Why foxing affects old paper and how to slow it

What foxing actually looks like

Foxing is the name we give to the reddish-brown speckles that appear on old paper — prints, maps, letters, book pages, watercolours and postcards. The spots are usually small and irregular, somewhere between a pinprick and a five-pence piece, and they often cluster around the edges of a sheet where moisture has lingered. The colour can range from pale tan through rusty orange to a deep chocolate brown.

It is worth learning to tell foxing apart from other kinds of marking, because the remedies differ. Foxing tends to sit in the paper rather than on it, and it usually shows as a soft-edged bloom rather than a sharp stain. Offset printing ink that has migrated, iron-gall ink corrosion, flyspecks, and tidelines from a historic flood can all look similar at a glance. If you hold the sheet up to a window and the spot seems to float inside the fibre, you are probably looking at foxing.

The term itself has nothing to do with animals. It most likely comes from the rusty, foxy colour of the marks, though some sources trace it to the old practice of "foxing" leather with ferrous sulphate. Either way, the name has stuck for two centuries and collectors have learned to live with it.

Where the spots come from

For a long time foxing was blamed on mould alone. Modern analysis suggests it is usually a combination of factors working together, which is why some sheets fox badly and identical sheets from the same batch stay clean.

• Moisture. Damp conditions allow mould spores to germinate and encourage chemical reactions inside the paper. Even short spells at high relative humidity can set foxing going.
• Metal particles. Iron and copper fragments entered paper through the rag stock, the water supply, or the machinery used in manufacture. These metals act as catalysts, and a speck of iron in the fibre can trigger a spot right above it.
• Mould metabolites. Certain fungi produce coloured by-products as they digest the sizing. The staining is often the residue of an infection that happened decades ago and has since gone dormant.
• Acidic sizing and alum-rosin. Papers sized with alum-rosin from the nineteenth century onward tend to be more vulnerable, because the sizing itself breaks down and releases acids.
• Airborne pollutants. Sulphur dioxide, nitrogen oxides and fine particulates all accelerate the process, which is why paper stored in smoky rooms or near busy roads often fox faster.

The important point is that foxing does not always mean the paper is actively decaying. Much of it is old damage that has stopped spreading. Spots that are dry, brown and stable are usually a cosmetic issue; spots that are soft, greyish, or accompanied by a musty smell suggest the environment is still working against you.

Why some collections fox faster than others

Two identical books, bought the same day and shelved in different houses, can age very differently. The variables that matter most are humidity, temperature, light and air quality, roughly in that order.

Attics are the worst offenders: they swing from cold to hot, and warm air holds more moisture than cool air, so condensation forms on paper surfaces as temperatures drop overnight. Basements and cellars are usually damp, and a damp cellar with no ventilation is a breeding ground. Rooms above a kitchen or bathroom, or against an unheated outside wall, can also run humid.

Light matters too, particularly ultraviolet. Sun will not create foxing from nothing, but it fades the surrounding paper and makes existing spots appear far more prominent, and it accelerates the breakdown of cellulose.

Slowing foxing with better storage

You cannot reverse foxing at home, and any attempt to bleach or wash spots out of a valuable print can do far more harm than good. What you can do is slow its progress, and that is largely a matter of controlling the microclimate around each item.

• Target 45–55% relative humidity. A simple hygrometer costs very little. Keep it near your collection rather than on the other side of the room.
• Keep temperatures moderate and stable. Somewhere around 16–20°C is comfortable for paper. Avoid rooms that fluctuate sharply, and never store paper against an outside wall.
• Use archival housing. Acid-free boxes, folders and envelopes made to archival standards will buffer your items against changes in the room. Avoid ordinary cardboard, which is itself acidic and will transfer damage.
• Interleave where it helps. Loose sheets can be separated with acid-free tissue or buffered paper. Do not interleave anything with a textured or absorbent surface right against an image.
• Choose the right sleeves. Polyester sleeves are inert and excellent for handling. Avoid PVC, which off-gasses and can fuse to paper over time.
• Keep air moving gently. A little ventilation discourages mould. Do not seal damp items into boxes; let them equilibrate first.

Handling, display and what to avoid

You can undo a lot of good storage work in five minutes of careless handling. Wash and dry your hands before touching paper, and use clean cotton gloves for metals and photographs. Support large sheets on a rigid board rather than letting them sag.

When framing, insist on acid-free mounts, spacers between the artwork and the glass, and UV-filtering glazing. Hang frames on internal walls away from radiators, direct sun and spotlights. Rotate displays so that nothing stays lit for years on end.

Do not store paper in plastic bags from the supermarket, do not press items under heavy books in a damp cupboard, and do not use household bleach, lemon juice, hairspray or sticky tape, however tempting. Tape is the single most common cause of irreversible damage we see in family collections.

If a piece is genuinely valuable — a signed map, an early watercolour, a family document with legal weight — a paper conservator can assess whether the foxing is active and whether washing, deacidification or local treatment is appropriate. For everything else, stable housing, steady humidity and a cool, dark shelf will do more than any clever trick. Foxing is a slow process. Slow it a little more and most collections will outlast us comfortably.