digital anastylosis

From virtual reconstruction to physical copy

 

The philological reconstruction of a work that has been destroyed by natural events or war is a complex operation. Although we live in a word where images (which for the most part are now digital) and three-dimensional models (from satellite maps) depict every place on the planet, monuments often lack documentation that is thorough enough to allow for their faithful reconstruction down to the last detail; this is especially true for at-risk areas, even those that are UNESCO World Heritage Sites. For example, there are over two million online images of the Coliseum in Rome, taken from every angle, in addition to drawings and relief models from every era, and no less than 10 three-dimensional architectural models created by universities, foundations, or private individuals. Palmyra, which has been a World Heritage Site since 1980, has been known since the 19th century, when archaeological digs also took place, and documentation of it is thus rather obsolete. In particular, there are only a few relief drawings of the Temple of Bel, published in the books “The ruins of Palmyra, otherwise Tedmor, in the desert” by Robert Wood (1753) and “Le temple de Bêl à Palmyre” by Henri Seyrig, Robert Amy and Ernest Will (1975). These are the only two reference works featuring drawings and engravings that describe the temple in detail; in addition there are the historic photographs taken between 1867 and 1876 by the French photographer Félix Bonfils. The digital reconstruction of the ceiling on the northern Thalamos was based on these sources, which document the state of the monument upon its discovery, and on the photographs taken by G. Degeorge in 1999, the only high-resolution photos of the site. (figures 1-2)
It took two technicians over a month to reconstruct the digital model. They had to carry out a careful iconographic analysis of the material they received from the archaeologists, make it philologically coherent, then put it together with enough detail to build the life-size replica. The relevant part of the ceiling of the northern Thalamos - which is 4 meters wide, 1.5 meters deep, and with a maximum height of 1.6 meters – is decorated by a rich array of sculptures, including numerous Greco-Roman capitals, kyma lesbio, acanthus leaves, human sculptures and sculptures of mythological animals, some headless, within a broader Greco-Roman architectural structure with decorated architraves.

 

Creation of the architraves

The methodology used to carry out the project called for a sequence of procedures (analysis – modelling – construction) to ensure that the replica would be built properly.
The first step is an architectural analysis of the monument itself, through a historical and artistic analysis of its decorative and architectural elements, so as to identify their main characteristics and the style in which they were built. (figure 3)

This is followed by an iconographic analysis of all available images – photographs and historical drawings and engravings – to create a database of the elements necessary to achieve as lifelike as possible a virtual replica. This entails re-modelling the parts that are visible in photographs, and using available data and analyses to re-create the parts that are not.

This follows a methodology that has been successfully tested over the years in similar digital and physical reconstruction projects. First of all, paper material is digitized, then the images are vectorised by turning them from raster graphics to two-dimensional vector CADs. The data are then cross-checked, and a two-dimensional file is created that takes into account all deformation and possible interpretations of the drawings and photographs. This makes it possible to obtain coherent maps, cross-sections, and perspective drawings that can be used to draft the 3D model. (figure 4)

Starting from file CAD 2D, which comprise mainly lines, curves, and two-dimensional polygons, another software is used to draft the model, with the lines, curves and polygons being modified by specific modelling functions to create a three-dimensional model. In order to maintain original proportions, a specific order is followed from the architecture to decorative details, and the model is fine-tuned for subsequent modifications, beginning with the overall architecture and ending with the tiniest details. (figure 5) In the case of the Temple of Bel we also made 3D prints of the decorative elements at the various modelling stages, from the first rough draft to the finished piece, in order to compare the various components and understand them better. Subsequently, the changes to be adopted in the final model are marked on the 3D prints themselves. This makes it possible to follow the evolution of the process step by step, including by constantly revising the draft versions with the help of the archaeologists. (figure 6)

Such an approach is necessary because the final product is a life-size 3D model – not a render image, but a tangible object. Additionally, the impossibility to compare the model directly with the original, which has been destroyed, required a new approach to modelling that was as objective as possible despite the initial constraints.

The virtual model looks like a newly-finished sculpture, as it shows no signs or wear or damage, whether natural or man-made. The only exception concerns the figures that decorate the dome, which were severely damaged, and would have made for an unrealistic reconstruction. This choice was made to ensure that the replica would match the original, including the correct architectural and decorative ratios. The replica was then aged manually by a technician who analysed the photographs and the wear patterns of the stones used to build the temple. By removing some of the materials and adding tints (some parts are blackened by soot and ash), he gave the replica its appearance in 2015, before it was destroyed in August. As we firmly believe in the importance of a 3D database for a monument of such great value, we made a 3D scan of the finished model after the end of all operations and final revision by experts. This meant that all of the project’s work - research, digital reconstruction, and manual reconstruction - came together in a single model to be preserved for future research and other works. (figure 7)

 

Creation of the physical copy

Starting with the virtual model – complete down to its decorations – a life-size physical replica was created. As this replica is quite large, the model had to be broken down into about 250 sub-section to be assembled. Additionally, since the replica needs to be transported and installed in the exhibition venue, it was made using lightweight materials (polyester and polyurethane) coated with a water-and wear-resistant resin.
In order to optimize the process, we decided to apply a number of different digital construction techniques, which exploited the model’s data through CNC machines to craft the various components.

> Hot wire cutting machines _ The general architectural model was made using a hot wire cutting machine that automatically shapes the parts exported from the CAD drawings: starting with a large block of polyester, extruded polystyrene, or similar material, the various elements were cut using a system of pulleys and other mechanisms that allow a hot metal wire to run through the material. This system was used to create 140 different elements, with the smallest having sides of 10 cm, and the largest 1.4 meters. (figure 8)

> Robotic arm for milling _ The part of the dome decorated with many sculptures was made of 5 parts that were milled using a robotic arm with six degrees of freedom. Starting with the digital model and using the processing software, the machine is programmed and the milling cutters work the way a sculpture would. (figure 9)
The robotic arm makes a first rough cut from the block of polyester, then by automatically changing the cutters, it carves it in more detail. (figure 10)

> 3D printing system _ The monument is decorated with a series of repetitive (figure 11), modular sculptures as follows: 11 modular decorations divided into 23 capitals, 25 cinquefoils and 30 six-lobed rosettes of various sizes; 12 modules of acanthus friezes with bull’s heads topped with a three-lobed kyma lesbio decoration and one frieze comprising 32 modules of acanthus decorations under a ribbon of biconvex whorls with oval pearls; these biconvex whorls are also present in a ribbon separating the capitals from the flat decorations.

Due to the high number of serial elements to be reproduced, and in order to optimize production times, we decided to use rapid prototyping techniques to create the ceiling decorations separately, then apply them to the base. Having obtained the modules from the overall model, we applied two different 3D printing techniques.

Complex, decoration-rich elements were made using a professional chalk-powder 3D printer, which can deliver a high degree of precision (0.01 mm for each layer)1, while simpler elements were made with a thermoplastic FDM printer whose degree of precision is sufficient for simple geometric shapes (0.06 mm for each layer). (figure 12) Once the modular components were printed in 3D, we used a silicone mould to make all necessary copies, which are faithful down to the last detail but made of a lighter material. (figure 13)

 

Assembly and finish

Having verified that all the components were present and having completed the dry assembly of the model, we used certified epoxy resins to add all of the parts that were printed and milled using CNC. We thus obtained a complete copy of the ceiling, whose morphology matches that of the temple’s ceiling before natural and human-generated wear. In order to make our replica match the appearance of the original right before it was destroyed, we added wear and tear both manually and mechanically to the decorations and some of the architectural parts, using the images we were provided as a model, and in constant contact with the experts from our scientific committee.

Once this phase was finished, we consolidated the decorative elements using a thin layer of synthetic chalk enhanced with oxides and sandstone powder (much like the original) to make the entire replica monolithic and give it a uniform tone. The back and sides were strengthened with a layer of fiberglass. (figure 14)

The final tint of each component was picked from a series of samples by the scientific committee in order to best match the effects of wear from time, wind, temperature changes, and human activities, such as the destruction of parts of the ceiling and soot from bivouacs inside the temple. (figure 15)

After balancing the colours of each component and completing all the various tints using suitable liquid and powdered acrylic and oxide-based pigments, the entire replica was coated with a transparent opaque acrylic protective layer. (figure 16)

The project highlighted the problems associated with safeguarding the memory of historic sites when they are struck by tragic events. The implementation of data collection and archiving projects, including through new technologies, would make it easier to study and share this issues, for both restoration and promotion of historic sites. More specifically, such an approach would limit the subjective interpretation of scholars in cases of replicas and reproduction – which bias is inevitable when consulting photographs and historical data – and ensure certified precision.

The replica described here is the outcome of a major collaborative effort between professionals from various sectors. It would have been impossible without the desire to raise awareness on these issues, and saw the involvement of the project promoters, its scientific committee, and the entire staff of TryeCo 2.0 srl, Alex P.O.P, and Andrea Fantini Studio. Along with us these companies actively contributed to the project’s success.

 

 

[by Matteo Fabbri e Roberto Meschini]

 

NOTE 1. 3D printers make three-dimensional objects through the overlap of two-dimensional layers, the thickness of which determines the precision of the final model.