The Oseberg collection consists of about 900 objects and fragments. They are in different states of preservation, varying from relatively well-preserved to very degraded. Today the wooden objects in the worst condition are extremely fragile and brittle. Their varnished surfaces disguise the highly disintegrated inner wooden fabric – a thin crust consisting of layers of resin and oil, remaining wood and alum crystals holds the objects together.
Wood composition
Wood consists of three main components: cellulose, hemicellulose and lignin. Its composition can change over time due to biological, physical and chemical degradation; wood is susceptible to the presence of various organisms, chemicals and changing climatic conditions. In some objects from the Oseberg find almost no cellulose and hemicellulose remains and even the structure of lignin has changed.
Wood degradation: alum-treatment
The main reason for such degradation is the alum-treatment; it is an active deterioration process that causes both chemical and mechanical damage to wood. During treatment in the early 1900s the wood was immersed in a hot alum solution; this heating caused decomposition of alum and consequent formation of sulphuric acid. Today the alum-treated wood presents high levels of acidity; the pH is around 1–2. When comparing alum-treated wood with other wooden objects from the Oseberg find, significant differences in their current chemical composition can be observed; the high acidity of alum-treated wood caused degradation of both carbohydrates (cellulose and hemicellulose) and lignin.
Furthermore, the uneven distribution of alum inside the wood and the generation of salt precipitates on its surface caused formation of cracks and voids (V); the high concentration of deposited alum crystals on the surface resulted in mechanical tensions at boundary zones having lower concentrations of alum crystals. This weakens the wooden structure and causes long-term stability problems.
Other consequences of alum treatment
Alum-treated objects can no longer be thought of as only wood, since the alum salt now makes up a large part of them. In many cases it is the main thing holding the wooden object together. The alum-treatment has also affected metal parts of the objects such as iron nails and pins. These have corroded in the acidic, alum-rich environment and reacted to form iron salts, which can potentially contribute to wood degradation by behaving as catalysts.
Detecting the damage: Imaging
Several imaging techniques are used to investigate the inner structure of the Oseberg objects, including:
- Light microscopy
- Scanning electron microscopy
- Conventional X-ray radiography
- Synchrotron X-ray microtomography
Understanding the damage: Chemical analysis
The degradation of wood components is investigated using analytical methods including:
- Fourier transform infrared spectroscopy
- Pyrolysis coupled with gas chromatography and mass spectrometry
Analyses show that very low relative amounts of holocellulose (cellulose and hemicellulose) remain in alum-treated woods from Oseberg. The lignin in these samples is also highly degraded, with unusually high contents of oxidized units, when compared to other samples. Inorganic materials (e.g. alum, iron salts) are detected by methods such as:
- Inductively coupled plasma optical emission spectroscopy
- X-ray diffraction
- Raman spectroscopy
- Scanning electron microscopy with energy dispersive X-ray spectroscopy
Analysis from an object shows aluminium, potassium and sulfur from alum-treatment, along with iron, copper and zinc from metal parts and storage in metal tanks.