In Erik the Red’s Saga we are told in chapter 14 that in saga times the bottoms of ships were painted with 'seal tar' in order that shipworms would not gnaw through the wood and thereby sink the vessels. Unfortunately, the saga does not tell us what 'seal tar' consists of. Fritzner’s Old Icelandic Dictionary suggests a form of tar made from seal blubber. In historical times, train oil rendered from seal blubber, was much used as a wood preservative as it contains polyunsaturated fat in much the same way as linseed oil. As a result, a working group at the Viking Ship Museum began a practical experimental project, which in the first instance was based on porpoise oil.
The first step was the construction of a chemical database containing all available information, formulae and potential sources with regard to biocidal paint and the preservation of wood, known so far from the Viking period and the Middle Ages. This database will naturally be kept updated as research results and new information become available.
The museum then came into contact with the firm of Olsen Design Aps in Skærbæk who were already involved in carrying out practical attempts to develop a modern non-environmentally damaging bottom paint that could protect wooden ships against shipworms. With that intention, a number of wooden plates with samples of paint were to be placed in various Danish harbours. The Viking Ship Museum was given the opportunity to contribute with a number of samples at no cost in exchange for the company being able to place one of their sample rafts in the Viking Ship Museum’s harbour. Therefore four small clinker-built “sections of hull” were built in the museum’s boatyard – two of oak and two of pine. After priming with undiluted wood tar, these were divided up into six vertical “stripes”, which were then painted with different treatments, all with a base of wood tar and porpoise oil, as it was not possible to obtain seal oil in autumn. The porpoise oil was “rendered” from blubber from a porpoise which died of natural causes, obtained via the Zoological Museum in Copenhagen. As a control, four planks of pine and oak were sawn off, primed with wood tar and then treated with a modern chemical preservative, Hempel “Classic”, which has copper as the active preservative substance and is the treatment normally applied to the Viking Ship Museum’s sailing reconstructions.
On the 18th December 2001 the museum’s hull sections and planks were hung up in pairs (pine and oak), respectively in Skærbæk harbour where the salt content of the water, which is crucial for the presence of shipworms, is normally low, and in Hirtshals harbour, where the salt content is very high by Danish standards. The experimental set up was left in place for about a year, during which the water temperature and salt content in Skagerrak and Little Belt were plotted at regular intervals from the Danish Meteorological Institute’s website.
Olsen Design Aps have produced a detailed report: Projekt Anti Pæleorm. Miljøprojekt 838, Miljøstyrelsen 2003 (Project Anti Shipworm. Environmental Project 838, Danish Environmental Protection Agency 2003). It can be reported that there are shipworms in all the experimental wood from both Hirtshals and Skærbæk, as the occurrence of shipworms in 2002 had extended unexpectedly far into the Baltic and all the way to Bornholm.
The porpoise oil treatment had, however, clearly hindered the attacks relative to the untreated pine planks. The attacks on the wood treated with porpoise oil are characterised as 'slight to moderate', whereas those on the untreated wood are 'serious'.