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12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 3D-LIGHTSCANNING OF THE TWO RUNE STONES IN JELLING, DENMARK - A TOOL IN THE DOCUMENTATION OF THE STATE OF PRESERVATION Susanne Trudsø The National Museum of Denmark, Department of Conservation, Brede, DK-2800 Kgs. Lyngby. E-mail: [email protected] Abstract The two important rune stones in Jelling had been exposed to the natural climatic conditions for more than a thousand year and there was a growing concern for their state of preservation. A condition survey concluded that there was an urgent need for protecting the stones from further degradation. Included in the survey project was a three dimensional documentation of the two rune stones and 3D light scanning was used for this purpose. Due to the existence of old copies of the rune stones light scans of originals and older copies could be compared. The result were maps, showing similarities and differences, which could be read directly from these maps. The comparative light scanning revealed small inequalities and confirmed that loss of surface was an ongoing process, but difficult to detect with the naked eye. In this way the 3D light scanning became an important tool, proving the deterioration as a slow, but still active and not acceptable process. As the stones could not be conserved by existing methods they now are sheltered in-situ in two showcases, first and foremost in order to avoid humidity and frost. Keywords: rune stones, 3D lightscanning, 3D documentation, in-situ stone monuments, copies, comparative light scanning, documentation of exfoliation 1. Introduction In recent years the Conservation Department at The National Museum in Denmark has had several projects, where the state of preservation of a number of very important stone monuments in Denmark has been investigated and evaluated. One of these projects concerned a condition survey and a three dimensional documentation of the two major rune stones in the village of Jelling in Denmark. Both rune stones are of gneiss and date back to the Viking Age about 950 and 965 AD. The rune stones are situated between two gigantic mounds and within the remains of the largest stone ship in the world, just in front of the entrance to the 11th century church. The small rune stone was erected by King Gorm the Old and has text on two sides, commemorating his Queen Thyra and the first known reference to the name ‘Denmark’. The large rune stone is erected by their son, King Harold Bluetooth. This rune stone has tree sides, all with both text and decorative reliefs and the rune stone is also known as "the birth certificate of Denmark" because DRAFThere King Harold emphasizes his royal deeds by the words "…and made the Danes Christian”. These words are located below the figure of Christ with cruciform halo and winding weeds. This motif is the oldest depiction of Christ in Scandinavia. The two other sides are one with mostly text and one with a large 1 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 advancing animal, surrounded by a snake. The Jelling site as a whole is inscribed in the UNESCO World Heritage List. Figure 1. The rune stones in Jelling. To the Figure 2. The rune stones in Jelling by winter left Gorm’s small rune stone dated 950 AD time. In the front Gorm’s rune stone and to the and to the right Harold’s larger rune stone back Harold’s rune stone. dated 965 AD. For more than 400 years there has been considerably historic interest and awareness for this location in Jelling, however an exact investigation of the state of preservation of the rune stones has never been carried out. A condition survey was undertaken in 2006- 2007 by The National Museum and it consisted by a number of in-situ investigations including documentation of geology, climate, fouling of lichens, the impact of a nearby linden, mapping the deterioration and in addition studies in the archives at the National Museum. The state of preservation for both rune stones was critical and especially big cavities below the surface caused worries. Included in the project was also a three dimensional documentation of the two rune stones including changes to their state of preservation over time using the 3D-documentation. The first question was how to provide this 3D-documentation. The projects in general did not include the testing of different monitoring methods and selecting the most suitable method. A number of methods were assessed based on written documentation of experience gained, on which basis the right choice of method had to be made. The considerations included high-resolution digital stereo photogrammetry, laser scanning and light scanning, of which 3D light scanning was chosen. 2. Three dimensional light scanning in practice Light scanning is an optical measurement system based on triangulation and scanning by a fringeDRAFT projector. The basic product is a digital set of coordinates where each measuring point is logged by an x-, y- and z-coordinate. All points are mutually related, whereby it is possible to use the information in an analytical context. 2 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 The equipment used was an Atos IIe light scanner and a Tritop camera. First, a local spatial coordinate system was established by fixed points, small circular stickers that were temporarily attached to the rune stones. These fixed points were placed evenly over the surface with a distance of 20-30 cm. In addition to these a number of larger measurement points were placed around the monument in the initial registration. With the handheld Tritop camera, a high resolution photogrammetric digital camera, the exact spatial positions of the fixed points as well as the registration marks were registered by optical measurements from different angles. The digital pictures looks at fixation points from different angles and compiles them into a software program to a well-defined 3D spatial coordinate system of the surface of the monument with an uncertainty of ±0.01 mm. The result of the scanning is a cloud of reference points from the surface of the monument with exact x-, y- and z- positions. Figure 3. Point cloud of fixed points Figure 4. The scanning of Harold’s rune stone in determined by a Tritop camera from many Jelling is underway with Atos IIe. On the top positions. Two examples show the waves surface, in the foreground, a number of small of the fixed points seen from two camera circular stickers are fixed. These fixation points positions and one example show one point around the entire surface together are the cloud of seen by many camera positions. points in the spatial coordinate system. Afterwards the Atos IIe light scanner emits stripes of white light of varying width from a light source, while two high-resolution cameras record the distance to the surface of the stone with an uncertainty of ±0.02 mm. On the basis of these observations the system automatically generates 3D coordinates for each camera pixels. From these generated 3D points the surface between the points is established by the PC. In this way it is possible to compare with a 3D curve model obtained by conventional surveying with the important difference that the accuracy is much greater, each square mm of the surface is registeredDRAFT with a number of points. In this case 9 measuring points per square mm were selected, which means that the absolute accuracy is ±0.022 mm. The equipment allows choosing a much higher density of measuring points than traditionally used. However, one must bear in mind that the amount of data subsequently becomes 3 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 gigantic if many square meters are to be light scanned. The limiting factor is the size and data power of the PCs in order to process the images afterwards. During the actual scanning the light scanner is positioned at different angles relatively to the surface, until the whole surface is scanned. Each measuring area is up to 40 x 50 square cm and must contain at least four fixed points. Data is collected continuously, whereby it is possible to follow the registration simultaneously on a PC. Data and points already recorded and registered are then diluted. Figure 5. In parallel with the scanning data was Figure 6. An area at the bottom of the animal collected in a PC and could immediately be side of the large rune stone is being scanned. seen on the screen. The vertical striped light is only barely visible in the bright light. All surfaces above the ground on both rune stones were light scanned. Gorm’s rune was scanned from a total of approx. 250 positions and Harold’s rune stone from approx. 600 positions corresponding to approx. 1 billion points. DRAFT Figure 7. Light scanning of Figure 8. Light scanning of the Figure 9. Light scanning of Gorm’s rune stone. text side of Harold’s rune stone. the side with Christ on Harold’s rune stone. 4 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 3. Comparative light scanning – degradation over time Repeated light scanning of the same object with intervals of some year would make it possible to compare scans in a software programme, whereby it would be possible to identify even the smallest change in the surface. However, having to wait for a decade or two before repeating and comparing the scans and perhaps finding a partial difference would be a long time to wait.