Often stains are caused by pictures getting damp and the soluble brown colouring from old paper moves with the water leaving tide lines across the paper. When this has happened washing the area with water will generally reduce the stain. Sometimes more permanent colours from the paper are moved as well which are harder to remove and require the use of alternative treatments.

Unfortunately when pictures get damp they often go unnoticed for some time leading to mould growing. The stains caused by mould are much harder to treat and can rarely be removed altogether. The mould also attacks the fabric of the paper leaving it considerably weakened in those areas. If a pictures gets wet it is always best to remove it from its frame as quickly as possible to let it dry. In warm conditions damp paper will grow mould very quickly and certain types of paper and pictures, particularly photographs, will stick to the glass if left.

Stains from sellotape and masking tape can usually be reduced with solvents but it can depend on the age of the tape and the medium used in the image.