Alum treated finds from the Oseberg Collection Wood preservation by conservation?

Transcription

1 Alum treated finds from the Oseberg Collection Wood preservation by conservation? Susan Braovac, PhD Museum of Cultural History, UiO, Saving Oseberg

2

3 ph ca 1

4

5 Problems with the alum treatment are similar in other collecctions: acidity, mechanical weakness, etc.

6 Saving Oseberg is an interdisciplinary research project based at the Museum of Cultural History (KHM). Runs from 2014 to aims to improve our understanding of alum-treated wood chemically, morphologically, hygroscopically, mechanically, which will allow us to develop safe methods to stabilize the finds for the future. The project is funded by the Ministry of Education and Research and the University of Oslo.

7 Saving Oseberg - people and institutions Core group: 5 chemists (+1 PhD student), 2.5 conservators. Home: Museum of Cultural History, Dept. of Collection Management University of Oslo, Dept. of Chemistry, Prof. Tore Benneche University of Pisa Dept. of Chemistry and Industrial Chemistry, Chemical Science for the Safeguard of Cultural Heritage Group, Prof. Maria Perla Colombini University of Florence/ CSGI (Research Center for Colloids and Nanoscience), Prof. Piero Baglioni University of Nottingham, School of Biosciences, NCMH (National Centre for Macromolecular Hydrodynamics), Prof. Stephen Harding NIBIO (Norwegian Institute of Bioeconomy Research), Erik Larnøy, PhD. We also involve expertise from other museums and specialists in our reference group.

8 Archaeological wood and its conservation Buried wood is usually found in a waterlogged state. It has undergone bacterial decay (anaerobic). Both lignin and carbohydrates are deteriorated to varying extents. Some factors affecting condition of wood upon excavation: wood type, soil type (porosity, ph), oxygen content of soil. It has often absorbed metal ions (Fe, Cu) from the soil or from metal objects lying in proximity. Metal ions can lead to chemical instability. Water supports the wood structure. If highly degraded wood is allowed to dry out, it would collapse. The weakened cells cannot withstand these forces.

9 buried 834 AD excavated in 1904 Objects conserved and reconstructed from ca

10 Oseberg woods Extent of deterioration was not only dependent on burial conditions, but also on wood type. In the Oseberg find, the oak, ash, pine, yew were in relatively good condition, and were essentially dried without treatment

11

12 The other wood types, such as maple, birch, beech?, hazel etc..they needed to be treated before drying at this point in time, there were few choices available, and alum salts were used.

13 The alum-method was used to conserve the most degraded wooden finds (mainly diffuse porous woods) Alum-treated Waterlogged Air-dried Conservation Fragments immersed in a hot (90 C), concentrated bath of alum salts for about 24 hours Alum = aluminum potassium sulfate - 12 hydrate KAl(SO 4 ) 2 12H 2 O

14

15

16

17

18

19 P A+L CT-scan of saddle

20 Alum did not penetrate completely: cracks and voids formed

21 Chemical degradation likely due to treatment: Heating alum solutions produces (alot of) acid! ca 20 C 2 KAl(SO 4 ) H 2 O 2 K + + Al(OH) 2+ + Al(SO 4 ) 1+ + H 3 O SO 4 2- ph 3.5 In hot solutions this reaction ALSO occurs 90 C 3 KAl (SO 4 ) H 2 O KAl 3 (SO 4 ) 2 (OH) K + (aq) + 4 SO 4 2- (aq) + 6 H 3 O + ph 2 30x MORE ACID is MADE AT 90C! Acid is absorbed by the wood and starts degrading it

22 Sound aspen Alum-treated Oseberg X-ray microtomography + 3D reconstruction, Paul Scherrer Inst. Villingen, Switzerland

23 X-ray image detail, Gustafson s sled, showing corroded metal pins used to hold fragments together in the reconstruction. This demonstrates the corrosiveness of the environment inside the wood.

24 Today, alum treated wood is as strong as crispbread Bending strength 2.7 Mpa 2.5 Mpa Young s Modulus 610 Mpa 380 Mpa [Hoffmann and Schwab, 2002]

25

26 % fragments / objects Distribution of the condition of the fragments and objects from the storage survey (n= ca 2300) % % 23% % 6% best worst Condition category (visual assessment)

27 Fresh birch Archaeological birch Untreated Alum-treated Oseberg sample

28

29 Increasing oxidation of remaining polymers (mainly lignin) Principle component analysis Comparing IR-spectra from several samples Alum-treated, heavily degraded birch Frag Frag-1A Frag-8 Frag-5 Alum-treated, less heavily degraded Untreated archaeological woods 185-2, alder aspen Decreasing holocellulose content

30 Further chemical characterization of the wood and inorganic components is still ongoing, to ensure we will have an accurate overall picture of the situation. Material chemical information will be combined with information gained from retreatment trials on original Oseberg material, currently underway. Our main focus is to address the high acidity and reduced strength of the wood. Main methods used in Saving Oseberg: Infrared and Raman spectroscopy Analytical pyrolysis (Py-GC/MS) Gas chromatography (GC-MS) Evolved gas analysis-mass spectroscopy (EGA-MS) Scanning electron microscopy energy dispersive Xray spectroscopy (SEM-EDS) Xray Diffraction (XRD) Inductively coupled plasma- optical emission spectroscopy (ICP-OES) Colour measurement, L*a*b* (spectrophotometer) RH testing (wood-water-polymer interactions) Tomographic techniques to document retreatment tests (3D scanning, photogrammetry) Xradiography Photography Mechanical testing.. Etc.

31 For example, we are testing alkaline nanoparticles (Ca(OH) 2 ) to reduce acidity in those cases we cannot immerse the fragment in water. We are also testing different consolidation materials to strengthen the wood both existing materials and new materials developed in SO. We must also develop methods for evaluation of retreatments. Test results will lead to a Retreatment Protocol for the collection.

32 Summary -Alum treated woods have shown extreme chemical and mechanical deterioration initiated by alum treatment. -Variable deterioration likely related to type of surface coatings and/or amount and type of reactive metal ions. However this work is being investigated further. -Retreatment tests are underway on original Oseberg material. Aim is to neutralize (by nanoparticles or through immersion) and strengthen wood (existing and new consolidants developed in Saving Oseberg ). -test results will be used to develop a retreatment protocol

33 Acknowledgements The entire Saving Oseberg team Ministry of Education and Research University of Oslo AND Thank you for your kind attention!