draftiscs.icomos.org/pdf-files/newyorkconf/trudso.pdf · 2014-10-01 · draft 12th international...

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12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012

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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 here 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

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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 left Gorm’s small rune stone dated 950 AD and to the right Harold’s larger rune stone dated 965 AD.

Figure 2. The rune stones in Jelling by winter time. In the front Gorm’s rune stone and to the back Harold’s rune stone.

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 fringe 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.

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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 determined by a Tritop camera from many positions. Two examples show the waves of the fixed points seen from two camera positions and one example show one point seen by many camera positions.

Figure 4. The scanning of Harold’s rune stone in Jelling is underway with Atos IIe. On the top surface, in the foreground, a number of small circular stickers are fixed. These fixation points around the entire surface together are the cloud of 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 registered 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

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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 collected in a PC and could immediately be seen on the screen.

Figure 6. An area at the bottom of the animal side of the large rune stone is being scanned. 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.

Figure 7. Light scanning of Gorm’s rune stone.

Figure 8. Light scanning of the text side of Harold’s rune stone.

Figure 9. Light scanning of the side with Christ on Harold’s rune stone.

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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. Another possibility could be to compare with already existing copies of the rune stones.

The rune stones in Jelling have been copied countless times. Way back in 1889

Harold’s rune stone was copied for the first time in order be an eye-catcher in the Danish pavilion at the World Exhibition 1889 in Paris. This copy is still in Paris, but it is in a sad state of preservation and not useful for comparison. The same is the case with the copies places in London, Rouen, Tokyo, Utrecht, Los Angeles and in three Danish provincial towns. However, the very successful copies that were prepared in 1984 and stored in The National Museum could be used. For this casting silicon rubber was used, and the support mould was made by epoxy reinforced with plywood. As a slip agent a thin layer of methyl cellulose was applied. After hardening of the epoxy shells the support moulds were separated and the rubber cast was cut and peeled off and the methyl cellulose was washed away again. The copies were made of epoxy adding crushed granite and reinforced with a plywood skeleton. The copies of Gorm’s and Harold’s rune stone were light scanned in the same way as the original stones

Figure 10. The copy in Paris. Due to obvious degradation it is unsuitable as the basis for a comparison.

Figure 11. The copy in London. Due to the copying process the material has a rough surface and thus unsuitable as a basis for a comparison.

For the comparison between originals and copies Atos software program, version

6.0.2 was used. The requirements for this comparison include

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• an area designated in accordance with agreed criteria, for example in areas with material loss and exfoliation, areas sanding or areas around cracks,

• a tolerance for deviation must be fixed before the smoothing, so that large deviations in small areas are not included in the smoothing. The tolerance was set at 1.5 mm between the surfaces.

• the smoothing is carried out, during which the differences within the tolerance are averaged

• deviances between the remaining surfaces, not included in the smoothing, are calculated

• a map of the comparison of the area is produced, by which is clear and consistent differences between the surfaces (the original stone and copy), are visualized as colour contour lines with a variation from material loos and material growth.

The maps with the colour curves have a variation from red to yellow and green to

blue. Green indicates that there is no difference between past and present. Yellow and turquoise shows that there could be a difference up until 1.0 mm. So beyond the smoothing tolerance happened no verifiable change has happened since 1984. Red and orange indicate different degrees of material loos since 1984, and similarly in the opposite end of the scale blue indicate that there is more material now than in 1984. This could occur if fillings had been carried out, black crust had been growing or fouling of lichens has taken place. It should be noted that marked differences in colour or grey colour in the edge of the map does not indicate a change, but rather indicates that it was not possible to select the exact same areas on the original as well as the copy. Data from the border areas must consistently be omitted in evaluating the comparative results. The white dots in the comparison areas are the fixing points from the light scans. All comparison areas are naturally selected to confirm or refute concerns regarding development of damage in the period between the time of the casting and today.

Figure 12. The top of the text side, where the rock is intersected by a myriad of cracks.

Figure 13. The base of the animal side with a convexity in the middle, which is adjacent to the cracks and an area of exfoliation.

Figure 14. The base of the Christ side with an area of exfoliation.

4. Results

On Gorm’s rune stone 12 areas were compared, seven areas from the front side and five areas from the back of the stone. Approx. 50% of the total surface area was

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compared. There has been no change along the cracks. At a single point it was found that there had been exfoliation of approx. 3.5 cm x 1,5 cm x 0.9 cm, namely at the front in the central rune line in an area adjacent to a very large area with exfoliation and at the same time just beside a very large cavity. Furthermore, during the mapping and the study of the state of preservation it was found that the area of exfoliation developed over centuries and in conjunction with the finding of a large cavity the change observed by the comparative light scanning caused great concern. The development of the deterioration is therefore still ongoing.

On Harold’s rune stone 19 areas were compared corresponding to approx. 20% of the total surface. Along the cracks were not detected changes beyond the tolerance of 1.5 mm on any of the rune stones three sides. This can be seen in figure 12-14.

Figure 15. Comparison of light scans on Gorm’s rune stone with material loss on the front side in the middle of the central rune lines.

Figure 16. The material lost has occurred close to a crack and a very large area of exfoliation.

Figure 17. Comparison of light scans of the text side of Harold's rune stone, where a fragment has been lost from the lower rune line.

Figure 18. The material loss had happened in an area, where there are many cracks along the carved runes.

The comparison of the light scanning are clear and unambiguous, as can be seen in

figure 17. There has been exfoliation of an area in the bottom line on the text side of

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approx. 3.0 cm x 3.0 cm x 0.9 cm. Subtracted the uncertainty is at least approx. 7 mm thick fragment of decreased has taken place since 1984.

As an extra benefit of the light scanning maps showing exfoliation may be turned in the PC, showing the exfoliation in two levels. At first superficial exfoliation only takes place to a certain depth, exposing new surface. This process is then repeated in the newly exposed area, but not necessarily the whole area, by which a second level of exfoliation is created. It is easier to observe this phenomenon on the PC than visually on the stone, because the colours in the scan are neutral and differs in depth land therefore appear more pronounced. 5. Discussion

The whole idea of comparing light scans of the rune stones in Jelling with light scans of old copies was very optimistic and the expectations high, but it turned out not to be quite as simple as that. Due of the large amount of data it was not possible to compare the original stones with the copies as a whole. In order to get manageable amounts of data the light scans had to be ‘cut up’ into smaller areas, corresponding to the approx. 30 x 40 square cm of the surface.

From other projects, it was known that it could be difficult to compare old plaster copies with cast burrs from the gypsum wedge mould. Therefore copies could only be compared in small areas, corresponding to the individual moulding parts. Furthermore, earlier the material for a casting would often have been clay and during the casting process these imprint in clay may become wavy or twisted slightly, which is also reflected in the contemporary comparisons of light scans.

The castings of the rune stones in Jelling in 1984 were immediately expected to be better than in previous comparisons of the scans, because there were very large casting areas without dividing lines, due to the use of silicone rubber. Another fact to remember is that epoxy expands slightly during curing with approx. 1%, wherefore it was necessary to reduce the data from the copies to the same level as the originals. Another factor was the application of a slip agent, which constitutes a small, perhaps only theoretically, source of inaccuracy.

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Figure 19. The side of Christ during casting. "Snaps" in the silicone rubber can be seen in the support mould, which is under construction.

Figure 20. Comparison of the area around the legs and feet of Christ. Differences of up to ±2-5 mm were found. The differences are not sudden and abrupt as the refusal, but undulating and thus show that the copy of this 12 m2 area is not exactly accurate, but still of very high quality.

The results of the comparisons was found to detect a further factor in the moulding

of the copy. The comparisons showed that the surfaces of the high quality copies waves slightly. This is absolute not visible to the naked eye on the major surfaces, and in fact it is just a few millimetres. One explanation for this wave seems to be that in order to ensure a good contact between the silicone rubber and the supporting moulds some "snaps" were established. Here contact was full, because the rubber blanket was fixed in the supporting moulds. These snaps were scattered over the surface with 20-30 cm intervals. Before the copy could be casted, the mould was placed horizontally. Probably some slim air pockets had arisen between the rubber blanket and the supporting moulds. However, there were no air pockets at the points, where the snaps were. The rubber blanket was not draped in full contact with the supporting moulds, but is raised a few millimetres over the surface in a low wave motion. Of course the weight of the cast material, the epoxy where had added crushed granite, to some extent pressured the rubber blanket been down in the supporting mould, and thereby diminishing the wave effect to a certain extent, but not completely. Another factor might be that since the whole copy of Harold’s rune stone was 12 m2 it was for practical reasons it was made in three parts. After the casting the three parts were combined together into a complete copy of Harold’s rune stone, and this could have resulted in some stresses and twisting of the surface of the copy.

The idea of ‘just’ to compare two scans - a snapshot of today with the contemporary current form and an earlier snapshot through a copy with the contemporary shape - in practice turned out to be not quite so simple. The hope was that the state of preservation physical extent, before and now could be compared. But despite the emerging problems the method still have given results and advantages. As long as the comparisons express

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waving variations this may be attributed to imperfections in the copy phase, and these variations must be considered as sources of error. However, if the variation in the comparison are sudden and abrupt it may be attributed to real change, which means material loss or gain. One should therefore not only look at the quantitative differences in the comparison of light scanning, but also look at the nature of the differences. The changes in Figure 15 and Figure 17 may be considered to be abrupt and therefore, effective exfoliation. Growing cracks do not cause major physical changes in material loss in the surface, which means that changes can easily be so minor that it is accommodated within tolerances.

As also suggested by others an optimal basis for comparison would, , be light scanning the original artefact with intervals of years and then compare the original with the original, however the waiting time before results are obtained can then be long. In Jelling the comparing light scanning useful proving that the deterioration had been going on for the last 25 year and could be expected to continue. This proved to be one of the most important arguments together with the climatic investigation, the mapping of the actual state of preservation and human factors as well. Therefore the decision was to make protect both rune stones in two separate showcases.

6. Conclusions

The light scannings has given a 3D documentation of the physical extent of both rune stone in the form of digital copies. The methodology has been non-destructive, except the fixing of small fixed points superimposed on the surface. These were removed again without loss of mineral grains. Unlike the previous copying the precision in the documentation is considerably greater.

Comparative light scanning of an original stone with an older copy is not a simple and easily accessible method, but a possibility. Practical experience shows that the copies can include some measuring barriers related to expansion, inaccuracies as twisting and "waves" and cast burrs, which crucially can define the comparison area. Despite these complications the methodology may generate results in cases where abrupt changes can be observed, because it demonstrates exfoliation that may otherwise be difficult to determine by other means. Furthermore, a comparison may also document when there has been no change within the tolerance. This allows comparative light scanning to give an indication on which the state of preservation can be evaluated to be stable relatively to exfoliating. Changes in the form of growing cracks cannot be detected due to the minimum material loss which is less than the tolerance. Overall, there comparing light scanning achieve certain information and documentation in the case of comparing the original with a copy.

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Figure 21. In 2011 two protective showcases were establish in-situ in Jellling.

Acknowledgements

The condition survey was supported by The Velux Foundation. References Boochs, F.; Huxhagen, U. & Kraus, K. 2008. ‘Potential of high-precision measuring for

the monitoring of surfaces from heritage objects’. In In situ monitoring of monumental surfaces, Proceedings, Firenze 2008, SMW08. Firenze: Edifir – Edizioni. 87-96.

Trudsø, S., Jensen, K.S., Madsen, P.K. 2008. ‘How bad is it and why. Study of the preservation of the Cat’s Head Door in Ribe Cathedral and the rune stones in Jelling’. In Nationalmuseets Arbejdsmark 2008, 239- 256. (English summery)

Trudsø, S. 2010. ‘3D lightscanning as documentation and comparative methodology’. In Meddelelser om konservering 2010, 2, 3-14. (English summary)

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