromy to have survived to the present day.

In most cases the colouring has been completely worn away over the centuries.

To illustrate this fact, it was decided to show the world how vibrant these works of art would have looked in their original form.

This involved creating reproduction pieces for a special travelling exhibition organised by the Glyptothek museum in Munich, Germany.

The project was led by archaeologist Professor Vinzenz Brinkmann.

It represents more than two decades of research on the polychromy of ancient sculpture, undertaken by museums around the world in collaboration with scholars from different countries.

Paint fragments were analysed using modern techniques such as infra-red spectroscopy and recreated with authentic pigments by Professor Brinkmann and his team.

With more than 20 full-size coloured reconstructions of important Greek and Roman works, Multicoloured Gods breaks new ground as the first large-scale effort to recreate the original appearance of ancient sculpture.

Starting off in Munich, the exhibition has toured major European cities and is now in the USA.

One of the exhibition's centrepieces is a section of the Alexander Sarcophagus attributed to the 4th Century BC Lydian King Abdalonymos.

An ally of Alexander, Abdalonymos had the marble sarcophagus adorned with bas-relief depictions of scenes from the life of his hero battling against the Persians.

This relic, discovered in Lebanon and now housed in the museum of Istanbul, was one of those rare finds containing fragments of original colours, which were painstakingly analysed and reproduced.

It is believed that the Alexander Sarcophagus, was painted by Nicias, a renowned artist of the period who showed Alexander in a vivid red tunic, magenta cape and golden lion-skin headdress.

The first challenge with the sarcophagus was to make a replica section that was precise in every detail.

The second challenge was to use a material with hardness and surface qualities similar to marble.

A silicone mould could not be used to make an impression due to the danger that the precious paint fragments would be removed by the mould.

It was therefore decide to use a scanning technology to generate a three dimensional data set.

This was then used to build a more or less consistent 3D file which could be used for producing the replica by stereolithography (SL), a process that uses photopolymer liquid resins which solidify when exposed to UV laser light.

A software program transfers the designer's 3D CAD model, or in this case a laser scanned file, into an electronic file for SL machines, composing the information into thin cross sections or layers.

A laser beam then traces each layer onto the surface of a vat of photopolymer resin, building the part in repeated layers until a solid replica of the original is completed.

Alphaform AG, a German based specialist service bureau was approached by Professor Brinkmann's team to reproduce the part.

Alphaform had previously created pieces of art for well known artists like Andrew Barov and the Bayrische See und Schlosserverwaltung, an institution responsible for preserving pieces of art and ancient buildings in the south of Germany.

As Alphaform Director Ralf Deuke said: "These kind of projects are far removed from our usual rapid prototyping work, for example for automotive and Formula 1 where we receive well designed files with good surfaces".

"The project generated a number of specific challenges: The scan contained a lot of defects due to a combination of the protective glass cover and the space limitations around the original piece in the Istanbul museum".

"Another big challenge was that the file generated thousands of supports".

"Some structural supports are necessary while the part is being manufactured using the SL process but not thousands".

"We therefore had to use our experience and know-how to find a good compromise and produce a file that the machines were able to handle and which showed minimal defects".

To faithfully reproduce the fine detail of the piece also required an SL material with hardness and surface qualities similar to marble.

Although Alphaform also use laser sintering techniques [SLS] they decided to use SL because of its superior surface finish and detail resolution.

SL photopolymer NanoTool from DSM Somos, a high modulus material designed for engineering applications in the automotive industry, as well as for rapid tooling.

NanoTool is filled with noncrystalline nanoparticles, allowing for faster processing.

Being a virtually zero-shrinkage polymer, build lines don't detract from the smooth finish.

"We have a lot of experience with NanoTool for the rapid prototyping of F1 aero sections and other parts that need high surface quality" Deuke said.

"It provides extremely fine detail resolution compared to other SL materials".

"Professor Brinkmann evaluated the material and found it easy to finish and paint - far superior to the plaster normally used to create replicas"".

"After first creating a small section less than half a metre wide, we move on to replicating a full three metre side of the sarcophagus".

"The complete piece was built in three sections which were then seamlessly fitted together".

"Without rapid prototyping it would have been impossible to create this part".

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