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Digital fabrication of cultural heritage artwork replicas. In the search for resilience and socio-cultural commitment
Digital Applications in Archaeology and Cultural Heritage 15 (2019) e00125
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Digital Applications in Archaeology and Cultural Heritage journal homepage: www.elsevier.com/locate/daach
Digital fabrication of cultural heritage artwork replicas. In the search for resilience and socio-cultural commitment M.J. Merchan a, *, P. Merchan a, S. Salamanca a, E. Perez a, T. Nogales b a b
Escuela de Ingenierías Industriales. Universidad de Extremadura, Avda. de Elvas s/n, Badajoz, Spain Museo Nacional de Arte Romano, C/ Jose Ramon Melida, s/n, Merida, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: 3D modelling Digital fabrication Cultural heritage Interpretive centre Education Social cohesion
The traditional approach for physical reproduction in Cultural Heritage requires the production of moulds on the very surface of the original artwork in an invasive, manual and usually time-consuming process that is intensely influenced by the complexity of the piece to be reproduced. The difficulties increase when changing the size of the copy is needed. But producing replicas is a necessity, mostly when it comes to mounting exhibitions within interpretive centres. The educational role of such institutions is beyond doubt. Furthermore, they have also become major drivers of social inclusion of at-risk groups. So, having perfect copies of archaeological remains without damaging them is a challenge that technology has been facing for the last years. This paper describes a procedure to obtain exact replicas of classical statues that reduces their manipulation, thus preventing them for further damages, which was applied to two Roman marble sculptures of Medellín (Badajoz, Spain).
1. Introduction The use of 3D technologies for digitizing cultural heritage has been developing rapidly and constantly in the last years, thanks in part to computer vision advances and to the improvement of technological devices. In this respect, the acquisition and storage of sites, monuments and works of art data can be carried out in a more effective and efficient way. This fact has provoked the arisen of repositories of 3D models with research purposes as well as virtual museums that have widened the applicability of these models. The availability of an accurate digital representation opens up a broad spectrum of possibilities for experts(Katz and Tokovinine, 2017). Therefore, there have been a lot of works in this field lately, either using photogrammetry techniques or active 3D scanning (3D laser scanning, structured light 3D scanner, phase shift and time of flight laser scanners …). One can distinguish between those works dedicated to scanning open spaces and monuments (e.g. Arav et al., 2016; Sapirstein, 2016; Xu et al., 2017; Zhang et al., 2017; Balsa-Barreiro and Fritsch, 2018; Perez et al., 2019); and those which are focused on a smaller-scaled research: artefacts (Banterle et al., 2017; Graciano et al., 2017; Banducci et al., 2018; Zhang et al., 2018), epigraphy (for instance, Ramírez et al., 2017; Di Paola and Inzerillo, 2018; Carrero-Pazos and Espinosa-Espinosa, 2018;
Andreu and Serrano, 2019), sculpture (Adan et al., 2012; Zhang et al., 2015; Malik and Guidi, 2018; Perez et al., 2018), etc. 3D models in cultural heritage can be used not only for documentation and visualization purposes but also as tools for carrying out several tasks. Among them, it is worth noting the manufacture of original artworks copies (Scopigno et al., 2011; Katz and Tokovinine, 2017). Digital manufacturing, defined as the physical reproduction of objects from their three-dimensional models, combines conventional manufacturing techniques with digital technology (Ribeiro da Silva et al., 2019). It has developed rapidly in recent years, as techniques, technology and materials have evolved (Bogue, 2013), and its scope of application has not ceased to grow. Thus, its use has been extended to the most diverse areas of daily life. Cultural heritage is one of the fields in which digital manufacturing is gaining more prestige. Within this branch, its uses are multiple and very diverse (for example, the reproduction of sculptures for sensorial exhibitions, manufacturing pieces to facilitate the labour of restorers, or a more general use: allowing the resilience of the patrimony assets throughout the years) (Scopigno et al., 2017). The digital fabrication techniques can be classified into two groups: on the one hand, the subtractive techniques that consist of carving a block by removing material to sculpt the desired piece, which is made by using a milling tool controlled by a computer. On the other hand, you can
* Corresponding author. E-mail addresses: [email protected] (M.J. Merchan), [email protected] (P. Merchan), [email protected] (S. Salamanca), [email protected] (E. Perez), [email protected] (T. Nogales). https://doi.org/10.1016/j.daach.2019.e00125 Received 11 June 2019; Received in revised form 30 September 2019; Accepted 2 October 2019 2212-0548/© 2019 Elsevier Ltd. All rights reserved.
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Digital Applications in Archaeology and Cultural Heritage 15 (2019) e00125
find the additive processes. As their name suggests, they consist of the deposition/addition of successive layers of material to “build” the object. These are the most spread techniques for 3D printing in the last years since they are more versatile, easy-to-use and less expensive than the others. A recent review of this type of manufacturing procedures (materials, methods, applications and challenges) can be found in Ngo et al. (2018). The decision to opt for one type of technique or another will depend on the type and size of the piece to be replicated, the accuracy required, the material to be used, the subsequent use of the copy, etc. Up to the apparition of 3D digitization techniques (and even nowadays), it was usual to make a mould on the very surface of the sculpture when replicating it, what implies the use of products that usually affected the epidermis of the material. It is needless to say that this technique, which can go back to the Roman time (Rigdway, 1984; Gazda, 1995), also exposes the archaeological remains to unacceptable physical risks. However, having perfect replicas of artworks has been always necessary. In previous epochs, they facilitated that artists, students and common people approached to the masterpieces of the ancient world. Nowadays, they become an essential resource for culture and education. Replicating archaeological objects with a higher standard of perfection is especially important in the case of interpretive centres. An interpretive centre is a hybrid concept between a museum and a visitors’ centre: they are thought to welcome, look after and guide visitors while introducing them in the knowledge of heritage through a museographic discourse with a pedagogical purpose (Martín Pi~ nol, 2013). Its aim is not to acquire, study or keep heritage pieces and goods, but to show how to appreciate and take care of them in an educational manner. The absence of original pieces, which must be necessarily preserved within a museum, is palliated by the exhibition of replicas to build a narrative and iconographic discourse that must be understandable and attractive at the same time (Izquierdo et al., 2005). Digital manufacturing becomes the ideal resource for generating the replicas needed in interpretative centres, since it provides with an efficient, accurate and quick working way that is also respectful with the original pieces as it does not require any contact. Besides, the reproduction costs have steadily decreased, favoured by the constant development of devices and technology. Two very interesting surveys on this subject are (Scopigno et al., 2011) and (Weigert et al., 2019). The aim of this paper is to emphasise the importance of producing perfect physical reproductions of statues to provide interpretive centres with the resources needed for their educational purpose. To do this, we will analyse the work done on two Roman sculptures made of marble that were found in Extremadura (Spain). Both statues were uncovered in the excavations held under the supervision of P. Mateos (Mateos and Picado, 2011), in the hyposcaenium of the Roman theatre of Medellín (Badajoz), the ancient colony Metellinum, in Lusitania (Hispania) in mid-2007. The rest of this paper is organised as follows. In Section 2, the archaeological study of the pieces that states the importance of the statues and the circumstances that led to the need to get high-quality replicas in a short period of time are exposed. Section 3 presents an overview of the procedure followed to obtain the 3D models. Section 4 offers a brief state of the art on digital fabrication technologies and shows how the physical reproductions were made in this case. In Section 5, the results obtained are shown and discussed. Section 6 states the conclusions.
her hand would be extended outwards, whereas the left one, attached to the side, is bent at the elbow to be projected forward. Both forearms would have been worked as separated pieces to be joined to the body. She wears a thin tunic or chiton buttoned over her shoulders, which forms abundant folds that appear in certain areas under a mantle wrapped around all her perimeter, drawing a balteus from the right side to the left shoulder. The rounded neck of the chiton defines the hole to insert a head, also worked in a separate piece of marble, just as the front part of both feet would have been. The rear side, less visible and for this reason less elaborated, has a large perforation over the gluteal area, probably to ensure her fixation since the weight of this statue (1000 kg approx.) and its narrow base make it quite unstable. The carving is exceptional therefore this statue becomes a top-level work of art due to the delicate modelling of its forms and the relief of the cloth, with almost transparent folds and a remarkable “sfumato” in the mantle. It probably represents one of the female members of the imperial family. The second statue is a smaller female figure (H: 120 cm; W: 45 cm; D: 28 cm), equally in a free-standing position (Fig. 1. Right). It was unearthed in two large fragments that were joined by the excavation team. This sculpture wears a tunic and a wrapping mantle, under which both hands appear. The right one is on the chest, grasping the cloth, and the left one is along the side, holding the mantle, a part of which disappears inside the fist. It preserves the conic-shape cavity at the base of the neck to fix the head, which was worked in a different piece of marble, as the front part of the left foot. Although it presents some erosions and losses in its surface, it is a piece of noteworthy quality that would possibly have represented a young private lady belonging to the elite of Metellinum, who was able to afford a statue of this type in such an outstanding place as the scaenae frons of the Theatre (Nogales and Merchan, 2018). Both of them seem to be works done in the first century of our Era. 2.2. Created necessities The work that led to the recovery and rehabilitation of the Roman theatre of Medellín favoured its reopening in 2013. This very year, it was awarded with one of the three Heritage Awards conferred by Europa Nostra thanks to the excellent work of preservation done. The opening of the site was accompanied by the remodelling of the adjacent church of Santiago as an interpretive centre, in which it was decided to show some of the most significant works of art in order to reinforce its pedagogical discourse. The quality and archaeological worth of our sculptures, just like the facilities of the church, advised against the idea of a new transfer to Medellín and their exhibition. So, it was proposed to show two exact replicas instead the originals. The contracting party established a set of requirements that conditioned the choice of methodology to be applied. The two main ones were: 1. the limitation of time, since the replicas had to be available less than a month after starting the work; and 2. the quality of the result, which was supposed to be superb, in order to continue with the self-imposed excellence criteria. The very statues forced a third condition: the danger of their manipulation. All that folds so masterfully worked are susceptible to break with any minimum contact. Likewise, the fact that the smaller statue was reconstructed from fragments that could be detached, together with the fragile parts of the largest one, such as the mantle under the arm that floats in the air, advised against excessive manipulation of the pieces. This third condition, although indirect, made them discard any type of invasive technique that could endanger the artworks from the beginning.
2. Materials and methods 2.1. Description of the statues
3. Generation of the 3D models
The largest piece is a freestanding sculpture bigger than life size (H: 167 cm; W: 64 cm; D: 36 cm) (Fig. 1. Left). Made in grey veined marble, it represents a woman who adopts an erected and frontal position, leaning on her left leg while the right, which wouldn’t support any weight, is marked under the tissue of her clothing. She elevates her right arm and
Our 3D digitization method followed the stages of the 3D modelling process (Bernardini and Rushmeier, 2002). Taking into account that in this case the stream for the texture is not needed, since the statues were 2
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Fig. 1. The two Roman statues to be reproduced.
partial views of each sculpture in a common reference system was implemented, as is usual, through geometrical transformations (rotation and translation) by using the kd-tree algorithm and point-to-point minimization (Rusinkiewicz and Levoy, 2001), a variation of the popular ICP registration technique. The merging stage was carried out by following the well-known mesh zippering algorithm (Turk and Levoy, 1994). The output of this second stage is a mesh fitted to the whole surface of the object. This single mesh is then subjected to a hole-filling algorithm in the third stage. We used the technique proposed in (Perez et al., 2008), an approach to fill any kind of holes in 3D meshes based on an image in-painting algorithm that uses the Fields of Experts (Roth and Black, 2009) framework. The last step is the analysis and correction of errors in the resulting surface. This step is mainly automatic, using the suitable tools of the specific software: detection of peaks, self-intersections, non-manifold elements, tunnels, etc …, but also includes a manual process where a visual inspection must detect small protrusions. These are produced by the accumulation of errors in all the stages (digitalization, registering and fusion). To solve it, a defeaturing tool that carries out elimination of the small protrusions and filling of the resulting holes are applied. The final geometrical models of the statues are shown in Fig. 3. The high-resolution mesh for the first statue (Fig. 3 above) has 3.5 million triangles whereas the mesh for the smaller statue has 2.2 million triangles.
going to be reproduced in a marble-like material, the conceptual map proposed in (Merch an et al., 2011) could be modified to the one shown in Fig. 2. As seen, the initial stage consists of data acquisition, either using photogrammetry techniques or 3D laser scanning, depending on the final purpose of the model. Following this, the well-known phases of registration, integration and post-processing of the overlapping scans are carried out to obtain the final geometrical mesh. Data acquisition planning is always a fundamental stage in 3D digitalization processes, but it is particularly important in the case of heritage pieces, since a loss of data due to poor planning might be hardly recovered. As commented, several 3D digitization technologies are available nowadays; the selection of the specific instrument is influenced by the characteristics of the 3D model required, in terms of accuracy and resolution, which depend on the intended use for the digital model. In the case of the two Roman statues under study, we decided to use laser scanning due the features of the sculptures themselves (roman clothes, reflection particularities of the marble, morphological complications, time constraints, etc.). We used a Konica Minolta Vivid 910 laser scanner on fine mode. In this configuration, its nominal accuracy is 0.10 mm and the precision 0.016 mm at a distance of 0.6 m. Pictures were also acquired through the system, but not processed into a final texture, as said before. The final geometrical mesh was created by following the stages specified in the conceptual map of Fig. 2. Internally developed (noncommercial) software was used in this process. The registration of the
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Fig. 2. Simplified conceptual map of the generation of a geometrical model from multiple overlapping scans.
4. Production of the replicas
Adapting 3D models to be used by digital manufacturing machines is often a relatively straightforward process. However, there is currently a very interesting line of work in which specific 3D mesh processing algorithms are being proposed to facilitate and improve the fabrication process. For example (Herholz et al., 2015), and (Alemanno et al., 2014) present a procedure to decompose a 3D model into pieces that can be manufactured either by traditional methods (casting or 3-axis CNC) or by 3D printers, respectively. With these procedures, the price of the replicas is reduced, compared to the use of 6-axis and 7-axis CNC, but at the cost of losing quality in reproduction, especially when the objects to be replicated are large. Both methods could also be applied in the case of extractive manufacturing techniques. Other methods propose making silicone moulds that allow the fabrication of objects with complex geometry, as in (Malomo et al., 2016) or in (Alderighi et al., 2018), where they obtain reusable moulds. Although these techniques are very promising, they are designed to be used with small objects that can be manufactured with a 3D printer, so they do not apply to the situation dealt here. In the case presented in this paper, due to the size of the pieces, the quality requirements, the intricacy of the folds, and the exhibition necessities, a 7-axis CNC machine robot arm was the option chosen to build intermediate replicas, which will be used to make moulds from which the final replicas are obtained. There are several reasons to accomplish an intermediate copy instead of manufacturing the final statue in stone directly. The first one is the price of the replica, which is minor using this methodology, because of the lower cost of the robot arm (since it is not necessary that it exerts the high torque required to sculpt stone) and the cheaper materials (marble or granite blocks versus high-density polyurethane and marmolina powder). The second factor refers to the weight of the copies obtained,
As said previously, the methods for digital manufacturing of replicas can be classified into two groups depending on the technique they utilized: subtractive or additive (Scopigno et al., 2017). Within the first group, Computer Numerical Control (CNC) machine tools employ a spindle to remove material of the original block to obtain the replica. When a perfect copy of complex sculptures is needed, as in the case proposed, 6-axis CNC or 6 DOF robots are the most common method (La Pensee et al., 2010; Hayes et al., 2015; Tucci and Bonora, 2007). But, even so, it is not always possible to have perfect replicas by this mean since there may be unreachable positions for the spindle. To solve this problem, solutions such as the addition of a 2 DOF platform, which moves the blank material, to the 6 DOF robot (Zhu et al., 2006) or, directly, using 7 DOF robots (Fernando, 2018) have been proposed. The extra degree of freedom is used to orient and position its end in the three-dimensional space and a much greater number of solutions can be obtained when calculating the inverse kinematics. This way, the milling tool can be placed in any position and orientation due to the new values of joint coordinates provided by this type of machine. Additive fabrication techniques consist of adding layers of some material that is solidified by some specific procedure. There are different manufacturing methods within the additive technology (Scopigno et al., 2017): fused deposition modelling (FDM), laminated object manufacturing (LOM), granular materials binding (GMB) or photopolymerization. The advantages of additive techniques over the extractive ones are their lower cost and ease of use. Nevertheless, not only the quality of replicas is inferior but the 3D printers do not allow to fabricate large parts currently, which makes additive manufacturing not suitable for the type of application proposed in this work.
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Fig. 3. Final geometrical models.
sculpture was used to produce and exact copy of each statue by sculpting a high-density polyurethane block. The robot arm works from minimum dimensions of about 500 mm up to 2500 mm in height in a single block. High-density polyurethane was used because of its ability to be trimmed and handled at will, but still maintaining its rigid form. Fig. 5 shows a
much lighter than if they were made in stone, which facilitate their transport. The procedure performed for the fabrication of 1:1 scale replicas is schematized in Fig. 4 and described below. First, a 7-axis Kuka robot arm adapted for milling and machining of
Fig. 4. Flowchart of the procedure used to obtain the replicas. 5
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Fig. 5. Sculpting of the intermediate replica with a robot arm.
moment of the sculpting process. After that, the intermediate replica generated with the robot arm was used to make a mould that will serve to obtain the final reproduction by a traditional casting process. Fig. 6 is a picture of the mould generated for one of the statues. In the end, the final replicas were obtained by pouring marble powder (marmolina) into this mould. This casting material makes it possible to create pieces that are very resistant, making them suitable for indoor and outdoor exposure. These statues are exact copies that have accuracy below 0.3 mm and a hyper realistic aspect. In Fig. 7 the final reproduction of one of the statues can been seen in the foreground. The intermediate replica sculpted by the robot arm appears in the background. Fig. 7. Final physical reproduction with its intermediate replica in the background.
5. Results and discussion 5.1. Results
3D model can be spoiled if the manufacturing process is inappropriate; in the same way, although we had the best machines, a poor digital model hinders getting good results. In our case, the fact of having produced such a perfect and tough mould, thanks to the last generation manufacturing robot arm, allowed making the copies in marmolina in just one piece, with a finish very close to the original one. The time needed to complete the whole process was about eight working days, distributed as shown in Fig. 8. In addition, the replicas are weather-resistant and durable. Another key point is the final weight of the replica. This is an important factor when dealing with manipulation and possible loaning to other institutions. Thus, while the original pieces needed a crane (the largest statue) or, at least, two operators (the smallest one) to be moved, copies can be transported by one person. Figs. 9 and 10 are two pictures of the final copies as they are currently exposed in the visitor interpretive centre (former church of Santiago) in Medellín (Spain). Concerning the second result, the fact that archaeologists and historians can have the virtual model appears as an opportunity for the exhaustive analysis of the piece. The traditional on-site autopsy has often been replaced by the study of the piece through photographs when the researcher cannot move to the place where it is. It goes without saying that these images had not only neither the size nor, in many cases, enough quality for the correct understanding of the object, since they always provide a two-dimensional vision of an artistic technique that arose, precisely, for conquering the space in its three dimensions. The advantages of these virtual models applied to archaeological research have been proved indispensable in recent years (Scopigno et al., 2011; Merchan et al., 2013).
There are two immediate results that can be highlighted in the accomplishment of this work: 1) the obtaining of the physical replicas themselves and 2) the digital models that are left, that can be used by the researchers for further study. In regard to the first result, it has been proven that digital fabrication is the most suitable option nowadays, mostly when the main challenge that has to be faced is the optimization of time and the quality of the results. In the process of digital manufacturing, it is equally important both the data acquisition and the fabrication method: one very accurate
Fig. 6. Moulds produced from the intermediate replica in high density polyurethane. 6
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Fig. 8. Time consumption in the digital fabrication of the two replicas.
Fig. 10. Picture of the replica of statue no.2 exposed in the visitor interpretive centre.
5.2. Analysis of replicas In order to demonstrate the effectiveness of the manufacturing method presented, the digital models of the physical replicas have been generated following the same procedure explained in section 3. Fig. 11 is made of the images of the 3D models of the original and the replica of both roman sculptures. Several measures have been taken to test the quality of the result. For each 3D point of the mesh of the original sculpture, the closest point of the 3D mesh of the replica has been searched and the distance between them computed. These distances form a vector, D, whose size is equal to the number of points of the 3D mesh of the original statue. Fig. 12 shows the histogram of that vector. Table 1 summarizes the mean, standard deviation and maximum and minimum values of D. As can be seen in Table 1, the errors are negligible, taking into account the sizes of the sculptures (167 cm tall for the first statue, and 120 cm tall for the second). Despite the above, it can be observed, both in the histograms and in the data shown in Table 1, that there are outliers in the distance vector. A second analysis has been carried out after eliminating these outliers. A distance value di 2 D is considered as an outlier if it verifies that: di > D þ 3 ⋅ σ D Fig. 9. Picture of the replica of statue no.1 exposed in the visitor interpretive centre.
(1)
where D is the mean of D and σ D is the standard deviation of D. The case 7
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Fig. 11. 3D models of the original (left) and replica (right) of the (a) first and (b) second statue.
Fig. 12. Histogram of the distances between the 3D points of the original statue and the replica. (a) Statue 1. (b) Statue 2.
The examination of outliers also allows carrying out a visual analysis of where the most significant errors occur, as shown in Fig. 13. In it, the areas with outliers have been marked in red. As can be seen, many of them correspond with folds, which happen to be the most difficult zones to scan and sometimes the application of filling-hole algorithms was necessary. There are also many outliers in the base of the 3D models because they were not acquired directly since the statues could not be lifted from the ground. In summary, the biggest errors between the meshes of the original statues and the replicas are due to the algorithms used in the creation of these models, and not to the manufacturing process used. Finally, in Fig. 14 vector D is represented onto the 3D models using a colour code. In this representation, the outliers have been removed. It can be observed that, besides the areas of more complex geometry, the sides of the statues show greater error than the rest of the model. This could be due to the joining of the two halves of the moulds used in the manufacture process. Anyway, it must be emphasized again that the maximum
Table 1 Mean, standard deviation and maximum and minimum values of.D
Statue 1 Statue 2
Mean distance (mm)
Standard deviation (mm)
Maximum distance (mm)
Minimum distance (mm)
3.53
3.55
40.94
0.0084
3.27
2.94
40.18
0.0092
in which the value of the distance is less than the mean of D minus three times the standard deviation has not been taken into account since, as seen in Table 1, there will be no data that verify it. Table 2 shows the number of outliers, the percentage related to the number of the 3D points of the model, the mean distance, the standard deviation and the maximum and minimum values when the outliers have been removed.
Table 2 Values when the outliers are remove from.D
Statue 1 Statue 2
Number of outliers
% of outliers
Mean distance (mm)
Standard deviation (mm)
Maximum distance (mm)
Minimum distance (mm)
25821 24444
2.29% 1.44%
3.19 3.04
2.72 2.15
14.18 12.10
0.0084 0.0092
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our history and culture known. Due to the possibility of replicating archaeological pieces, it is feasible to reconcile two basic principles: 1. that blind people need to have direct contact with objects to facilitate their exploration through touch, and 2. museums must take into account the conservation of materials. Here is where the digital fabrication supposes a huge qualitative leap with respect to other exhibitions carried out previously: it offers a quicker and cheaper non-invasive way of reproduction, as well as, since the techniques and materials to be used are so accurate, so wide and so varied, it allows replicating the least imperfection, the texture and even the common temperature of the material in which they were made originally by choosing the proper one in every case. It makes the sensory experience through touch very realistic. 7. Conclusions During the last years, several papers have hypothesized about the advantages and disadvantages of using one of the different technologies available for the acquisition of 3D information and its subsequent physical reproduction. But only through the experience provided by particular case studies it will be possible to come up with useful and reliable conclusions for researchers. In this paper, we have analysed a specific case: the physical reproduction of two Roman female sculptures using laser scanner for 3D model generation and digital manufacturing. This is a particularly significant example because of the morphological features that this type of statues presents, due to the protuberances and shadow areas that produce the folds of the dresses. With the aim of reducing statues manipulation and preventing for further damages, we propose a procedure that is an alternative to traditional casting. We have used laser scanning and computer vision techniques to generate the accurate 3D models needed. These models are utilized to produce intermediate replicas by means of a sculpting robot arm. After that, the final reproductions are obtained by pouring marble powder into the moulds generated with the intermediate replicas. Digital fabrication is very useful and versatile when needing to produce exact replicas of pieces belonging to the Cultural Heritage, especially those that are fragile, since it is a non-invasive and non-destructive tool. The subsequent use of the copies in interpretive/visitors’ centres is very important for the correct understanding of their exhibition discourse and for the deep knowledge of the History of the place where located. Even more, this type of centre, as well as museums, can offer new possibilities for those groups of people that have any kind of visual disabilities, providing new experiences that encourage their social inclusion. An additional advantage, not directly related to the purpose of this paper, is the obtaining of high-resolution 3D models of the sculptures, which can be used by researchers in their investigations. These 3D models offer much richer information that the traditional photographs used to date for this kind of archaeological pieces. Furthermore, they can be used to create VR experiences to approach Cultural Heritage to the people. These experiences have also a social component since they allow disfavoured groups, such as wheelchair users or people with mental impairment, among others, to know and enjoy the cultural heritage assets. It is a way to virtually and effectively demolish the barriers that, unfortunately, still exist for them. Undoubtedly, this is a way to preserve the memory of Antiquity that guarantees its resilience over the years, providing, in turn, the tools to make it accessible to all people, without distinction of any kind.
Fig. 13. Zones with outliers (marked in red) of the 3D models of the statues. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
error is less than 1.5 cm, which is a negligible value for sculptures of this size. 6. Discussion Possibly, the most relevant result that can be outlined through this case study is that both the 3D digital models and the physical replicas are perfect tools for education and social cohesion. On the one hand, in concern to the digital models, they are powerful resources for formal and informal learning. The use of 3D models of monuments, sculptures, artefacts …, either directly shown on a screen or through Virtual Reality recreations, allows building educational environments in an attempt of achieving students’ involvement in their own learning process in a more effective and fun way. The idea is to accomplish new educational strategies in which, through individual or collective activities with 3D technology support, they actively participate in the acquisition of knowledge, even without noticing they do (Merchan et al., 2018). On the other hand, the importance of having perfect copies with a pedagogical purpose has been explained when talking about the educational role of interpretive centres. However, its significance goes a step beyond since the replicas constitute an important element of social integration and cohesion. This is because Cultural Heritage is considered by EU as a perfect resource to achieve the real integration of those socio-cultural groups that are still not. Thus, thanks to the physical reproduction of the archaeological remains, visually disabled people, one of those groups that are nowadays not sufficiently integrated in society, can learn and enjoy our past by touch. The World Health Organization estimates that currently 217 million have moderate to severe vision impairment and 36 million people more are blind (https://www.who.int/news-room/fact-sheets/detail/blindn ess-and-visual-impairment). Besides the barriers they have to cope in their day-to-day, they discover new ones when facing Cultural Heritage. For instance, the access of these people to the archaeological collections of museums has been quite limited to date. It is a collective that is caught in the midst of the dilemma that any museum has: the logical need for the preservation of irreplaceable pieces and the diffusion of its funds to make
Funds This work has been supported by the project IB16162 from Junta de Extremadura and European Regional Development Fund (ERDF) “A way to make Europe”. 9
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Fig. 14. Colour representation of the distance of the 3D model of the original sculpture and the replica. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Declaration of competing interest
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The authors declare that they have no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References Ad an, A., Salamanca, S., Merchan, P., 2012. A hybrid human-computer approach for recovering incomplete cultural heritage pieces. Comput. Graph. 36 (1), 1–15. Alderighi, T., Malomo, L., Giorgi, D., Pietroni, N., Bickel, B., Cignoni, P., 2018. Metamolds: computational design of silicone molds. ACM Trans. Graph. 37, 1–13. Alemanno, G., Cignoni, P., Pietroni, N., Ponchio, F., Scopigno, R., 2014. Interlocking pieces for printing tangible Cultural Heritage replicas. EUROGRAPHICS Work. Graph. Cult. Herit. Andreu, J., Serrano, P., 2019. Contributions of the digital photogrammetry and 3D modelling of Roman inscriptions to the reading of damaged tituli: an example from the Hispania Tarraconensis (Castiliscar, Saragossa). Digit. Appli. Archaeol. Cult. Herit.. 12, 1–7. Arav, R., Fili, S., Avner, U., Nadel, D., 2016. Three-dimensional documentation of masseboth sites in the ‘Uvda Valleyarea,southern Negev, Israel. Digit. Appli. Archaeol. Cult. Herit. 3, 9–21. Balsa-Barreiro, J., Fritsch, D., 2018. Generation of visually aesthetic and detailed 3D models of historical cities by using laser scanning and digital photogrammetry. Digit. Appli. Archaeol. Cult. Herit. 8, 57–64. Banducci, L.M., Opitz, R., Mogetta, M., 2018. Measuring usewear on black gloss pottery from Rome through 3D surface analysis. Internet Archaeol. 50 https://doi.org/ 10.11141/ia.50.12. Banterle, F., Itkin, B., Dellepiane, M., Wolf, L., Callieri, M., Dershowitz, N., Scopigno, R., 2017. VASESKETCH: automatic 3D representation of pottery from paper catalog drawings. In: 14th IAPR International Conference on Document Analysis and Recognition (ICDAR), pp. 683–690. Bernardini, F., Rushmeier, H.E., 2002. The 3D model acquisition pipeline. Comput. Graph. Forum 21 (2), 149–172. Bogue, R., 2013. 3D printing: the dawn of a new era in manufacturing? Assemb. Autom. 33 (4), 307–311. Carrero-Pazos, M., Espinosa-Espinosa, D., 2018. Tailoring 3D modelling techniques for epigraphic texts restitution. Case studies in deteriorated roman inscriptions. Digit. Appli. Archaeol. Cult. Herit. 10, 1–12. Di Paola, F., Inzerillo, L., 2018. 3D reconstruction-reverse engineering-digital fabrication of the Egyptian palermo stone using by smartphone and light structured scanner. Int. Arch. Photogramm. Remote Sens. Spat. Inf. Sci. XLII-2, 311–318. Fernando, S., 2018. The culture of crafting: exploring the relationship between the hand and the machine in digital stone sculpture. In: Proc. 1st Annu. Des. Res. Conf. Sydney 2018. University of Sydney, pp. 509–516. Gazda, E.K., 1995. Roman sculpture and the Ethos of emulation: reconsidering repetition. In: Harvard Studies in Classical Philology. Department of the Classics, Harvard University, pp. 121–156. Graciano, A., Ortega, L., Segura, R.J., Feito, F.R., 2017. Digitization of religious artifacts with a structured light scanner. Virtual Archaeol. Rev. 8 (17), 49–55. Hayes, J., Fai, S., Kretz, S., Ouimet, C., White, P., 2015. Digitally-assisted stone carving of a relief sculpture for the parliament buildings national historic site of Canada. Int. Arch. Photogramm. Remote Sens. Spat. Inf. Sci. 2, 97–103. Herholz, P., Matusik, W., Alexa, M., 2015. Approximating free-form geometry with height fields for manufacturing. Comput. Graph. Forum 34, 239–251. Izquierdo, P., Tresserras, J.J., Matamala, J.C., 2005. The Hicira Handbook. Heritage Interpretation Centres, Barcelona. Katz, J., Tokovinine, A., 2017. The past, now showing in 3D: an introduction. Digit. Appli. Archaeol. Cult. Herit. 6, 1–3. La Pens ee, A., Parsons, J., Cooper, M., Broadbent, S., Bingham, R., Watts, R., Myers, J., Berry, J., 2010. The use of 3D laser scanning and 3D modelling in the realisation of an artistic vision; production of large scale public art in tudor square, sheffield. Int. Arch. Photogramm. Remote Sens. Spat. Inf. Sci. - ISPRS Arch. 38, 375–380. Malik, U.S., Guidi, G., 2018. Massive 3D digitization of sculptures: methodological approaches for improving efficiency. IOP Conf. Ser. Mater. Sci. Eng. 364, 12–15.
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Digital Applications in Archaeology and Cultural Heritage journal homepage: www.elsevier.com/locate/daach
Digital fabrication of cultural heritage artwork replicas. In the search for resilience and socio-cultural commitment M.J. Merchan a, *, P. Merchan a, S. Salamanca a, E. Perez a, T. Nogales b a b
Escuela de Ingenierías Industriales. Universidad de Extremadura, Avda. de Elvas s/n, Badajoz, Spain Museo Nacional de Arte Romano, C/ Jose Ramon Melida, s/n, Merida, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: 3D modelling Digital fabrication Cultural heritage Interpretive centre Education Social cohesion
The traditional approach for physical reproduction in Cultural Heritage requires the production of moulds on the very surface of the original artwork in an invasive, manual and usually time-consuming process that is intensely influenced by the complexity of the piece to be reproduced. The difficulties increase when changing the size of the copy is needed. But producing replicas is a necessity, mostly when it comes to mounting exhibitions within interpretive centres. The educational role of such institutions is beyond doubt. Furthermore, they have also become major drivers of social inclusion of at-risk groups. So, having perfect copies of archaeological remains without damaging them is a challenge that technology has been facing for the last years. This paper describes a procedure to obtain exact replicas of classical statues that reduces their manipulation, thus preventing them for further damages, which was applied to two Roman marble sculptures of Medellín (Badajoz, Spain).
1. Introduction The use of 3D technologies for digitizing cultural heritage has been developing rapidly and constantly in the last years, thanks in part to computer vision advances and to the improvement of technological devices. In this respect, the acquisition and storage of sites, monuments and works of art data can be carried out in a more effective and efficient way. This fact has provoked the arisen of repositories of 3D models with research purposes as well as virtual museums that have widened the applicability of these models. The availability of an accurate digital representation opens up a broad spectrum of possibilities for experts(Katz and Tokovinine, 2017). Therefore, there have been a lot of works in this field lately, either using photogrammetry techniques or active 3D scanning (3D laser scanning, structured light 3D scanner, phase shift and time of flight laser scanners …). One can distinguish between those works dedicated to scanning open spaces and monuments (e.g. Arav et al., 2016; Sapirstein, 2016; Xu et al., 2017; Zhang et al., 2017; Balsa-Barreiro and Fritsch, 2018; Perez et al., 2019); and those which are focused on a smaller-scaled research: artefacts (Banterle et al., 2017; Graciano et al., 2017; Banducci et al., 2018; Zhang et al., 2018), epigraphy (for instance, Ramírez et al., 2017; Di Paola and Inzerillo, 2018; Carrero-Pazos and Espinosa-Espinosa, 2018;
Andreu and Serrano, 2019), sculpture (Adan et al., 2012; Zhang et al., 2015; Malik and Guidi, 2018; Perez et al., 2018), etc. 3D models in cultural heritage can be used not only for documentation and visualization purposes but also as tools for carrying out several tasks. Among them, it is worth noting the manufacture of original artworks copies (Scopigno et al., 2011; Katz and Tokovinine, 2017). Digital manufacturing, defined as the physical reproduction of objects from their three-dimensional models, combines conventional manufacturing techniques with digital technology (Ribeiro da Silva et al., 2019). It has developed rapidly in recent years, as techniques, technology and materials have evolved (Bogue, 2013), and its scope of application has not ceased to grow. Thus, its use has been extended to the most diverse areas of daily life. Cultural heritage is one of the fields in which digital manufacturing is gaining more prestige. Within this branch, its uses are multiple and very diverse (for example, the reproduction of sculptures for sensorial exhibitions, manufacturing pieces to facilitate the labour of restorers, or a more general use: allowing the resilience of the patrimony assets throughout the years) (Scopigno et al., 2017). The digital fabrication techniques can be classified into two groups: on the one hand, the subtractive techniques that consist of carving a block by removing material to sculpt the desired piece, which is made by using a milling tool controlled by a computer. On the other hand, you can
* Corresponding author. E-mail addresses: [email protected] (M.J. Merchan), [email protected] (P. Merchan), [email protected] (S. Salamanca), [email protected] (E. Perez), [email protected] (T. Nogales). https://doi.org/10.1016/j.daach.2019.e00125 Received 11 June 2019; Received in revised form 30 September 2019; Accepted 2 October 2019 2212-0548/© 2019 Elsevier Ltd. All rights reserved.
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find the additive processes. As their name suggests, they consist of the deposition/addition of successive layers of material to “build” the object. These are the most spread techniques for 3D printing in the last years since they are more versatile, easy-to-use and less expensive than the others. A recent review of this type of manufacturing procedures (materials, methods, applications and challenges) can be found in Ngo et al. (2018). The decision to opt for one type of technique or another will depend on the type and size of the piece to be replicated, the accuracy required, the material to be used, the subsequent use of the copy, etc. Up to the apparition of 3D digitization techniques (and even nowadays), it was usual to make a mould on the very surface of the sculpture when replicating it, what implies the use of products that usually affected the epidermis of the material. It is needless to say that this technique, which can go back to the Roman time (Rigdway, 1984; Gazda, 1995), also exposes the archaeological remains to unacceptable physical risks. However, having perfect replicas of artworks has been always necessary. In previous epochs, they facilitated that artists, students and common people approached to the masterpieces of the ancient world. Nowadays, they become an essential resource for culture and education. Replicating archaeological objects with a higher standard of perfection is especially important in the case of interpretive centres. An interpretive centre is a hybrid concept between a museum and a visitors’ centre: they are thought to welcome, look after and guide visitors while introducing them in the knowledge of heritage through a museographic discourse with a pedagogical purpose (Martín Pi~ nol, 2013). Its aim is not to acquire, study or keep heritage pieces and goods, but to show how to appreciate and take care of them in an educational manner. The absence of original pieces, which must be necessarily preserved within a museum, is palliated by the exhibition of replicas to build a narrative and iconographic discourse that must be understandable and attractive at the same time (Izquierdo et al., 2005). Digital manufacturing becomes the ideal resource for generating the replicas needed in interpretative centres, since it provides with an efficient, accurate and quick working way that is also respectful with the original pieces as it does not require any contact. Besides, the reproduction costs have steadily decreased, favoured by the constant development of devices and technology. Two very interesting surveys on this subject are (Scopigno et al., 2011) and (Weigert et al., 2019). The aim of this paper is to emphasise the importance of producing perfect physical reproductions of statues to provide interpretive centres with the resources needed for their educational purpose. To do this, we will analyse the work done on two Roman sculptures made of marble that were found in Extremadura (Spain). Both statues were uncovered in the excavations held under the supervision of P. Mateos (Mateos and Picado, 2011), in the hyposcaenium of the Roman theatre of Medellín (Badajoz), the ancient colony Metellinum, in Lusitania (Hispania) in mid-2007. The rest of this paper is organised as follows. In Section 2, the archaeological study of the pieces that states the importance of the statues and the circumstances that led to the need to get high-quality replicas in a short period of time are exposed. Section 3 presents an overview of the procedure followed to obtain the 3D models. Section 4 offers a brief state of the art on digital fabrication technologies and shows how the physical reproductions were made in this case. In Section 5, the results obtained are shown and discussed. Section 6 states the conclusions.
her hand would be extended outwards, whereas the left one, attached to the side, is bent at the elbow to be projected forward. Both forearms would have been worked as separated pieces to be joined to the body. She wears a thin tunic or chiton buttoned over her shoulders, which forms abundant folds that appear in certain areas under a mantle wrapped around all her perimeter, drawing a balteus from the right side to the left shoulder. The rounded neck of the chiton defines the hole to insert a head, also worked in a separate piece of marble, just as the front part of both feet would have been. The rear side, less visible and for this reason less elaborated, has a large perforation over the gluteal area, probably to ensure her fixation since the weight of this statue (1000 kg approx.) and its narrow base make it quite unstable. The carving is exceptional therefore this statue becomes a top-level work of art due to the delicate modelling of its forms and the relief of the cloth, with almost transparent folds and a remarkable “sfumato” in the mantle. It probably represents one of the female members of the imperial family. The second statue is a smaller female figure (H: 120 cm; W: 45 cm; D: 28 cm), equally in a free-standing position (Fig. 1. Right). It was unearthed in two large fragments that were joined by the excavation team. This sculpture wears a tunic and a wrapping mantle, under which both hands appear. The right one is on the chest, grasping the cloth, and the left one is along the side, holding the mantle, a part of which disappears inside the fist. It preserves the conic-shape cavity at the base of the neck to fix the head, which was worked in a different piece of marble, as the front part of the left foot. Although it presents some erosions and losses in its surface, it is a piece of noteworthy quality that would possibly have represented a young private lady belonging to the elite of Metellinum, who was able to afford a statue of this type in such an outstanding place as the scaenae frons of the Theatre (Nogales and Merchan, 2018). Both of them seem to be works done in the first century of our Era. 2.2. Created necessities The work that led to the recovery and rehabilitation of the Roman theatre of Medellín favoured its reopening in 2013. This very year, it was awarded with one of the three Heritage Awards conferred by Europa Nostra thanks to the excellent work of preservation done. The opening of the site was accompanied by the remodelling of the adjacent church of Santiago as an interpretive centre, in which it was decided to show some of the most significant works of art in order to reinforce its pedagogical discourse. The quality and archaeological worth of our sculptures, just like the facilities of the church, advised against the idea of a new transfer to Medellín and their exhibition. So, it was proposed to show two exact replicas instead the originals. The contracting party established a set of requirements that conditioned the choice of methodology to be applied. The two main ones were: 1. the limitation of time, since the replicas had to be available less than a month after starting the work; and 2. the quality of the result, which was supposed to be superb, in order to continue with the self-imposed excellence criteria. The very statues forced a third condition: the danger of their manipulation. All that folds so masterfully worked are susceptible to break with any minimum contact. Likewise, the fact that the smaller statue was reconstructed from fragments that could be detached, together with the fragile parts of the largest one, such as the mantle under the arm that floats in the air, advised against excessive manipulation of the pieces. This third condition, although indirect, made them discard any type of invasive technique that could endanger the artworks from the beginning.
2. Materials and methods 2.1. Description of the statues
3. Generation of the 3D models
The largest piece is a freestanding sculpture bigger than life size (H: 167 cm; W: 64 cm; D: 36 cm) (Fig. 1. Left). Made in grey veined marble, it represents a woman who adopts an erected and frontal position, leaning on her left leg while the right, which wouldn’t support any weight, is marked under the tissue of her clothing. She elevates her right arm and
Our 3D digitization method followed the stages of the 3D modelling process (Bernardini and Rushmeier, 2002). Taking into account that in this case the stream for the texture is not needed, since the statues were 2
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Fig. 1. The two Roman statues to be reproduced.
partial views of each sculpture in a common reference system was implemented, as is usual, through geometrical transformations (rotation and translation) by using the kd-tree algorithm and point-to-point minimization (Rusinkiewicz and Levoy, 2001), a variation of the popular ICP registration technique. The merging stage was carried out by following the well-known mesh zippering algorithm (Turk and Levoy, 1994). The output of this second stage is a mesh fitted to the whole surface of the object. This single mesh is then subjected to a hole-filling algorithm in the third stage. We used the technique proposed in (Perez et al., 2008), an approach to fill any kind of holes in 3D meshes based on an image in-painting algorithm that uses the Fields of Experts (Roth and Black, 2009) framework. The last step is the analysis and correction of errors in the resulting surface. This step is mainly automatic, using the suitable tools of the specific software: detection of peaks, self-intersections, non-manifold elements, tunnels, etc …, but also includes a manual process where a visual inspection must detect small protrusions. These are produced by the accumulation of errors in all the stages (digitalization, registering and fusion). To solve it, a defeaturing tool that carries out elimination of the small protrusions and filling of the resulting holes are applied. The final geometrical models of the statues are shown in Fig. 3. The high-resolution mesh for the first statue (Fig. 3 above) has 3.5 million triangles whereas the mesh for the smaller statue has 2.2 million triangles.
going to be reproduced in a marble-like material, the conceptual map proposed in (Merch an et al., 2011) could be modified to the one shown in Fig. 2. As seen, the initial stage consists of data acquisition, either using photogrammetry techniques or 3D laser scanning, depending on the final purpose of the model. Following this, the well-known phases of registration, integration and post-processing of the overlapping scans are carried out to obtain the final geometrical mesh. Data acquisition planning is always a fundamental stage in 3D digitalization processes, but it is particularly important in the case of heritage pieces, since a loss of data due to poor planning might be hardly recovered. As commented, several 3D digitization technologies are available nowadays; the selection of the specific instrument is influenced by the characteristics of the 3D model required, in terms of accuracy and resolution, which depend on the intended use for the digital model. In the case of the two Roman statues under study, we decided to use laser scanning due the features of the sculptures themselves (roman clothes, reflection particularities of the marble, morphological complications, time constraints, etc.). We used a Konica Minolta Vivid 910 laser scanner on fine mode. In this configuration, its nominal accuracy is 0.10 mm and the precision 0.016 mm at a distance of 0.6 m. Pictures were also acquired through the system, but not processed into a final texture, as said before. The final geometrical mesh was created by following the stages specified in the conceptual map of Fig. 2. Internally developed (noncommercial) software was used in this process. The registration of the
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Fig. 2. Simplified conceptual map of the generation of a geometrical model from multiple overlapping scans.
4. Production of the replicas
Adapting 3D models to be used by digital manufacturing machines is often a relatively straightforward process. However, there is currently a very interesting line of work in which specific 3D mesh processing algorithms are being proposed to facilitate and improve the fabrication process. For example (Herholz et al., 2015), and (Alemanno et al., 2014) present a procedure to decompose a 3D model into pieces that can be manufactured either by traditional methods (casting or 3-axis CNC) or by 3D printers, respectively. With these procedures, the price of the replicas is reduced, compared to the use of 6-axis and 7-axis CNC, but at the cost of losing quality in reproduction, especially when the objects to be replicated are large. Both methods could also be applied in the case of extractive manufacturing techniques. Other methods propose making silicone moulds that allow the fabrication of objects with complex geometry, as in (Malomo et al., 2016) or in (Alderighi et al., 2018), where they obtain reusable moulds. Although these techniques are very promising, they are designed to be used with small objects that can be manufactured with a 3D printer, so they do not apply to the situation dealt here. In the case presented in this paper, due to the size of the pieces, the quality requirements, the intricacy of the folds, and the exhibition necessities, a 7-axis CNC machine robot arm was the option chosen to build intermediate replicas, which will be used to make moulds from which the final replicas are obtained. There are several reasons to accomplish an intermediate copy instead of manufacturing the final statue in stone directly. The first one is the price of the replica, which is minor using this methodology, because of the lower cost of the robot arm (since it is not necessary that it exerts the high torque required to sculpt stone) and the cheaper materials (marble or granite blocks versus high-density polyurethane and marmolina powder). The second factor refers to the weight of the copies obtained,
As said previously, the methods for digital manufacturing of replicas can be classified into two groups depending on the technique they utilized: subtractive or additive (Scopigno et al., 2017). Within the first group, Computer Numerical Control (CNC) machine tools employ a spindle to remove material of the original block to obtain the replica. When a perfect copy of complex sculptures is needed, as in the case proposed, 6-axis CNC or 6 DOF robots are the most common method (La Pensee et al., 2010; Hayes et al., 2015; Tucci and Bonora, 2007). But, even so, it is not always possible to have perfect replicas by this mean since there may be unreachable positions for the spindle. To solve this problem, solutions such as the addition of a 2 DOF platform, which moves the blank material, to the 6 DOF robot (Zhu et al., 2006) or, directly, using 7 DOF robots (Fernando, 2018) have been proposed. The extra degree of freedom is used to orient and position its end in the three-dimensional space and a much greater number of solutions can be obtained when calculating the inverse kinematics. This way, the milling tool can be placed in any position and orientation due to the new values of joint coordinates provided by this type of machine. Additive fabrication techniques consist of adding layers of some material that is solidified by some specific procedure. There are different manufacturing methods within the additive technology (Scopigno et al., 2017): fused deposition modelling (FDM), laminated object manufacturing (LOM), granular materials binding (GMB) or photopolymerization. The advantages of additive techniques over the extractive ones are their lower cost and ease of use. Nevertheless, not only the quality of replicas is inferior but the 3D printers do not allow to fabricate large parts currently, which makes additive manufacturing not suitable for the type of application proposed in this work.
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Fig. 3. Final geometrical models.
sculpture was used to produce and exact copy of each statue by sculpting a high-density polyurethane block. The robot arm works from minimum dimensions of about 500 mm up to 2500 mm in height in a single block. High-density polyurethane was used because of its ability to be trimmed and handled at will, but still maintaining its rigid form. Fig. 5 shows a
much lighter than if they were made in stone, which facilitate their transport. The procedure performed for the fabrication of 1:1 scale replicas is schematized in Fig. 4 and described below. First, a 7-axis Kuka robot arm adapted for milling and machining of
Fig. 4. Flowchart of the procedure used to obtain the replicas. 5
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Fig. 5. Sculpting of the intermediate replica with a robot arm.
moment of the sculpting process. After that, the intermediate replica generated with the robot arm was used to make a mould that will serve to obtain the final reproduction by a traditional casting process. Fig. 6 is a picture of the mould generated for one of the statues. In the end, the final replicas were obtained by pouring marble powder (marmolina) into this mould. This casting material makes it possible to create pieces that are very resistant, making them suitable for indoor and outdoor exposure. These statues are exact copies that have accuracy below 0.3 mm and a hyper realistic aspect. In Fig. 7 the final reproduction of one of the statues can been seen in the foreground. The intermediate replica sculpted by the robot arm appears in the background. Fig. 7. Final physical reproduction with its intermediate replica in the background.
5. Results and discussion 5.1. Results
3D model can be spoiled if the manufacturing process is inappropriate; in the same way, although we had the best machines, a poor digital model hinders getting good results. In our case, the fact of having produced such a perfect and tough mould, thanks to the last generation manufacturing robot arm, allowed making the copies in marmolina in just one piece, with a finish very close to the original one. The time needed to complete the whole process was about eight working days, distributed as shown in Fig. 8. In addition, the replicas are weather-resistant and durable. Another key point is the final weight of the replica. This is an important factor when dealing with manipulation and possible loaning to other institutions. Thus, while the original pieces needed a crane (the largest statue) or, at least, two operators (the smallest one) to be moved, copies can be transported by one person. Figs. 9 and 10 are two pictures of the final copies as they are currently exposed in the visitor interpretive centre (former church of Santiago) in Medellín (Spain). Concerning the second result, the fact that archaeologists and historians can have the virtual model appears as an opportunity for the exhaustive analysis of the piece. The traditional on-site autopsy has often been replaced by the study of the piece through photographs when the researcher cannot move to the place where it is. It goes without saying that these images had not only neither the size nor, in many cases, enough quality for the correct understanding of the object, since they always provide a two-dimensional vision of an artistic technique that arose, precisely, for conquering the space in its three dimensions. The advantages of these virtual models applied to archaeological research have been proved indispensable in recent years (Scopigno et al., 2011; Merchan et al., 2013).
There are two immediate results that can be highlighted in the accomplishment of this work: 1) the obtaining of the physical replicas themselves and 2) the digital models that are left, that can be used by the researchers for further study. In regard to the first result, it has been proven that digital fabrication is the most suitable option nowadays, mostly when the main challenge that has to be faced is the optimization of time and the quality of the results. In the process of digital manufacturing, it is equally important both the data acquisition and the fabrication method: one very accurate
Fig. 6. Moulds produced from the intermediate replica in high density polyurethane. 6
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Fig. 8. Time consumption in the digital fabrication of the two replicas.
Fig. 10. Picture of the replica of statue no.2 exposed in the visitor interpretive centre.
5.2. Analysis of replicas In order to demonstrate the effectiveness of the manufacturing method presented, the digital models of the physical replicas have been generated following the same procedure explained in section 3. Fig. 11 is made of the images of the 3D models of the original and the replica of both roman sculptures. Several measures have been taken to test the quality of the result. For each 3D point of the mesh of the original sculpture, the closest point of the 3D mesh of the replica has been searched and the distance between them computed. These distances form a vector, D, whose size is equal to the number of points of the 3D mesh of the original statue. Fig. 12 shows the histogram of that vector. Table 1 summarizes the mean, standard deviation and maximum and minimum values of D. As can be seen in Table 1, the errors are negligible, taking into account the sizes of the sculptures (167 cm tall for the first statue, and 120 cm tall for the second). Despite the above, it can be observed, both in the histograms and in the data shown in Table 1, that there are outliers in the distance vector. A second analysis has been carried out after eliminating these outliers. A distance value di 2 D is considered as an outlier if it verifies that: di > D þ 3 ⋅ σ D Fig. 9. Picture of the replica of statue no.1 exposed in the visitor interpretive centre.
(1)
where D is the mean of D and σ D is the standard deviation of D. The case 7
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Fig. 11. 3D models of the original (left) and replica (right) of the (a) first and (b) second statue.
Fig. 12. Histogram of the distances between the 3D points of the original statue and the replica. (a) Statue 1. (b) Statue 2.
The examination of outliers also allows carrying out a visual analysis of where the most significant errors occur, as shown in Fig. 13. In it, the areas with outliers have been marked in red. As can be seen, many of them correspond with folds, which happen to be the most difficult zones to scan and sometimes the application of filling-hole algorithms was necessary. There are also many outliers in the base of the 3D models because they were not acquired directly since the statues could not be lifted from the ground. In summary, the biggest errors between the meshes of the original statues and the replicas are due to the algorithms used in the creation of these models, and not to the manufacturing process used. Finally, in Fig. 14 vector D is represented onto the 3D models using a colour code. In this representation, the outliers have been removed. It can be observed that, besides the areas of more complex geometry, the sides of the statues show greater error than the rest of the model. This could be due to the joining of the two halves of the moulds used in the manufacture process. Anyway, it must be emphasized again that the maximum
Table 1 Mean, standard deviation and maximum and minimum values of.D
Statue 1 Statue 2
Mean distance (mm)
Standard deviation (mm)
Maximum distance (mm)
Minimum distance (mm)
3.53
3.55
40.94
0.0084
3.27
2.94
40.18
0.0092
in which the value of the distance is less than the mean of D minus three times the standard deviation has not been taken into account since, as seen in Table 1, there will be no data that verify it. Table 2 shows the number of outliers, the percentage related to the number of the 3D points of the model, the mean distance, the standard deviation and the maximum and minimum values when the outliers have been removed.
Table 2 Values when the outliers are remove from.D
Statue 1 Statue 2
Number of outliers
% of outliers
Mean distance (mm)
Standard deviation (mm)
Maximum distance (mm)
Minimum distance (mm)
25821 24444
2.29% 1.44%
3.19 3.04
2.72 2.15
14.18 12.10
0.0084 0.0092
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our history and culture known. Due to the possibility of replicating archaeological pieces, it is feasible to reconcile two basic principles: 1. that blind people need to have direct contact with objects to facilitate their exploration through touch, and 2. museums must take into account the conservation of materials. Here is where the digital fabrication supposes a huge qualitative leap with respect to other exhibitions carried out previously: it offers a quicker and cheaper non-invasive way of reproduction, as well as, since the techniques and materials to be used are so accurate, so wide and so varied, it allows replicating the least imperfection, the texture and even the common temperature of the material in which they were made originally by choosing the proper one in every case. It makes the sensory experience through touch very realistic. 7. Conclusions During the last years, several papers have hypothesized about the advantages and disadvantages of using one of the different technologies available for the acquisition of 3D information and its subsequent physical reproduction. But only through the experience provided by particular case studies it will be possible to come up with useful and reliable conclusions for researchers. In this paper, we have analysed a specific case: the physical reproduction of two Roman female sculptures using laser scanner for 3D model generation and digital manufacturing. This is a particularly significant example because of the morphological features that this type of statues presents, due to the protuberances and shadow areas that produce the folds of the dresses. With the aim of reducing statues manipulation and preventing for further damages, we propose a procedure that is an alternative to traditional casting. We have used laser scanning and computer vision techniques to generate the accurate 3D models needed. These models are utilized to produce intermediate replicas by means of a sculpting robot arm. After that, the final reproductions are obtained by pouring marble powder into the moulds generated with the intermediate replicas. Digital fabrication is very useful and versatile when needing to produce exact replicas of pieces belonging to the Cultural Heritage, especially those that are fragile, since it is a non-invasive and non-destructive tool. The subsequent use of the copies in interpretive/visitors’ centres is very important for the correct understanding of their exhibition discourse and for the deep knowledge of the History of the place where located. Even more, this type of centre, as well as museums, can offer new possibilities for those groups of people that have any kind of visual disabilities, providing new experiences that encourage their social inclusion. An additional advantage, not directly related to the purpose of this paper, is the obtaining of high-resolution 3D models of the sculptures, which can be used by researchers in their investigations. These 3D models offer much richer information that the traditional photographs used to date for this kind of archaeological pieces. Furthermore, they can be used to create VR experiences to approach Cultural Heritage to the people. These experiences have also a social component since they allow disfavoured groups, such as wheelchair users or people with mental impairment, among others, to know and enjoy the cultural heritage assets. It is a way to virtually and effectively demolish the barriers that, unfortunately, still exist for them. Undoubtedly, this is a way to preserve the memory of Antiquity that guarantees its resilience over the years, providing, in turn, the tools to make it accessible to all people, without distinction of any kind.
Fig. 13. Zones with outliers (marked in red) of the 3D models of the statues. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
error is less than 1.5 cm, which is a negligible value for sculptures of this size. 6. Discussion Possibly, the most relevant result that can be outlined through this case study is that both the 3D digital models and the physical replicas are perfect tools for education and social cohesion. On the one hand, in concern to the digital models, they are powerful resources for formal and informal learning. The use of 3D models of monuments, sculptures, artefacts …, either directly shown on a screen or through Virtual Reality recreations, allows building educational environments in an attempt of achieving students’ involvement in their own learning process in a more effective and fun way. The idea is to accomplish new educational strategies in which, through individual or collective activities with 3D technology support, they actively participate in the acquisition of knowledge, even without noticing they do (Merchan et al., 2018). On the other hand, the importance of having perfect copies with a pedagogical purpose has been explained when talking about the educational role of interpretive centres. However, its significance goes a step beyond since the replicas constitute an important element of social integration and cohesion. This is because Cultural Heritage is considered by EU as a perfect resource to achieve the real integration of those socio-cultural groups that are still not. Thus, thanks to the physical reproduction of the archaeological remains, visually disabled people, one of those groups that are nowadays not sufficiently integrated in society, can learn and enjoy our past by touch. The World Health Organization estimates that currently 217 million have moderate to severe vision impairment and 36 million people more are blind (https://www.who.int/news-room/fact-sheets/detail/blindn ess-and-visual-impairment). Besides the barriers they have to cope in their day-to-day, they discover new ones when facing Cultural Heritage. For instance, the access of these people to the archaeological collections of museums has been quite limited to date. It is a collective that is caught in the midst of the dilemma that any museum has: the logical need for the preservation of irreplaceable pieces and the diffusion of its funds to make
Funds This work has been supported by the project IB16162 from Junta de Extremadura and European Regional Development Fund (ERDF) “A way to make Europe”. 9
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Fig. 14. Colour representation of the distance of the 3D model of the original sculpture and the replica. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Declaration of competing interest
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