The conservation problems associated with prints and drawings are often described as being either “internal” or “external”. Internal conservation problems can afflict any work of art on paper if it has been created using chemically unstable materials or in a way that is inherently unsound. Examples include poor-quality wood-pulp papers, bleaching residues from paper manufacture, acidic sizings, light-sensitive dyes, or a combination of incompatible media. Given proper care, however, the negative consequences of these internal sources of degradation can be significantly lessened.
External conservation problems are those caused by outside factors, such as improper mounting, matting, and framing, overexposure to light, dangerous temperature and humidity fluctuations, and, too often, human error. These take a far greater toll on prints and drawings than do internal problems. By eliminating external sources of degradation, most damage can be prevented or, at least, minimized.
Most harm occurs in mounting, matting, and framing prints and drawings using commonly available, inexpensive and labor-saving materials such as wood-pulp mat board, corrugated cardboard, rubber cement, white glue, dry-mounting adhesives, masking and “Scotch” pressure-sensitive tapes, brown gummed tape, and the like. One characteristic these materials share is their chemical instability. As they deteriorate over time, acidity is generated and migrates into the naturally absorbent paper of the artwork with which they are in contact. Acids attack the chemical bonds of the long flexible cellulose molecules that make up most papers and break them into shorter and shorter segments. As a result, the paper darkens and becomes brittle in these areas. “Mat burns”, tape stains, and dark striations from corrugated cardboard are evidence that the artwork is in contact with an acidic material. In order to protect prints and drawings from such damage, they should be matted only with rag board, buffered rag board or “conservation” board, all of which have a neutral or alkaline pH (a pH of 7 or above). As the name implies, rag board is made from cotton fibers; it usually has a neutral pH. If it has been improperly sized or stored under poor conditions, however, 100 percent rag board can become acidic. Buffered rag board is alkaline because a small percentage of calcium carbonate has been added. Conservation board, also called archival or acid-free mounting board, is made from wood pulp that has been chemically purified and buffered. Buffered boards are effective longer because their alkaline reserves maintain a higher pH as they neutralize acid from the surrounding environment. Thus they can help slow down the internally generated degradation of drawings done on acidic wood-pulp papers.
Wood-pulp mat board--a stiff, bonded sheet of bleached wood pulp compressed between two sheets of facing paper--should be avoided at all costs. Though it is attractive for matting inexpensive reproductions, it should never be used to mat valued prints and drawings. If silk mats or decorative “French” mats are used, the side of the mat that touches the artwork should be lined with acid-free mat board or paper.
Conservators generally prefer to “hinge” drawings into mats, using small strips of folded long-fibered Japanese tissue applied using an adhesive of purified rice or wheat starch paste. These papers and the starch necessary to make the paste can be purchased at many art- or library-supply stores. Pressure-sensitive tapes, double-
sided tapes, brown gummed paper, and all synthetic adhesives should be avoided. Recently a new family of tapes, advertised as being archival, has been introduced. Although their adhesives are significantly improved in terms of their chemical stability, with time they become insoluble in water and require organic solvents for their removal.
Photo corners made of clear polyester film (Mylar ) or acid-free paper are especially useful for attaching prints and drawings into mats because no adhesives come into direct contact with the artwork. These can be made at home or purchased.
If matting a collection of prints and drawings is not feasible, at the very least they can be stored between neutral glassine or tissue or slipped into folders made from acid-free paper. These can then be put into a sturdy solander box (specifically made for museum storage) or one constructed from acid-free corrugated cardboard. Artworks on paper should never be wrapped in cellophane, wax paper, newspaper, stored against wood or cardboard, or curled inside tubes.
Frames provide additional physical protection for prints and drawings. The matted artwork is fitted into a frame with the remaining space filled with acid-free corrugated cardboard or acid-free polystyrene core board. The perimeter of the backboard is sealed with gummed-paper tape to prevent dust penetration.
When prints and drawings are framed it is important that they not come into contact with the glass or acrylic sheet used for glazing. A standard 4-ply mat board window is usually sufficient for this purpose; however, if the drawing is buckled, a thicker mat made from 8-ply or two pieces of 4-ply may be necessary. Spacers or fillets fitted around the rabbet of the frame (the “shelf” on which the glazing rests) can provide a breathing space for unmatted or “floated” prints and drawings. This breathing space is crucial for the dissipation of trapped humidity. In areas where the drawing is in contact with the glass or acrylic sheet, moisture can easily condense and foster mold growth. Foxing, characteristic small brown spots, can result.
The choice of glass or acrylic sheet for glazing should also be made with conservation considerations in mind. Drawings done with powdery media such as pastel, charcoal, chalk, or heavy graphite are susceptible to the static charge created when acrylic is wiped. Loose pigment particles can be literally transferred from the paper to the plastic glazing. Drawings with flaking paint, as is common with heavily applied gouache, should likewise not be glazed in proximity to acrylic sheeting.
Environmental factors such as temperature and humidity levels should be as constant as possible. Drastic fluctuations in both are more dangerous than gradual shifts in climate. A temperature range of 60-70 degrees Fahrenheit and a relative humidity level of 45-55 percent plus or minus five percentage points are more reasonably maintained in a home than one exact level. When relative humidity rises above 65 percent, action should be taken since the danger of mold growth is great.
The effects of overexposure to light on paper are both drastic and irreversible. Cellulose deteriorates and colors fade or change completely. Light-sensitive media include the paints used in hand-colored prints, the inks in Japanese prints, iron-gall inks, pastels, watercolors, and the colored dye-based inks of felt-tip and ballpoint pens. Papers can become dark brown or fade depending upon their constituents.
When controlling light, it is useful to consider the three factors which will determine the degree of damage exposure may or may not cause.
While all light is dangerous, certain wavelengths are more harmful to paper and colorants than others. A portion of the near wavelength spectrum, ultraviolet light is invisible to humans but is especially destructive to both paper and organic media. UV light is found in high proportions in daylight and in slightly smaller amounts in fluorescent lighting. Incandescent (tungsten) light bulbs do not produce appreciable amounts of UV light. Tungsten halogen lights, currently in favor in museums and galleries, contain more UV light than tungsten incandescent lamps. The rule of thumb is, the bluer or “cooler” the light, the more UV it contains.
Methods of removing UV light from rooms where works of art on paper are exhibited are straightforward. Physically blocking out all daylight with curtains, removing fluorescent tubes and substituting incandescent light is one logical way. Filtration is another method. Filtration is accomplished by the use of plastic filters, which are available in many forms. Exterior windows and skylights can be treated with UV-filtering film applied directly to the glass. Flexible blinds can be hung in windows. Panels of UV-filtering acrylic can be positioned in front of windows or suspended below skylights. Cylinders of similar filtering material can be slipped around fluorescent tubes. Finally, prints and drawings can be framed behind UV-filtering acrylic sheet except in cases where powdery media make this inadvisable. These ultraviolet-filtering rigid acrylic sheets are sold under the names UF-3 , UF-4 (Rohm and Haas) and OP-2 (Cyro Industries). Ultraviolet-filtering glass is now also available, as are a variety of nonreflective versions of the glass.
The intensity of light and the amount of exposure time should also be considered when displaying prints and drawings. In the United States light intensity is usually measured in footcandles; in Europe it is measured in lux or lumen units. The safest level of illumination for works of art on paper has been much debated. It is generally agreed that 5 to 8 footcandles of incandescent light with no UV present from other sources is acceptable for limited amounts of time. This figure represents an uneasy compromise between the need to exhibit an object, the ability to see it, and the desire to protect it.
The effects of exposure to light of any type or level are cumulative. For this reason prints and drawings should never be permanently displayed. Three months of exposure time per year per drawing is a prudent policy adopted by many museums. For private collectors, regular and frequent rotation of prints and drawings is advisable. Once again, these recommendations are based upon the conditions that can reasonably be met and maintained. Remember the three factors of light: TYPE, INTENSITY, and EXPOSURE TIME. Reduce any one factor and damage to paper will be reduced. Better yet, reduce all three! If these lighting restrictions seem excessive, it is for good reason -- light damage is irreversible, and when it is finally noticed it is too late to lower the lights.
