This dissertation discusses issues related to a tide-line on one of Edvard Munch s large-scale paintings, The Source, in the Oslo University Assembly Hall, the Aula. The Aula paintings were produced in the period 1909-1916, executed in oil on primed canvas. Both paint and priming layers are thin and lean, and the paint surfaces have never been varnished. The main focus and question for the study has been whether the materials in the water damaged area of the painting might have been affected more than visually, and whether change might have occurred to the material composition in the area. It might be possible that, varying with the medium and pigment composition of the paint, water may have a profound impact on the paintings materials. It is likely that tide-lines on a medium-poor surface, as on The Source, are the result of particles transported by water. The study has therefore focused on analyses of particles in the tide-line area, in an attempt to determine possible alterations in the affected area. The results from the analyses of the tide-line on The Source indicate that the content of some particles in the paint layers, like Zn, Al and Ca containing particles, varies between unaffected and affected areas of the painting, in terms of decreased or elevated levels of elements, thus demonstrating the effects of water on the painting materials. This may be related to the hygroscopic qualities of the pigments containing these elements (like zinc white and ultramarine) as well as to the fact that little binding medium binds and protects the particles from water. The reaction of painting materials to water might thus be both a chemical reaction, according to the hygroscopic properties of the present elements, and a more physical reaction where leanly bound particles are transported by the water. It is further suggested that broader analyses should be carried out to achieve a more in-depth understanding of the tide-line phenomenon in order to be able to make the right treatment decisions for these kinds of damages.