Yellowish-brown burn lines are caused by acids released as the Lignin in pulp products breaks down Alum, an acidic binding agent often used in board manufacture, accelerates the breakdown. The acids cause artwork to discolour and become brittle. Dirt Air-borne pollutants combine with moisture and produce acids readily absorbed by the artwork. Mounting Damage Pressure sensitive tapes contain solvent. If they are used for framing, the solvent will soak into the artwork.
Some tapes lift from the artwork causing it to fall from the mount.
Other tapes permanently bond causing damage if they ever have to be removed. Foxing Reddish brown patches caused by a combination of metallic salts, mould, high humidity and high temperatures. Condensation and bleed Moisture from condensation behind the glass promotes mould growth and can cause colours to bleed.