An unusual surface change on the Viking ships exhibited in Roskilde

Five Viking ships, conserved with PEG, have been displayed for over 50 years in an exhibition hall with a large window section. Daylight could reach peak levels of up to 30,000 lux in summer. An unexplained phenomenon of a thin bluish-white layer forming on surfaces has been observed on the ship planks. It is distributed everywhere but most intensely on surfaces facing the windows. Light and UV were suspected to cause this, and the phenomenon was investigated so that a sufficient light level policy can be formulated. Monitoring of light and UV revealed annual light doses between 2 and 8 Mlux-hours. Blue-wool scales faded to step 5 in one year. Color measurements of areas cleaned and then exposed to exhibition light showed a color change (CIELAB1976 Delta E) of about 3 after one year, and about 7 after three years. A UV-filter retarded this change. The phenomenon was imitated on PEG impregnated wood samples by accelerated light and UV exposure tests. It is assumed that the discoloration is due to light and UV-radiation and that UV is at least equally contributing, and probably the main driver of it. An annual light exposure limit of 0.6 Mlux-hours with no UV-content was recommended.