3D laser scanning in digitization, reconstruction and replication of sculptural monuments

Parfenov Vadim Aleksandrovich ORCID: 0000-0002-2048-4677 Doctor of Technical Science Professor, Photonics Department, St.Petersburg Electrotechnical University 197376, Russia, Saint Petersburg, ul. Professor Popov, 5 vadim_parfenov@mail.ru

 IntroductionThe use of laser technology in the field of preservation of cultural and historical heritage is a new, but rapidly developing scientific and technical direction.

Its founder is the American physicist John Asmus, who in 1972 for the first time demonstrated the possibility of using lasers in restoration, as well as for non-destructive testing and 3D visualization of historical monuments ^[1],[2].  In his works, J.Asmus used laser cleaning technology and holography, but today many other laser methods are also used in museum work, including raman spectroscopy and laser-induced plasma, laser Doppler vibrometry, digital speckle interferometry, 3D scanning, etc. ^[3],[4],[5].

The main advantage of 3D scanning technology is that, unlike holography, it allows not only to capture a three-dimensional image of objects, but also provides high-precision information about their dimensions ^[3],[5] – ^[8]. As a result of scanning, a so–called point cloud is created – a point computer model of the surface of the original object, which is its virtual copy (in fact, its high-precision electronic passport). The image of 3D models obtained as a result of scanning can be viewed on a computer screen from any angle of view (as if "flying around" the monument from all sides) and with any magnification. Since a 3D model is a computer file in one of the graphic formats (usually STL or OBJ), it can be easily exported to all major 3D data exchange standards for subsequent processing. Due to this, with the help of specialized computer programs, various calculations can be performed, for example, to determine the volume and area of the expanded surface of an object, to obtain its dimensional drawings, etc.

The information contained in the 3D model of monuments can be simply irreplaceable in case of their damage or death as a result of wars, natural disasters or vandal attacks. 3D models can be used to create all kinds of computer animations, for example, for historical reconstructions of palaces, park ensembles, streets of long-vanished cities, etc. Another interesting possibility is associated with the creation of virtual museums. In this case, in a single "gallery" (accessible to its visitors via the Internet), you can collect the most famous monuments from one particular museum or even museums from around the world. In addition to increasing the accessibility of museum collections, this opens up wide opportunities for the exchange of information between museum specialists from different countries by introducing a large number of monuments into full scientific circulation.

In addition, 3D models can be used for computer reconstruction of sculptures, bas-reliefs and architectural objects, including their individual fragments. For example, in the case of damaged monuments, using computer graphics methods, it is possible to reconstruct their intended original appearance, or simulate various options for restoration "completions" without touching the monument itself. This approach allows you to plan the entire complex of necessary works in the process of future restoration. 3D scanning can also be useful for solving the problem of changing the dimensions of the monument when casting sculptures made of bronze, lead and other metals. In restoration practice, there are often situations when damaged parts of sculptures have to be made anew. These new parts, cast into molds made according to the original sculpture, due to shrinkage of the molding material may differ significantly from it in size. This problem can be easily solved by creating a mold with an "allowance" by scaling the 3D model taking into account shrinkage (for this, in the original model, it is enough to increase the corresponding fragment of the monument in the required proportion).

Another important and promising application is monitoring the condition of exterior (i.e. exposed outdoors) monuments. As a result of periodic (with an interval of several months to a year) scanning and creation of 3D models of the surface of the same monument (as a whole or its individual fragments), it is possible to monitor the progress of its destruction processes ^[9],[10]. For example, you can monitor the increase in the size of cracks, the area of gypsum crusts and biofouling (we are talking about colonies of microorganisms – biodestructures that lead to the destruction of monuments), etc. Similarly, you can check the condition of monuments before and after the completion of restoration work.

A separate area of application of 3D scanning is the creation of high-precision physical copies and reconstruction of damaged and/or lost monuments ^[5],[12]. 

A number of the above-mentioned works performed in St. Petersburg under the guidance of the author will be discussed in this article.

 3 D-scanning in copying sculptural monuments Like all monuments exhibited outdoors, sculptures are not eternal, since any materials used in their creation (be it marble, bronze, or even granite) are gradually destroyed over time.

In this situation, the issue of the need to gradually replace exterior sculptures with copies, followed by the transfer of originals to closed museum premises, has long been on the agenda. As world practice shows, today this is the only opportunity to preserve for posterity the creations of masters of the past, at least the most valuable of them. And although the idea of replacing originals with copies has always caused and still causes an ambiguous attitude in society and even among museum workers, there is no other way to save monuments from destruction these days.

            This is the path that has long been followed in many European cities, and in recent years in our country.  However, today, when curators, art historians and restorers are gradually coming to an agreement on the need to copy monuments in principle, the question of what technologies can (and most importantly, is permissible) to use when creating copies becomes more and more urgent? 

       For many centuries, copies of stone monuments were made by hand from historical (author's) material. In the XX century, new copying technologies appeared. At first, they were based on the manufacture of a plaster model followed by the casting of a copy of cement or stone substitute. Later, a technology appeared in which copies were made by pouring or stuffing a composition based on a synthetic binder (polyester resin) and a mineral filler (marble chips with the addition of dye) into a mold made of silicone sealant. However, a very serious disadvantage of this technology is that it is contact. In the process of making a copy (at the stage of creating the form), the original sculpture is subjected to significant chemical and physico-mechanical influences, which can damage the state of preservation of the monument. So, after using silicone sealants, hard-to-remove traces in the form of yellow-brown streaks, pieces of molding plasticine and flakes of silicone itself often remain on the surface of white marble. Another problem is that when the reinforced silicone mold is disassembled, mechanical damage may appear, manifested in the form of chips and even fractures of individual small parts of the sculpture (fingertips, hair curls, etc.).

       As a result, the copied sculpture may require unscheduled restoration, and, consequently, a truly paradoxical situation arises. Copies are created to preserve the originals, but as a result of copying, irreparable harm can be done to the original sculpture!

However, the use of 3D scanning creates a very effective alternative to contact copying of sculptural monuments. We are talking about a method of creating copies, which is based on an innovative technology based on a combination of 3D scanning and stone milling machines with numerical control (CNC). This allows you to make copies of sculptural monuments made of natural stone (marble, limestone, granite, etc.).

For the first time such copying of monuments was carried out in the UK ^[13]. The main advantage of this method is that it is contactless and does not have any negative impact on the original monument. Due to this, laser copying technology is now widely used in a number of European countries (in Italy, France and Austria). However, in Russia its application is still in its infancy.

For the first time, the technology of contactless copying of sculptural monuments in our country was used to create a copy of the marble bust "Primavera" (XVIII century, Italy) from the collection of the State Museum of Fine Arts "Tsarskoye Selo" (Pushkin, St. Petersburg). This work was carried out by the St. Petersburg restoration company "Resstroy" with the participation of specialists of SPbSETU "LETI", who were engaged in the creation of a 3D computer model of this monument (see Fig. 1).

Fig. 1. Sculpture "Primavera": on the left – the process of 3D laser scanning, on the right – the finished 3D model on the computer screen The replica of the sculpture itself was made using a robot milling machine made of white Carrara marble in Italy (Fig. 2) and was installed on the site of the original monument near the Catherine Palace.

More detailed information about this project is given in ^[14],[15].

Fig.2. Sculpture "Primavera": on the left – the process of making a copy on a CNC machine (October 2009); on the right – a finished copy (July 2010) Another example of contactless copying of sculptural monuments in Russia is the creation of a copy of the marble bust of the Russian Emperor Peter I from the collection of the Peterhof Museum-Reserve in St. Petersburg.

This sculpture was presented as a gift to the French Academy of Sciences in Paris in connection with the 300th anniversary of Peter I's visit to France celebrated in 2017. The idea of this project was proposed by A.K. Orlov, the Ambassador Extraordinary and Plenipotentiary of Russia to France in 2008-2017, and found active support from the Director of the Peterhof GMZ E. Ya. Kalnitskaya, who together were able to find sponsors of this project.

The scientific group of the author of this article again took an active part in creating a copy of the bust of Peter the Great.  The work on copying this monument began with its 3D scanning in the storage facility of the Peterhof State Museum of Fine Arts.  Then, as in the case of Primavera, a copy was made from a block of marble on a CNC machine in Italy in Carrara. All the details of this project are described in ^[16], and in Fig. 3 you can see photos of the original monument and its copies.

The results of the work on the creation of copies of the marble busts of Peter I and "Primavera" are convincing evidence of the high efficiency of the laser contactless method of creating physical copies of sculptural monuments made of stone. Its relatively low cost, speed of execution and ensuring the complete preservation of the original in the process of work are the most significant advantages of this method. Fig. 3. Marble bust of Peter I: on the left – the original; in the center – the process of making a copy on a CNC machine; 

on the right – a finished copy

Application of 3 D-scanning for reconstruction and restoration of historical monumentsIt was mentioned above about the possibility of using 3D scanning for virtual reconstruction of monuments.

An interesting example of this kind of work is a project to reconstruct a completely lost sculpture from the former Sergievka estate in the suburbs of St. Petersburg, which in the XIX century was the country residence of the Duke of Leuchtenberg, the stepson of Napoleon Bonaparte.

The Duke of Leuchtenberg was a collector of antiquities and had a rich collection of works of art. Unfortunately, almost all of this unique collection perished during the Great Patriotic War, as Sergievka was in the epicenter of hostilities. Among the lost works of art was a zinc sculpture of the XIX century "Eve at the spring".

This sculpture was made in the XIX century by the method of fragmentary casting from a spy in the famous Moritz Geiss foundry in Berlin ^[5]. Spiatr, which is an alloy of zinc with tin, is a relatively inexpensive substitute for bronze. Due to its properties, this material was widely used in the XIX century in the manufacture of sculptural monuments. "Eve at the Spring" depicted a reclining nude figure of a young woman and was a copy of the marble sculpture of the same name, created in 1818 by the English sculptor Edward Hodges Baily (1788-1867). It is believed that this sculpture represents the "Paradise Lost". For many years, the "Eva" from the Sergievka estate was considered completely lost, but its fragments were unexpectedly discovered in 2007 in the ground during construction work.

Fig. 4. Fragments of the sculpture "Eva"

These fragments were a pile of scattered fragments (Fig. 4), which did not allow the reconstruction of this monument by traditional methods. Although the individual fragments of "Eve" were well "articulated" with each other, it was impossible to reconstruct the sculpture as a whole (even using the finishing mass – the traditional method of restoring dilapidated monuments), since due to significant losses of the sculpture it was not possible to correctly orient them relative to each other in space. However, as a result of using laser 3D scanning and subsequent computer 3D modeling, it was possible to carry out a virtual reconstruction of this sculpture (Fig. 5).

Fig. 5. Virtual reconstruction of the sculpture "Eva" (dark gray surface areas show the location of the preserved fragments of the sculpture)

At the final stage of this project, a replica of the sculpture "Eva" was made using FDM (Fused deposition modeling) 3D printing technology, which allows us to judge how the original of this monument looked (Fig. 6). More detailed information about this project is given in ^[21].

Fig. 6. Replica of the sculpture "Eve"

It should be noted here that the mentioned FDM technology is one of the varieties of the so-called additive technologies, which are based on the creation of three-dimensional objects as a result of their layer-by-layer "cultivation" from various materials (plastic, photopolymer, metal powder, etc.). The starting point for creating copies of objects using 3D printing methods is their 3D computer models, which can be obtained as a result of 3D scanning or 3D modeling.

 Currently, additive technologies are being increasingly used in the creation of replicas of cultural heritage objects ^[18] – ^[21].  Usually replicas are made using FDM technology or using laser stereolithography technology. However, a fundamentally new direction in the application of additive technologies in museum work may be the restoration of monuments made of metals. To compensate for the losses of such objects, laser surfacing technology can be used, which allows you to recreate the lost parts of the monument from metal powders by melting them under the action of high-intensity laser radiation.  In one of the recent works of the author of this article ^[22], the possibility of restoring a monument made of cast iron as a result of the combined use of powder surfacing and laser welding was demonstrated (Fig. 7). This work is the first case of using laser surfacing to compensate for the loss of historical monuments made of metals.     

Fig. 7. A decorative element of a cast-iron fence of the XIX century from the Necropolis of the Alexander Nevsky Lavra in St. Petersburg: on the left is a general view, on the right is a reconstruction of one of the lost tips of the "star" as a result of laser surfacing

Conclusion

This article describes the results of the most interesting works on the restoration and reconstruction of sculptural monuments in St. Petersburg, performed using 3D laser scanning. The examples given in the article show the possibility of effectively solving complex tasks of modern museum work when using 3D scanning in combination with other innovative technologies. This opens up prospects for the widespread use of 3D laser scanning for digitization, virtual reconstruction, restoration and creation of copies of cultural and historical heritage objects.

The author expresses the hope that in the near future 3D scanning technology will be even more widely used in museum practice in our country.