Multidisciplinary Investigation of the Imperial Gates of the 17th Century Wooden Church in S ă lis , ca, Cluj County, Romania

: One of the most beautiful elements of cultural heritage in Romanian villages is the Transylvanian wooden churches. Unfortunately, these heritage elements have experienced considerable deterioration. This paper presents the results of a project whose main goal was the scientiﬁc investigation of the Imperial Gates belonging to the wooden church in S ă lis , ca, Cluj County, Romania. To digitally preserve and document the Imperial Gates, a scientiﬁc investigation into the wooden support and the painting materials (ground, pigments) were performed using Fourier transform infrared (FTIR) and x-ray ﬂuorescence (XRF) spectroscopy. FTIR spectroscopy provided information about the wooden support, whereas XRF and FTIR methods were used to structurally characterize the painting materials. These structural data were correlated with the artistic, theological, and historical analysis of the religious heritage monument. Starting from the information acquired about the wooden support and the painting materials, the Imperial Gates were three-dimensional (3D) scanned using state-of-the-art laser scanners. The digitization of the Imperial Gates and the interior of the church was performed to maintain the current preservation status and disseminate this information to the public using online instruments.


Introduction
Romania was a country with lot of forests, leading to the creation of an enduring wood civilization that stretched throughout millennia. This fact is supported by the bas-reliefs visible on Trajan's Column where the houses of the Dacian people are shown. The columns are built from horizontal timber beams placed circularly; the same method that was preserved in popular Romanian architecture until present times. The same architectural style used by folk craftsmen to build cottages can also be found in buildings belonging to the Christian cult. The Romanian wooden churches represent a meeting place between popular and high art, containing records of civilization and cultural history, demonstrating the synthesis of the medieval spirituality of these people, and preserving treasures of Christian spirituality. These monuments of popular architecture must be considered as representing unity through diversity, The development of complex applications as previously presented [27,30] requires advanced knowledge in fields such as restoration and conservation [31], reverse engineering and new product development [32,33], and programming and 3D modeling [34], that can only be accomplished with the help of interdisciplinary teams.
The aim of this paper was to scientifically investigate the painting materials of the Imperial Gates of the wooden church from Sălișca to preserve and restore them in the digital environment. FTIR, XRF spectroscopy and 3D scanning procedures were applied for this purpose.

Investigation Methods
To scientifically evaluate the Imperial Gates of the Sălişca wooden church, an interdisciplinary team was created. The team included theologians, experts in the Romanian sacred art, experts in the arts, chemists, and engineers. Based on the methodology presented in Figure 2, the team followed a series of stages that allowed the identification of the pigments from the paint layer and the type of wood used to build the Imperial Gates. The working methodology is compatible with state-of-the-art procedures for digitization, restoration, maintenance, and digital reconstruction previously presented [38][39][40][41].  The development of complex applications as previously presented [27,30] requires advanced knowledge in fields such as restoration and conservation [31], reverse engineering and new product development [32,33], and programming and 3D modeling [34], that can only be accomplished with the help of interdisciplinary teams.
The aim of this paper was to scientifically investigate the painting materials of the Imperial Gates of the wooden church from Sălis , ca to preserve and restore them in the digital environment. FTIR, XRF spectroscopy and 3D scanning procedures were applied for this purpose.

Investigation Methods
To scientifically evaluate the Imperial Gates of the Sălişca wooden church, an interdisciplinary team was created. The team included theologians, experts in the Romanian sacred art, experts in the arts, chemists, and engineers. Based on the methodology presented in Figure 2, the team followed a series of stages that allowed the identification of the pigments from the paint layer and the type of wood used to build the Imperial Gates. The working methodology is compatible with state-of-the-art procedures for digitization, restoration, maintenance, and digital reconstruction previously presented [38][39][40][41]. The development of complex applications as previously presented [27,30] requires advanced knowledge in fields such as restoration and conservation [31], reverse engineering and new product development [32,33], and programming and 3D modeling [34], that can only be accomplished with the help of interdisciplinary teams.
The aim of this paper was to scientifically investigate the painting materials of the Imperial Gates of the wooden church from Sălișca to preserve and restore them in the digital environment. FTIR, XRF spectroscopy and 3D scanning procedures were applied for this purpose.

Investigation Methods
To scientifically evaluate the Imperial Gates of the Sălişca wooden church, an interdisciplinary team was created. The team included theologians, experts in the Romanian sacred art, experts in the arts, chemists, and engineers. Based on the methodology presented in Figure 2, the team followed a series of stages that allowed the identification of the pigments from the paint layer and the type of wood used to build the Imperial Gates. The working methodology is compatible with state-of-the-art procedures for digitization, restoration, maintenance, and digital reconstruction previously presented [38][39][40][41].  The methodology also focused on the digitization, digital restoration, and the promotion of the cultural heritage assets using online dissemination methods. In the first stage, the conservation status was analyzed to determine if the current state would allow their intervention and manipulation without damaging the Imperial Gates and whether they would be appropriate for classic or digital restoration. The second stage of the working methodology involved the theological and artistic analysis of the Imperial Gates to certify their value as a patrimony asset and to certify them as original assets. At this stage, we also identified the possible interventions of the restoration process of the Imperial Gates that could be performed in time. Simultaneously, the digitization and analysis of the wood support and the painting layer were performed. The digitization operation targeted both the context of the gate (the inside of the church and especially the altar) and the Imperial Gates themselves.
The analysis of the painting layer and of the wood support identified the pigment constituents to obtain these pigments by methods that are specific to the restoration process. XRF measurements were performed using an INNOV-X Alpha-6500 (Max Analyzer, Inc., Jakarta, Indonesia) portable instrument (35 kV voltage, 15 µA intensity, 3 mm filter, Be window, 2 square mm spot size, and PIN Si detector). Integration time was set for 60 s in two consecutive runs of 30 s each.
FTIR spectra were registered with a resolution of 4 cm −1 using a JASCO 6100 FTIR (JASCO 6100, Tokio, Japan) spectrometer in the 4000 to 400 cm −1 spectral domain by using the potassium bromide (KBr pure spectral powder) pellet technique. The spectra were processed by Spectral Analysis and Origin 8.0 software.
The 3D model resulting from the digitization, and the pigments identified in the physico-chemical analysis stage, represent the input data of the digital restoration stage, where the painting layer was restored using two scenarios: a digital restoration using a similar method to the classic state and a restoration of the painting layer with the original colors. The dissemination of the cultural heritage represented by the Imperial Gates was performed using online tools that enable real time visualization and interactivity with the 3D model to a wide audience.

Results
The Imperial Gates subjected to investigation ( Figure 3) are made from pieces of lime boards. By comparing its FTIR spectrum with those of lime in Figure 4 of Marutoiu et al. [21], the boards close to the middle of the border casements are made from the same material. On the upper part of the left door, the Holy Archangel Gabriel is painted, and on the lower part is Matthew the Evangelist. On the upper part of the right door, the Virgin Mary is painted, and on the lower part is Luke the Evangelist. The painter Popovici employed a small number of colors: red, white, green, black, and blue. The pigments were investigated using XRF and FTIR spectroscopy, whereas the wood was investigated only using FTIR spectroscopy.

XRF Results
X-ray fluorescence measurements were performed using an INNOV-X Alpha-6500 portable instrument (35 kV voltage, 15 µ A intensity, 3 mm filter, Be window, 2 square mm spot size, and PIN Si detector). Integration time was set for 60 s in two consecutive runs of 30 s each. Table 1 presents the XRF data for the investigated Imperial Gates. Gate no. 1 is on the left and Gate no. 2 is on the right.

XRF Results
X-ray fluorescence measurements were performed using an INNOV-X Alpha-6500 portable instrument (35 kV voltage, 15 µ A intensity, 3 mm filter, Be window, 2 square mm spot size, and PIN Si detector). Integration time was set for 60 s in two consecutive runs of 30 s each. Table 1 presents the XRF data for the investigated Imperial Gates. Gate no. 1 is on the left and Gate no. 2 is on the right.

XRF Results
X-ray fluorescence measurements were performed using an INNOV-X Alpha-6500 portable instrument (35 kV voltage, 15 µA intensity, 3 mm filter, Be window, 2 square mm spot size, and PIN Si detector). Integration time was set for 60 s in two consecutive runs of 30 s each. Table 1 presents the XRF data for the investigated Imperial Gates. Gate no. 1 is on the left and Gate no. 2 is on the right.

FTIR Results
FTIR spectra were registered with a resolution of 4 cm −1 using a JASCO 6100 FTIR spectrometer in the 4000 to 400 cm −1 spectral domain by using KBr (KBr pure spectral powder) pellet technique. The spectra were processed by Spectral Analysis software.
Copper ions were not detected, having small concentrations (<100 mg/kg). Based on this analysis (Table 1), the following composition of the painting materials is proposed: green results from the mixture of Prussian blue and yellow (orpiment; red-iron red, realgar, and lead red; brown-iron oxide; black-iron oxide; hallo-silver foil and bolus; and ochre-lead yellow). FTIR spectra of the investigated painting materials, together with the characteristic absorption bands of different components, are presented in Figures 4-7, including the whole spectral range and in several specific spectral domains.

FTIR Results
FTIR spectra were registered with a resolution of 4 cm −1 using a JASCO 6100 FTIR spectrometer in the 4000 to 400 cm −1 spectral domain by using KBr (KBr pure spectral powder) pellet technique. The spectra were processed by Spectral Analysis software.
Copper ions were not detected, having small concentrations (<100 mg/kg). Based on this analysis (Table 1), the following composition of the painting materials is proposed: green results from the mixture of Prussian blue and yellow (orpiment; red-iron red, realgar, and lead red; brown-iron oxide; black-iron oxide; hallo-silver foil and bolus; and ochre-lead yellow). FTIR spectra of the investigated painting materials, together with the characteristic absorption bands of different components, are presented in Figures 4-7, including the whole spectral range and in several specific spectral domains.    Table 2.    Table 2. Vibrational frequencies assignment was completed by comparing the obtained spectra with the standard spectra for ground, pigments, and binders that we obtained. The presence of proteins is evidenced by the absorptions at 3000-2800 cm −1 and~1645 cm −1 , also identifying the presence of egg yolk.
The employed painting materials included lead carbonate for white; red lead, arsenic red, and iron red for red; traces of Prussian blue; gypsum as ground; and proteins as binder.
The presence of gypsum and of oxalates was identified on, and in the wood (gypsum characteristic absorptions at 1157, 1120, and 700-500 cm −1 ) (Figure 8  Vibrational frequencies assignment was completed by comparing the obtained spectra with the standard spectra for ground, pigments, and binders that we obtained. The presence of proteins is evidenced by the absorptions at 3000-2800 cm −1 and ~1645 cm −1 , also identifying the presence of egg yolk.
The employed painting materials included lead carbonate for white; red lead, arsenic red, and iron red for red; traces of Prussian blue; gypsum as ground; and proteins as binder.
The presence of gypsum and of oxalates was identified on, and in the wood (gypsum characteristic absorptions at 1157, 1120, and 700-500 cm −1 ) (Figure 8, right side). The oxalate presence in the wood-lichens sample was demonstrated, especially when the 1800-1300 and 900-500 cm −1 spectral domains were analyzed (Figure 8, left side). The oxalate presence in the wood-lichens sample was demonstrated, especially when the 1800-1300 and 900-500 cm −1 spectral domains were analyzed ( Figure 8, right side).

Digitization
To obtain a detailed 3D textured model of the Imperial Gates, the structured light scanner Creaform GO!50 (2015, Creaform, Lévis-Québec, Canada) was used. The main purpose of this step was to acquire an accurate 3D model of the Imperial Gates that can be used for digital restoration and dissemination purposes. After the first phase of digitization, the result was processed within CAD software (Catia V5) and digital content creation software (3D Studio Max) to obtain the final 3D model. The final model represents a digital replica of the Imperial Gates that can be successfully used in various virtual reality/mixed reality applications. The first step in the preservation of Imperial

Digitization
To obtain a detailed 3D textured model of the Imperial Gates, the structured light scanner Creaform GO!50 (2015, Creaform, Lévis-Québec, Canada) was used. The main purpose of this step was to acquire an accurate 3D model of the Imperial Gates that can be used for digital restoration and dissemination purposes. After the first phase of digitization, the result was processed within CAD software (Catia V5) and digital content creation software (3D Studio Max) to obtain the final 3D model. The final model represents a digital replica of the Imperial Gates that can be successfully used in various virtual reality/mixed reality applications. The first step in the preservation of Imperial Gates was the digitization of the church interior using a Z+F IMAGER ® 5010X (2015, Zoller + Fröhlich, Wangen, Germany) terrestrial laser scanner (TLS) (Figure 9), with a resolution range of 0.1 mm, 320 • × 360 • field-of-view, with an HDR camera. The scanning of the interior of the church only served to digitally preserve the actual state of conservation of the wooden church. If the digitization of churches and their outside areas is required, digitization techniques must be combined. Normally, the TLS can be raised to a height of 5 m on the tripod, whereas an unmanned aerial vehicle (UAV) can be used for the upper elements of the church that are out of the scanner sight photogrammetry techniques, or other methods as previously presented. Gates was the digitization of the church interior using a Z+F IMAGER ® 5010X (2015, Zoller + Fröhlich, Wangen, Germany) terrestrial laser scanner (TLS) (Figure 9), with a resolution range of 0.1 mm, 320° × 360° field-of-view, with an HDR camera. The scanning of the interior of the church only served to digitally preserve the actual state of conservation of the wooden church. If the digitization of churches and their outside areas is required, digitization techniques must be combined. Normally, the TLS can be raised to a height of 5 m on the tripod, whereas an unmanned aerial vehicle (UAV) can be used for the upper elements of the church that are out of the scanner sight photogrammetry techniques, or other methods as previously presented. The scanning parameters were: resolution-super high, quality-high, 42 images acquired for texture mapping, 80 Mpx texture quality, points acquired-approximatively 6 million/scan position, and Smartlight activated. Seven points were used for scanning the interior, as shown in Figure 10. The first two positions overlapped; we used different resolutions and height for a better digitization of the Imperial Gates.  The scanning parameters were: resolution-super high, quality-high, 42 images acquired for texture mapping, 80 Mpx texture quality, points acquired-approximatively 6 million/scan position, and Smartlight activated. Seven points were used for scanning the interior, as shown in Figure 10. The first two positions overlapped; we used different resolutions and height for a better digitization of the Imperial Gates. Gates was the digitization of the church interior using a Z+F IMAGER ® 5010X (2015, Zoller + Fröhlich, Wangen, Germany) terrestrial laser scanner (TLS) (Figure 9), with a resolution range of 0.1 mm, 320° × 360° field-of-view, with an HDR camera. The scanning of the interior of the church only served to digitally preserve the actual state of conservation of the wooden church. If the digitization of churches and their outside areas is required, digitization techniques must be combined. Normally, the TLS can be raised to a height of 5 m on the tripod, whereas an unmanned aerial vehicle (UAV) can be used for the upper elements of the church that are out of the scanner sight photogrammetry techniques, or other methods as previously presented. The scanning parameters were: resolution-super high, quality-high, 42 images acquired for texture mapping, 80 Mpx texture quality, points acquired-approximatively 6 million/scan position, and Smartlight activated. Seven points were used for scanning the interior, as shown in Figure 10. The first two positions overlapped; we used different resolutions and height for a better digitization of the Imperial Gates.  The results of digitization can be seen in Figure 11. The results of digitization can be seen in Figure 11. The second step in digital preservation of the Imperial Gates was their digitization using a high accuracy laser scanner. The GO!Scan 50 ( Figure 12) scanner was used for a high-accuracy digitization, generating a mesh with 50 µ m precision on which the texture applied was 268 MPx. The result of the digitization process can be seen in Figure 13 (also Video S1). Both sides of the Imperial Gates were scanned using a single scanning session using the positioning targets ( Figure 12) and advance scanning methods.  The second step in digital preservation of the Imperial Gates was their digitization using a high accuracy laser scanner. The GO!Scan 50 ( Figure 12) scanner was used for a high-accuracy digitization, generating a mesh with 50 µm precision on which the texture applied was 268 MPx. The result of the digitization process can be seen in Figure 13 (also Video S1). Both sides of the Imperial Gates were scanned using a single scanning session using the positioning targets ( Figure 12) and advance scanning methods. The results of digitization can be seen in Figure 11. The second step in digital preservation of the Imperial Gates was their digitization using a high accuracy laser scanner. The GO!Scan 50 ( Figure 12) scanner was used for a high-accuracy digitization, generating a mesh with 50 µ m precision on which the texture applied was 268 MPx. The result of the digitization process can be seen in Figure 13 (also Video S1). Both sides of the Imperial Gates were scanned using a single scanning session using the positioning targets ( Figure 12) and advance scanning methods.   The next step was the digital restoration of the painted layer. Figure 14 illustrates the results of the digital restoration of the painting layer. The digital restoration accounted for two working hypotheses: one in which the painting layer is restored with the current nuances and one that uses the initial colors obtained based on the physico-chemical analyzes. Figure 14 also shows the result obtained for the second hypothesis. Having completed the digital restoration process, the texture was processed and remaped on the mesh of the model to integrate the digital model of the Imperial Gates into a virtual museum. The virtual museum (Figure 15) was developed in Unity and can be accessed on smart devices like interactive tables, phones, tablets, augmented glasses, and virtual reality headsets.  Figure 14 illustrates the results of the digital restoration of the painting layer. The digital restoration accounted for two working hypotheses: one in which the painting layer is restored with the current nuances and one that uses the initial colors obtained based on the physico-chemical analyzes. Figure 14 also shows the result obtained for the second hypothesis. The next step was the digital restoration of the painted layer. Figure 14 illustrates the results of the digital restoration of the painting layer. The digital restoration accounted for two working hypotheses: one in which the painting layer is restored with the current nuances and one that uses the initial colors obtained based on the physico-chemical analyzes. Figure 14 also shows the result obtained for the second hypothesis. Having completed the digital restoration process, the texture was processed and remaped on the mesh of the model to integrate the digital model of the Imperial Gates into a virtual museum. The virtual museum (Figure 15) was developed in Unity and can be accessed on smart devices like interactive tables, phones, tablets, augmented glasses, and virtual reality headsets. Having completed the digital restoration process, the texture was processed and remaped on the mesh of the model to integrate the digital model of the Imperial Gates into a virtual museum. The virtual museum (Figure 15) was developed in Unity and can be accessed on smart devices like interactive tables, phones, tablets, augmented glasses, and virtual reality headsets. Another form of promoting this cultural heritage was using the platform Sketchfab.com, where the users can visualize and interact with the 3D models using hot spots ( Figure 16). This platform also offers support for immersive visualization using a virtual reality headset.

Discussion and Conclusions
In our daily lives, we receive a mix of information from various digital media sources and the presence in this informational flux can be a success factor key to achieve a large audience. We believe that the presence of the Imperial Gates in the digital world can be the first step in their classic conservation and assist in the rescue and preservation of the wooden churches. Digitizing and publishing the Imperial Gates in an online 3D format increases the number of potential visitors to the churches where the Imperial Gates are located. We propose the digitization of as many Imperial Gates and wooden church interiors as possible so that they can be used in the online environment as marketing tools. Based on the 3D models available online and according to the interest of the tourists, theme cultural heritage trips could be organized.
The methodology presented is interdisciplinary in nature, and in this stage, the process cannot be automatized as it requires a high level of technical expertise, which, together with the cost factor, 3D data management could be a problem for large scale implementation [35].
FTIR spectroscopy established that lime was the wood used for the Imperial Gates. Oxalate presence was also detected in wood, including lichens systems, identifying the fungal attack on the wood. Based on XRF and FTIR data analyses, the composition of the painting materials was confirmed as: lead carbonate for white; red lead, red iron, and red arsenic for red; traces of Prussian Another form of promoting this cultural heritage was using the platform Sketchfab.com, where the users can visualize and interact with the 3D models using hot spots ( Figure 16). This platform also offers support for immersive visualization using a virtual reality headset.  Another form of promoting this cultural heritage was using the platform Sketchfab.com, where the users can visualize and interact with the 3D models using hot spots ( Figure 16). This platform also offers support for immersive visualization using a virtual reality headset.

Discussion and Conclusions
In our daily lives, we receive a mix of information from various digital media sources and the presence in this informational flux can be a success factor key to achieve a large audience. We believe that the presence of the Imperial Gates in the digital world can be the first step in their classic conservation and assist in the rescue and preservation of the wooden churches. Digitizing and publishing the Imperial Gates in an online 3D format increases the number of potential visitors to the churches where the Imperial Gates are located. We propose the digitization of as many Imperial Gates and wooden church interiors as possible so that they can be used in the online environment as marketing tools. Based on the 3D models available online and according to the interest of the tourists, theme cultural heritage trips could be organized.
The methodology presented is interdisciplinary in nature, and in this stage, the process cannot be automatized as it requires a high level of technical expertise, which, together with the cost factor, 3D data management could be a problem for large scale implementation [35].
FTIR spectroscopy established that lime was the wood used for the Imperial Gates. Oxalate presence was also detected in wood, including lichens systems, identifying the fungal attack on the wood. Based on XRF and FTIR data analyses, the composition of the painting materials was confirmed as: lead carbonate for white; red lead, red iron, and red arsenic for red; traces of Prussian

Discussion and Conclusions
In our daily lives, we receive a mix of information from various digital media sources and the presence in this informational flux can be a success factor key to achieve a large audience. We believe that the presence of the Imperial Gates in the digital world can be the first step in their classic conservation and assist in the rescue and preservation of the wooden churches. Digitizing and publishing the Imperial Gates in an online 3D format increases the number of potential visitors to the churches where the Imperial Gates are located. We propose the digitization of as many Imperial Gates and wooden church interiors as possible so that they can be used in the online environment as marketing tools. Based on the 3D models available online and according to the interest of the tourists, theme cultural heritage trips could be organized.
The methodology presented is interdisciplinary in nature, and in this stage, the process cannot be automatized as it requires a high level of technical expertise, which, together with the cost factor, 3D data management could be a problem for large scale implementation [35].
FTIR spectroscopy established that lime was the wood used for the Imperial Gates. Oxalate presence was also detected in wood, including lichens systems, identifying the fungal attack on the wood. Based on XRF and FTIR data analyses, the composition of the painting materials was confirmed as: lead carbonate for white; red lead, red iron, and red arsenic for red; traces of Prussian blue; gypsum as ground; proteins as binder; yellow-orpiment; and halo-silver. The corresponding pigments for each color were identical for both sides of the Imperial Gates.
As shown in Figure 14, the original colors were obtained in the laboratory, then digitized and processed as in the methodology proposed by Korytkowski and Olejnik-Krugly [39]. Using a commercial solution for image processing, we repainted the missing part of the painting with the actual colors used for "classic style" restoration. We replaced the current colors with the original colors, which we synthetized on the basis of physico-chemical analyses. Having these two versions, we could observed the footprint of time and external factors that influenced the painting over the centuries.
The digitization of the Imperial Gates using laser scanners was the first step in the digital restoration procedure and an effective method of preserving the cultural heritage represented by the Imperial Gates. The highly accurate model of the digitized Imperial Gates, combined with the physico-chemical analysis, provided a complete dataset that can describe the current conservation state of the Imperial Gates. Digitization is required for preserving this current state in digital format and for digital restoration. The high-accuracy model of the digitized Imperial Gates can also be used for dissemination of cultural heritage using the Internet. The 3D model can be seen on the project website or on Sketchfab: digitization of Imperial Gates (https://skfb.ly/IS89) and digital restoration version (https://skfb.ly/6ozOn).
The preservation and restoration of the wooden churches enable the introduction of the Romanian villages into international travel tours.