Often, when artwork arrives for treatment at my studio, washing and stain reduction are required to reduce overall discoloration, as well as foxing and staining due to tape adhesives and moisture incursion.
Staining can be caused by a variety of factors, including:
Inherent Vice – Perhaps the paper itself or media contain impurities that trigger staining as the artwork ages.
Adhesives – Adhesive residue from tape, hide and starch-based glues used to stabilize tears and attach the artwork to its frame housing can cause staining over time.
Water Damage – If the work of art somehow got wet, related staining from water and mold damage may be present.
Acidic Framing Materials – I see this issue frequently. Backing boards made of cardboard or wood, and acidic mat board can all cause staining to artwork as they lay adjacent to it in its framed housing.
What can you do? Hire a professional to evaluate your artwork. If washing and stain reduction are warranted, a treatment plan can be determined to restore your work of art to its former glory, so it can be appreciated for many more years to come.