The effect of ageing and the effect of the conservation treatments of washing and bleaching with the reduction agent sodium borohydride were investigated. Qualitative evaluation of the degradation of the paper at the location of the brown lines is done in daylight, under UV at 366 nm and staining with methylene blue dye. The results showed that any wet/dry interface, even a brief exposure, may be a potential cause of degradation. Ageing resulted in a drastic darkening of the brown line and insolubility of the brown compounds. The compounds formed at the wet/dry interface were identified as containing carbonyl and acid groups. Reduction with borohydride was found effective to decrease the browning of freshy formed brown lines exclusively. Washing was also found effective only on recent brown lines and had to be done in the liquid that originated the brown line (water or organic solvents). Results showed that conservation processes involving local or global wetting of paper documents are a potential source of browning. Practical recommendations are given for the conservator, such as the frequent observation of paper artifacts under UV wavelengths to detect where browning could appear on ageing.