Polyetylene glycol (PEG) treatment

When the Vasa was salvaged in 1961, no safe established method was available to prevent shrinkage when drying waterlogged wood (Håfors 2001). A filling material that would not evaporate should replace the water in the wood, especially when degraded by bacterial erosion processes. Morén and Centerwall at Mo och Domsjö AB had developed a method to prevent crack formation in fresh wood by impregnation with polyethylene glycol (PEG) in aqueous solution (Morén 1960). This method seemed suitable also for marine-archaeological wood, but had not been tested in larger scales, and the Vasa was the first major object for which PEG was used.

Polyethylene glycol, PEG, in aqueous solution was sprayed for 17 years on the Vasa's hull in the temporary shelter "Wasavarvet". The PEG molecule penetrates degraded cells of waterlogged wood, replaces the water and hinders shrinkage when drying. The molecular formula above shows PEG with short chain length, n = 5 in: HO-(CH₂CH₂O)_n-H (molecular mass = 238), which efficiently can penetrate the wood structure. Photo: The Vasa Museum

During the spray treatment of the Vasa a large number of core samples were analysed for PEG content, to establish suitable PEG concentrations and molecular mass in the conservation lliquid (Barkman 1975, Håfors 1990). In the 5-10 cm thick oak board planks, the analyses showed an overall mean PEG ratio of about 30%, evaluated from the weight of PEG divided by the dry fibre weight. In the much thicker heavy timbers the mean PEG ratio was found to be less than 10%. PEG treatments are now commonly applied, also in modern freeze-drying conservation procedures, to stabilise the dimensions of waterlogged wood. The museum shipwrecks of the Bremen Cog, Henry VIII's flagship the Mary Rose, the East Indiaman Batavia in Western Australia, and the Skuldelev Viking ships in Roskilde, Denmark, are examples of shipwrecks impregnated with PEG.

Properties of PEG

The polymeric PEG molecules posess several unusual characteristics. 1. Water solubility: Oxygen atoms (O) join -CH₂CH₂- units into a chain with terminal hydroxyl (-OH) groups, HO-(CH₂CH₂O)_n-H, see Figure above. Pure polyethylene glycol with a mean molecular mass higher than about 600 (PEG 600 has n ~ 13) is a waxy solid at room temperature, but the large number of oxygen atoms in the chain makes also longer PEG molecules water-soluble. 2. Ion-conductor: Even solid PEG can act like a solvent and has been tested as an ion-conducting medium for solid-state batteries (Gray 1997). The moist surface layer of the wood of the Vasa with high PEG content probably acts as a gel electrolyte - a solid solution that allows ion transport. Thus, salt precipitation can occur at the surface when water evaporates at decreasing humidity.

When the PEG chain is short its ability to penetrate waterlogged wood increases. However, wood impregnated with short chain PEG is more hygroscopic with higher tendency to take up and retain water. PEG 4000 (trade name for a PEG mixture with about 90 units in the chains, n ~ 90) and PEG 1500 (n ~ 34) in aqueous solution were sprayed during the first years (1962-1971) on the exposed surfaces of the Vasa's hull, followed after 1971 by PEG 600 (n ~ 13) to increase penetration. The PEG concentration was increased gradually, from an initial 10% to a final 45% before ending the spray treatment in 1979. No PEG was discarded in the closed-circuit automatic spray system operating from 1965 to 1979, and during that time 240 tonnes of PEG was consumed. In the final stage the outside surface of the hull was hand-sprayed with a 45% PEG 4000 solution (Håfors 2001). Pure PEG 4000 is a white, wax-like solid that melts at about 56 ^oC. It is not very hygroscopic, even though the solubility is about 50 % in water at room temperature. Surplus PEG 4000 on the Vasa's wooden surfaces was melted away by means of a hot-air blower, but can still be visible as white streaks and spots, mostly on the inside of the hull.

The mass spectrum is obtained for a surface sample from a PEG 4000 treated board on the Vasa’supper gun deck Each peak corresponds to a certain molecular mass. The difference between the major peaks is 44 mass units, which corresponds to one -CH₂CH₂O- entity (n ± 1) in the PEG chain. The three clusters of peaks with mean values of about 615, 1450 and 3920 mass units show that commercial compounds labelled PEG 600, PEG 1500, and PEG 4000 consist of a distribution of molecules, and that the PEG 600 from inside the board has penetrated into the PEG 4000 surface layer.