Oil media on paper: investigating the morphological and chemical changes of linseed oil-impregnated paper mock-ups upon ageing

Books, archival material and works of art on paper, present specific types of damage, when the oil binders in oil paints and traditional oil-based inks are absorbed by the paper supports, such as discolouration, loss of mechanical strength and drop of pH. These problems have been recorded in diverse case studies, differing in intensity and extent, setting questions for the input of materials, paper and oil type. The combination of paper and oil could raise substantial concerns for the preservation of works upon ageing, since the condition of the oiled areas of the paper support may, deteriorate leading to local or overall fragmentation. A recent PhD research project involved the investigation of the changes in the optical, mechanical and chemical properties that occur on the paper–linseed oil system upon ageing, aiming to the interpretation of damage recorded on works of art and printed materials, due to absorption of oil into the paper substrate [1,2]. Results showed that linseed oil has the predominant role in the changes of the system upon ageing, but the pulp content of the paper has a significant input to changes development. Application of Transmittance Fourier-transform infrared spectroscopy (FTIR) analysis provided more information on the chemical changes that occur upon ageing and the rate of deterioration, depending both on the linseed oil formulation and the types of paper. Analysis indicated that the presence of an alkaline buffer to the paper pulp, reduces the rate of deterioration, as it deters the formation of acids that are, possibly, consumed to formulate metal soaps of fatty acids [3]. Research also involved the study of the morphological properties of the system, to record the alterations that happen upon the deterioration of the system and support the changes in the optical and mechanical changes. SEM images demonstrated that the application of linseed oil after drying creates a united mass with the fiber net, while a film covers the surface of the impregnated paper, which gradually reduces into the fibre net upon ageing, exposing the fibres at the final stages. However, the rate and the extent of the reduction of the linseed oil, as well as the changes on the surface of the film and the evident deterioration of the system, differ between the sets of mock-ups impregnated, having first been prepared with three linseed oil formulations and three paper types. The results were supplemented by images of optical microscopy and digital microscopy.