The long soak
The conservation of six Viking Age wooden objects from the Oseberg ship burial took place 50 years after their excavation in 1904. As the delicate carvings could not be properly preserved by methods available in the early 1900s, these objects had to wait in tanks of water until a better method came along some time in the 1950s.
From 1904 to 1913 hundreds of wooden finds from the Oseberg collection were conserved using different methods, based on their extent of degradation from burial. The oak ship and a small number of other objects of oak, pine, yew and ash were basically allowed to dry slowly, as they survived burial better. Most objects of other wood types were so degraded that they had to be treated with alum salts – except for six. These six were not treated until the mid-1950s. Why was this? How were they treated? How did it go in the end? This is the story about the conservation of these objects, and of the disastrous fate of one, the animal head post, featured in the 2013 action film Gåten Ragnarok.
Objects recovered from the Oseberg ship were meant to sustain needs in the afterlives of the two women buried within. For the archaeologists, these remarkable objects divulged new insight about both the everyday and ceremonial lives of the Vikings. For conservators, the Oseberg collection also embodies the – at times – bumpy path of conservation history. Although these methods are no longer used today, they are nonetheless worthwhile to understand, as some have had a huge impact on how well the objects have withstood change over time, as with the ‘alum-treatment’, currently scrutinized in the Saving Oseberg project. Other methods were early versions of treatments currently in use.
Although a good method for most, the alum treatment – used at the National Museum of Denmark since the 1860s – blurred fine surface carvings, as shown in before and after alum-treatment photographs of the animal head post, named the ‘Carolingian’ (Figure 2).
A temporary solution
So, Professor Gabriel Gustafson (Figure 3), museum head and excavation leader, decided that the conservation of the 6 objects bearing the finest carvings – 4 animal head posts and 2 sled pulls – had to be postponed until a better method came along. This did not happen until the 1950s! The whole time they had to be kept wet.
Gustafson assumed that it would take a while before a good treatment would be developed for these. But his wish to display these beautiful objects drove him to think up an ingenious solution. He commissioned two specially designed aquariums to display two of the animal head posts in the new Oseberg salon in the Historic Museum in 1912 (Figure 4).
Over time, flourishing mould repeatedly clouded the water. Struggles in keeping the water clear are candidly expressed in notes Prof. Gustafson prepared for a talk he gave to the Chemistry group of the Polytechnical Society in 1913. His long list of chemicals tested includes ones which are rather dangerous, containing mercury and formaldehyde.
Tanks were diligently replenished over the years, even during World War II, when museum objects were packed and stored out of harm’s way. It was during the War that one tank sprung a leak, completely destroying the animal head post it held, the 'Baroque master's last' (Baroque head no.2, Figure 5). Luckily, thanks to Gustafson’s foresight, it had already been documented through a copy, carved by Jørgen Eriksen (Figure 6).
Fifty years(!) later, chemist and conservation scientist Annemor Rosenqvist was faced with the challenge of conserving the remaining 5 objects. What did she do? Once again, the National museum of Denmark inspired the use of a new treatment.
In Copenhagen, groundbreaking research on treatments for highly degraded wood led to the use of a new drying technique – using low pressures (vacuum). This was a far gentler technique than conventional air drying as it can remove substances from the frozen, solid state directly as a gas. That is, we completely avoid the liquid phase, which can cause irreversible damage – evidenced by the destruction of the animal head post, back in the 1940s.
Due to limitations of drying chambers available to museums in the 1950s direct removal of the water in the wood was not possible – it had to be replaced by something else. For our five remaining Oseberg objects, tertiary butanol was used. Unlike water, t-butanol does not change volume upon freezing. This property, along with others made it an ideal choice for the Oseberg objects.
Warm baths of liquid t-butanol penetrated the wet wood, replacing the water. Removing the object from the bath cooled it, ‘freezing’ it, and thus kept the shape of the object. Under vacuum drying, the t-butanol was then removed, directly from its frozen state (Figure 7 and 8).
Wait a minute…if the t-butanol was removed, what was left supporting the wood? Well…the answer is nothing! So after treatment, Rosenqvist brushed on dilute adhesive solutions to strengthen it more (Figure 9). But these adhesives mainly stayed on the surfaces, leaving the inner wood without any extra support. Needless to say, they are extremely lightweight and fragile. She estimated that when newly made, the animal head post weighed between 2.3 kg. After treatment, they weighed less than 1 kg, due to degradation from being buried for almost 1100 years.
Rosenqvist completed the treatment of these 5 objects in time for the opening of the new Oseberg wing at the Viking Ship Museum in 1957. Today we can enjoy these final additions alongside the remaining treasures from the Oseberg ship, including the animal head post treated with alum.
The Oseberg collection has many stories to tell, depending on how you look at it. The conservation story is a fascinating one, as it demonstrates wise judgement – the decision to wait for a new treatment was, in my eyes, rather exceptional, as was the decision to invest in copies of the five animal head post, which apparently cost a mint. This tells about the often invisible research behind the scenes, to develop new and better preservation techniques. Finally, it tells about the importance of displaying the real thing. Gustafson could have chosen the far easier route and displayed the wet animal head posts through photographs – but he didn’t. From these early experiences, research in conservation has moved forward. Drying chambers today can freeze water-based mixtures. Polyethylene glycols (PEGs) are most commonly used today to replace most of the water in the wood. PEGs are non-toxic and much safer than t-butanol, which is actually highly flammable! Moreover, PEG remains in the wood, supporting it even after drying.
And what about the destroyed animal head post? Well, it mainly lives in its box in the museum stores. However it was briefly displayed in connection with the Gåten Ragnarok premiere in 2013. Despite its fragmented state – and its supernatural associations devised for film – it remains a very useful object for conservation research: both in the 1950s as test pieces for surface coatings, but also more recently when samples were extracted for advanced chemical analyses in régi of the Saving Oseberg project, as a reference to understand more about the chemical degradation caused by the alum-treatment in other objects.
Come out to the Viking Ship Museum and see for yourself! Can you pick out the animal head posts treated by freeze drying? Can you find the one treated with alum, the one with ‘blurred’ carvings?
- Brøgger, A. W., H. Shetelig & H. Falk (1917): Osebergfundet. Oslo: Distribuert ved Universitetets Oldsaksamling.
- Christensen, B. B. (1970): The conservation of waterlogged wood in the National Museum of Denmark: with a report on the methods chosen for the stabilization of the timbers of the viking ships from Roskilde Fjord, and a report on experiments carried out in order to improve upen these methods. Copenhagen: The National Museum of Denmark.
- Gustafson, G. (1913): Foredrag i Polyteknisk forenings kjemikergruppes plenumsmøte, tirsdag 4 November 1913. Museum of Cultural History Archives.
- Hougen, B. (1957): En skisse av Vikingskipshusets historie. Museumsnytt, 3, 2-8.
- Rosenqvist, A. M. (1957): Nypreparering av tresakene i Oseberg fundet. Museumsnytt, 3, 12-15.
- Rosenqvist, A. M. (1959): The Stabilizing of Wood Found in the Viking Ship of Oseberg: Part II. Studies in Conservation, 4, 62-72.