Deterioration of Paper: Causes and Prevention
The assumption is often made that if a paper is 100% rag, it is also acid-free and permanent But this is not necessarily so, and the reasons require some explanation. It is also assumed that an acid-free paper will not deteriorate, but acidity is only one of several factors which act on the destruction of paper.
The internal factors which affect paper deterioration are established during the manufacturing process. They are: type and quality of the fibers, sizing material and the presence of acidic compounds. A paper described as 100% rag is made of cotton fibers, cooked and beaten into a consistency papermakers call rags. The beating process causes cotton fibers to interlock, thus creating the paper's strength. Cotton fibers are nearly pure cellulose.
In order to assure fiber bondage in the presence of moisture, most papers undergo internal sizing. The sizing determines the paper's resistance to water penetration and abrasion, creasability, finish, porosity, printability and surface bonding strength. This is accomplished with a number of different agents, such as rosin, animal glue and gelatin, starch, modified celluloses, synthetic resins, etc.
At one time, sizing was done primarily with rosin, held to the fibers by an astringent crystalline substance called alum (aluminum sulfate). Alum introduced acid to the previously neutral fibers. For a 100% rag paper to be acid-free, it must be sized synthetically, or with a process which requires no alum.
The potential permanence of paper is the responsibility of the manufacturer. In order to ensure this, the following ingredients should be excluded from the papermaking process:
- Ground wood fibers
- Concentrations of iron
- Copper minerals
- Alum/rosin sizing
- Residual bleaching chemicals
- Acidic compound
The symbol pH is a measure of the hydrogen ion concentration of a water solution and substance, and denotes if it is acid or alkaline. A substance with a pH value of 7 is neutral. Acids have pH values below 7 (1-6). Alkalis have pH values above 7 (8-14.) So-called permanent papers are weakly alkaline, with a common pH from 8-10.
Most permanency specifications call for a paper with a pH of 6.5 or higher. An acid-free matboard, for example, has an actual pH of 6-7. If there are no residual alkaline salts in the board, it may become increasingly acidic with time. To increase longevity, the board can be buffered, a process which adds in an alkaline earth carbonate (e.g. calcium carbonate) at the pulp stage.
Buffered boards have a pH of about 8.5 and an alkaline reserve of approximately 1-3%, and thus have more potential permanence.
While pH is the most crucial internal factor responsible for a paper's permanence, it should not be considered alone. Pulp type and quality, initial sheet strength, sizing agent and alkalinity must also be checked to ensure paper permanence.
Just because a paper is not 100% cotton, acid-free or buffered, it does not necessarily mean that it will not survive a long life. It also does not mean that you should no longer buy, or enjoy creating art, on papers which do not meet strict archival standards.
Besides the internal factors inherent in the papermaking process, there are also external factors which contribute to the deterioration of artists' papers, such as light, temperature and humidity, air pollution and insects.
Paper, like all organic material, is subject to the conditions under which it is used and stored. Therefore, the longevity of works of art on paper, given that the quality of the paper is good, is ultimately the responsibility of the artist and the collector. Due to the expense of handmade, acid-free and buffered paper products, it is certainly worth taking note of the paper enemies which reside within our homes, studios and galleries. tight. With all other factors being equal, works of art on paper will last longer if kept in the dark. Since they cannot be enjoyed and appreciated without light, it is wiser to think of ways in which direct light, with the irreversible chemical changes it imposes on paper, can be minimized.
Light has visible and invisible effects on paper, both of which can cause embrittlement and eventual deterioration. The most obvious visible effect of light is bleaching-the whitening and fading of paper color and some of the colors used to create the image. All light fades works of art on paper and fading is not reversible. Less light means less fading, and not no fading.
While this is occurring, invisible damage is also taking place. The ultraviolet rays in sunlight and fluorescent light cause chemical changes in the paper and accelerate the process of fading. Most of the light energy entering a room is converted to heat, but the minute fraction which is absorbed by paper is responsible for a process called photocatalyzed degradation. Paper which has been sized and exposed to dyes, pigments and dirt is vulnerable to complex photocatalyzed reactions. After oxidation occurs, the fibers in the paper are broken down until they are too short to stay bonded and embrittlement ensues.
The effects of deterioration initiated by light will continue even after the paper is removed from direct light. To minimize the effects of light, consider the following tips:
- Avoid hanging pictures on a wall directly opposite windows, since the light there is greater.
- Use louvered blinds or translucent curtains to moderate or redirect sunlight during the brightest hours.
- Rotate the position of your pictures around the house every year to decrease the chances of fading.
- Never work on, or hang, pictures in direct sunlight.
- Cover your fluorescent lights with protective sleeves which filter out radiation.
- Replace your picture frame glass with ultraviolet-filtering Plexiglas.
- Varnish paintings with picture varnish containing a UV-absorber.
These two factors work as a team on the destruction of paper. Evidence proves that the lower the storage temperature of paper, the longer it lasts. The useful life of paper is approximately doubled with every decrease of 10' F. For homes and studios, temperatures between 65'-70' F are optimal Relative humidity is the ratio of the amount of moisture in the air, at any temperature, to the amount required at that temperature to saturate the air.
Current research suggests that paper will last the longest when kept at a relative humidity of 45%. Ranges of 50-70% are considered acceptable for private homes and studios. Some climates may require a winter humidifier and summer dehumidifier to maintain this standard.
If humidity is too low, it can cause the desiccation and eventual embrittlement of paper. Too much humidity accelerates the growth of mold and the internal decomposition of paper.
Mold is nourished by sizing and paper fibers, and can also feed on the binders used in pastels. The presence of rusty-colored patches indicates that foxing -- chemical action of mold on the metallic salts in the paper due to prolonged, high humidity -- is occurring. Exposure to direct sunlight for 1 hour, or enclosure in a closed container with thymol crystals for 2-3 days, will kill mold. Thymol is a fungicide, available at chemical supply houses and some drug stores. Since thymol softens oil paint, it should not be used on oil paintings.
While low temperatures are best suited to the storage of paper products, consistency of temperature is critical. Temperature/humidity fluctuations, called cycling, can often be more detrimental to paper than consistently high temperatures.
Cycling weakens, and eventually breaks down, the fibers of paper by causing expansion and contraction due to water contained within it.
Changes in temperature/relative humidity should be minimal (10' F and 10%), and gradual Some ways to prevent mold and minimize deterioration due to heat and humidity are:
Maintain the lowest, most consistent temperature you possibly can between 60-70' F.
Keep humidity between 45-70%.
- Avoid hanging pictures on the outside facing walls, especially if they feel cold or damp.
- Avoid framing pictures directly against glass or Plexiglas without a spacer or mat, as this provides an excellent surface for moisture and mold.
- Dust framed pictures regularly, as airborne mold spores are contained within dust.
- Keep frames away from the wall by inserting small cork squares in the two lower corners of the frame. This creates good circulation and reduces chance of mold growths.
- Avoid leaving pictures in a closed room or house for extended periods of time without some form of circulation or dehumidification.
- Never hang pictures over a heating register, air duct or fireplace.
Large urban and industrial areas promote a process called sulfation, which occurs when flue gas constituents (water, carbon monoxide, sulfur dioxide, etc.) react to form sulfuric acid. Sulfur dioxide gas is absorbed by paper, where it reacts with moisture to create a destructive sulfuric acid problem within the fibers.
This is called acid hydrolysis -- the reaction of an organic compound with water in the presence of acid. This process breaks down the cellulose, causing discoloration, embrittlement and disintegration of the paper fibers. At normal temperatures, it occurs very slowly, but at elevated temperatures, the process is greatly accelerated.
Sulfuric acid does not evaporate or leave the paper, even after removal from the contaminated area.
Other major pollutants include industrial smoke and particulate matter, photochemical oxidants and haze, motor vehicle emissions, solar radiation, temperature inversions and sodium chloride (prevalent in coastal areas). Solid particles found in polluted air accelerate the deterioration of paper. Dust and grime are abrasive to the surface of paper. In the presence of moisture and in combination with the acid droplets around dirt, they promote a corrosive process which penetrates the fibers as well.
Many compounds are not dangerous themselves, but they promote paper deterioration because they form acids when they mix with atmospheric moisture.
If you live in the urban environment, the only way to combat air contaminants is through an air conditioning and filtration system.
While it is initially an expensive outlay, air conditioning with proper filtration will reduce restoration costs and capital losses of objects, and prove a good investment in time.
Paper contains ingredients, such as gelatin and glue sizing, wood pulp and flour paste, which are appealing to insects. Cockroaches, silverfish, termites and woodworms are the most common destroyers of paper. Silverfish love dark places and can make nice little homes inside frames as they nibble on sizing and wood pulp. Termites and woodworms like anything made of cellulose, as well as wood, so a rag paper picture in a wood frame is susceptible. Cockroaches can cause damage to paper by eating glue sizing or any painting media containing sugar. To help prevent infestation of harmful insects:
- Clean picture frames regularly.
- Never store works of art in dark, damp places such as basements or attics.
- Use screens on windows and doors to minimize insects.
- Use powdered or aerosol insecticides if signs of insects appear.