How Light and Humidity Damage Antique Paper

I have a small collection of antique maps of Yorkshire hanging on the walls of my study. I have owned them for about fifteen years, and in that time three of them have faded visibly, one has developed a crack across the paper along a fold line, and one has softened at the corners to the point of needing conservation work. The other seven, on the opposite wall, are more or less unchanged. The difference is that the seven face north, and the three that faded face south. That is what light does to paper, and it is worse than most people realise.

Handling gets blamed for a lot of paper damage that is really caused by environmental factors. Light and humidity are the two main culprits, and both of them work slowly enough that the damage is hard to see until it is done. This piece is about what actually happens, what the safe ranges are, and how to display antique paper without destroying it.

What light actually does to paper

Light causes two related types of damage: photochemical breakdown of the paper fibres, and fading of any inks or pigments on the surface. Both are cumulative, both are irreversible, and both are caused primarily by the ultraviolet and blue-violet components of light rather than by light in general.

The paper fibres in antique paper are largely cellulose, which absorbs UV energy and undergoes chemical reactions that weaken the fibre structure. Over decades this causes the paper to become brittle, yellowed, and eventually to lose its physical integrity. Modern paper has additives that slow this process; genuine antique paper, particularly rag paper from before 1850, has no such protection.

Inks fade at different rates depending on their chemistry. Iron gall ink, standard for handwritten manuscripts and many early printed works, fades from black to brown over centuries and continues to fade in light exposure. Printer's ink from copper plate engravings is more stable but still fades if strongly lit. Watercolour and hand-tinting on maps and prints is often the most vulnerable, particularly the yellows and reds, which can fade dramatically within a decade of exposure to daylight.

The damage is dose-based. A print hung in a bright south-facing room for ten years receives more total light energy than one hung in a dim north-facing room for a hundred years, and the damage scales accordingly. There is no safe minimum below which paper is unaffected; there are only more and less damaging conditions.

What humidity does, which is worse

Humidity damage is often more severe than light damage, and it works both up and down.

Excessive humidity, meaning relative humidity above about 65 percent, causes several problems. Paper absorbs moisture and becomes physically weaker. The cockling and rippling you see on damp prints is the fibres swelling unevenly. Mould can grow on damp paper, particularly in warm conditions, and mould damage is often irreversible. Foxing, the small brown spots on antique paper, is a form of mould-related damage and worsens in humid conditions.

Excessive dryness, meaning relative humidity below about 35 percent, causes the opposite problem. Paper loses moisture and becomes brittle. Folds crack, edges tear, and the paper loses flexibility. This is why antique paper survives better in temperate British houses than in either the dry heat of centrally-heated modern American houses or the humidity of tropical climates.

Fluctuating humidity is worse than either extreme. When paper expands and contracts repeatedly with changing humidity, the fibres are physically stressed at every cycle. Over years this leads to structural failure. A piece kept at a steady 45 percent humidity will survive better than a piece kept in an environment that swings between 30 and 70 percent seasonally.

The safe target for antique paper storage is roughly 45 to 55 percent relative humidity, with as little fluctuation as possible. This is easier to maintain in a dedicated storage space than in a working room, but it is achievable in most homes with a hygrometer to monitor conditions and a humidifier or dehumidifier to correct significant drift.

What the safe ranges actually are

Museum standards for paper storage and display are more stringent than most private collectors need to match, but they give useful guidance:

Storage conditions, meaning where paper is kept when not on display, should be at 18 to 22 degrees Celsius and 45 to 55 percent relative humidity. Light should be minimal or absent.

Display conditions, meaning where paper is exhibited on walls or in cabinets, should be at similar temperature and humidity, but with light levels limited to about 50 lux for sensitive items and up to 200 lux for less sensitive material. Sunlight through a normal window can exceed 10,000 lux on a bright day, which is far above what any museum would allow for paper display.

Practical implications for a private collector are:

Do not hang antique paper on walls that receive direct sunlight, ever. North-facing walls are usually safe; south-facing walls almost never are. East and west walls have brief periods of direct sunlight and need protection or should be avoided.

Rotate displayed items every six months or so if you have more paper than wall space. This gives each item recovery time in dark storage between exposures, and it lets you enjoy more of the collection over time.

Use UV-filtering glass or acrylic glazing for framed pieces. This reduces the damaging ultraviolet component of light substantially, though it does not eliminate visible-light damage. Good UV-filtering glass adds perhaps 30 pounds to the framing cost per piece and is money well spent.

Keep your home humidity monitored and stable. A good hygrometer costs 20 to 50 pounds and will tell you what conditions your paper is actually living in.

The advice that is well-meant but wrong

You will read, in home decorating magazines, that antique prints look best in bright sunny rooms where their colours can be appreciated. This advice destroys prints, and it is one of the most common ways that family collections are diminished over generations.

You will also read, in some conservation guides, that antique paper should never be displayed at all, and should always be kept in dark archival storage. This is technically the most conservative approach but it takes the point out of owning the material. Paper made for display was made to be looked at, and looking at it is part of what makes a collection worth keeping. The compromise is thoughtful display: rotating pieces, using good glazing, controlling the environment, and accepting that some slow damage is the price of enjoyment.

The final piece of common advice that misleads is the recommendation to conserve rather than replace. For high-value pieces, this is correct. For lower-value pieces, conservation cost often exceeds replacement cost, and a fading modern reproduction of a common map can often be replaced more cheaply than the original can be restored. Judge conservation spending by the value of the piece, not by an abstract principle of preservation.