Acid migration from adjacent materials is a common source of damage to paper. Over the years, chemically unstable materials used for framing take their toll. These materials include many of the cardboards, tapes, and adhesives used to mount artifacts before framing. Framing is supposed to protect, but if not done properly, it will cause damage instead.
Although framers in general know far more today than they did years ago, some are still unaware of best preservation procedures and materials. A paper conservator or a museum can help you find a framer familiar with the special requirements of artworks or historic artifacts. With any framer, discuss your matting and framing requirements to make certain appropriate materials and mounting procedures are used.
The window mat is the standard mount for a paper artifact that is to be framed. Mats are also used for storage, especially for prints, drawings, and other works of art on paper. Some institutions simplify their framing and storage operations by using mats with standard outer dimensions that fit inside standard-sized storage boxes or modular frames.
The typical museum mat is composed of a window and a backboard (Figure 1). The two boards are held together with a strip of cloth tape along one edge, usually the top but sometimes along the left side. When an object is matted but not framed, it should have a protective sheet over its face. Clear polyester film such as Melinex, an archival-quality polyester, is often used for the cover sheet because it is transparent and chemically and dimensionally stable. Polyester carries a static charge, however, and is therefore suitable only for media that are securely attached to the paper. Archival tissue paper is more appropriate for delicate media such as pastel, charcoal, soft graphite pencil, or opaque watercolor. Acid-free glassine can also be used, but this becomes acidic in time and must be replaced every few years.
The board recommended for preservation matting may be either traditional ragboard, which is usually 100 percent cotton, or high-quality, wood-derived archival board free of lignin, a substance that can lead to the formation of acid. Both types are usually buffered with an alkaline material to ensure that they will not be adversely affected by acidic surroundings. Both types are stocked by conservation framers, conservation suppliers, or large art supply stores in several shades of white and in a limited assortment of colors. New products are always coming onto the market, and some may not be appropriate for works of art or historic artifacts. If in doubt about matting or mounting materials, ask a knowledgeable framer or conservator or check the product literature.
The mat window and its backboard should be the same size and fit the frame exactly. The window portion of the mat must be deep enough to ensure that the glazing is not in contact with the object. Four-ply board usually suffices, but thicker mats are required for large sheets, for those that may cockle or ripple, or for works with thick media, seals, or other raised elements. Archival boards thicker than four-ply are available commercially, or they can be made by laminating two or more sheets of four-ply board. Attractive multilayered stepped mats can be made using one or more colors. With any layered mat, all layers should be made of archival board. If a very deep mat is needed, a sink mat may work best. Sink mats (see Figure 3) are constructed by adhering strips of conservation board to the backboard to make a recess, or "sink", in which the object is mounted. The walls of the sink are hidden by the window portion of the mat.
The methods and materials for attaching the object to the mount are as important as the mount itself. The object must be mounted on the backboard of the mat, never on the reverse of the window. Under no circumstances should it be glued directly to the backboard. The customary method is to use paper hinges and an appropriate adhesive. In recent years corner supports or edge strips have become popular since they do not require that adhesive be applied to the object. Hinges, corner supports, and edge strips all allow the artifact to be easily removed from the mat if necessary.
Hinges are small rectangles of strong, archival-quality paper, preferably Japanese kozo. Part of the hinge adheres to the reverse of the object and part to the backboard. Two common types of hinges are shown in Figures 1 and 2. Folded hinges are recommended when the edges of the sheet will be shown inside the mat window.
Hinges must be attached with an adhesive that is nonstaining, permanent, and reversible. Conservators recommend homemade starch-based paste.
Hinges are usually applied to the top corners of the artifact, although with larger or heavy sheets, additional hinges may be added at other points along the top edge. If the object is to be “floated” (displayed with the edges exposed), additional hinges at the bottom corners or along the other three edges are desirable.
The paper most often recommended for hinging, pure Japanese kozo, is sometimes referred to as mulberry paper or, erroneously, rice paper. Papers made from 100 percent kozo fibers are lightweight, lignin-free, and long-fibered. They age well, remaining strong and flexible for many years. Although these papers were once made only by hand, some are now machine made. Kozo papers are available from conservation suppliers in different weights and in varying shades of white.
The adhesive preferred by conservators is homemade starch-based paste. This paste has the qualities demanded of a conservation adhesive: sufficient strength, good aging properties, no tendency to discolor, and reversibility. Conservators also use methyl cellulose. Animal glues (mucilage) or rubber cement are not recommended as they darken on aging and can stain paper. Synthetic adhesives, such as the ubiquitous white household glue, may not stain but are not recommended because they become irreversible as they age. The self-adhering "archival" tapes that have come onto the market in recent years are nonstaining, at least in the short run, but their aging properties are still not known, and with time they are difficult to remove.
Nonadhesive methods of mounting may be used instead of hinges. One of these methods is to use corner supports, which can be either paper or plastic envelopes folded over the corners of the artifact and adhered to the backboard. Another method of nonadhesive attachment uses edge strips, that is, lengths of paper folded over the edges of the artifact. To hold the sheet, these nonadhesive supports must overlap the front edges of the object. They can be concealed if they are covered by a mat, which at the same time also covers the edges of the artifact. Corner supports or edge strips can be made of paper or polyester film. The small corners available commercially for mounting photographs are acceptable for photographs and for small paper objects. Larger paper artifacts need larger corners.
Mats are not always appropriate. Certain contemporary works look odd if matted, and mats are not historically correct for early prints displayed in original frames. When a mat is not used, the object should be hinged to a backing of archival board and framed so that it is not in contact with the glazing or the frame interior. A spacer, which can be at least partially hidden under the frame rabbet, assures a space between the glazing and the object. Like all materials inside frames, a spacer must be nonacidic and chemically stable. Strips of ragboard make effective spacers. Ragboard strips can be attached to the glazing under the frame rabbet with archival double-sided tape, such as 3M Scotch brand double-sided tape #415. If painted black, the spacer is less apt to be visible. An acrylic paint should be used. Spacers deeper than 4-ply can be made by laminating two or more strips together. Another option is to ask your framer to construct a frame with a built-in spacer.
For a framed work on paper, glazing is essential to protect the fragile, porous paper surface from air-borne dirt and pollutants. Because moisture can condense on the inside of picture glazing, the glazing should not touch the artifact. The best glazing materials for works of art and historic artifacts are designed to filter out the damaging (UV) component of light. An acrylic sheet, UF-3 Plexiglas, made by Rohm and Haas, has been used by museums for several decades. More recently, other companies have introduced acrylic and glass sheets that filter UV radiation. When choosing glazing, be certain to select a product with a high UV-filtering capacity, at least 90 percent. Most glass and many acrylics do not block UV radiation, and others filter only a small amount.
Acrylic sheets carry a static charge and must not be used for pastels, charcoal drawings, or objects with loosely-attached media. UV-filtering glass should be used instead.
Sometimes it is important to retain the original handblown glass in an old frame. In such a case, a double-glazing system can be used with UV-filtering glazing next to the object (but not touching it) and the old glass on top. The second glazing layer will not be obvious to the viewer.
For added protection, the frame should be sealed, and there should be at least one additional layer of sturdy archival board behind the one on which the object is mounted. This backing layer, for physical and thermal protection should be an all-paper, lignin-free board. This is preferable to wood or foam board, which may release chemicals as they age. For even greater protection, a moisture barrier should be inserted between the backing sheets or attached to the back of the frame with 3M double-sided tape #415. Four- or five-mil Melinex can be used as a moisture barrier, although Marvelseal®, a laminate of aluminum foil and inert plastic, is even better since it is more impermeable to both moisture and gases.
The frame must be deep enough to accommodate all layers. The contents should not protrude behind the frame and touch the wall. Ideally, all layers are recessed within the frame so there is air space between them and the wall. When ordering new frames, make certain they are deep enough. Existing wood frames can be deepened by building up the back of the frame with wooden strips, screwed or glued into place.
The frame should be as air-tight as possible to keep out dirt and pollutants and to stabilize the interior against short-term fluctuations in temperature and relative humidity. The contents of the frame should be firmly held in place with brads or other metal hardware. The final board layer should be covered with an archival paper dust cover attached to the back of the frame with framer’s sealing tape or double-sided tape. Another possibility is to make a sandwich of the contents (glazing, mat, object, and backing layers) and seal the edges with tape. The sandwich is then placed in the frame as a unit and held in place with hardware.
Wood can give off volatile substances damaging to paper. While this is especially true of freshly cut wood, even old wooden frames will off-gas. Ideally the wood should be an inch or more away from the object. Distance, together with an alkaline buffer in the mat or mounting board, should protect the artifact. When a frame has a tighter fit, the inside of the rabbet should be lined with a barrier material such as polyester film, Marvelseal®, or impermeable self-adhesive frame sealing tape. Marvelseal® is heat sensitive on one side and can be attached to the interior of the frame with a small heat tool.
Even if ultraviolet-filtering glazing is used, paper objects should be hung in areas of subdued lighting. Because light at any level is potentially damaging, museum conservators advise that no paper-based work of art be kept on permanent display. In addition, storage and display areas should be cool and dry, with minimal fluctuations of temperature and relative humidity. Climatic fluctuations not only weaken paper over time but can cause unsightly rippling or distortion of the sheet. Proper framing buffers a work of art against minor short-term climate changes but does not protect against seasonal changes or long periods of high humidity. As is the case with all works of art and historic artifacts on paper, the environment surrounding framed objects is crucial to their preservation.
Clapp, Anne F. Curatorial Care of Works of Art on Paper. New York: Nick Lyons Books, 1987.
Phibbs, Hugh. "Building Space Into the Frame" Picture Framing Magazine, Feb. 1995.
___. "Preservation Matting for Works of Art on Paper," Supplement to Picture Framing Magazine, Feb. 1997.
Smith, Merrily A. Matting and Hinging of Works of Art on Paper. Washington, D.C.:Library of Congress, 1981.
NEDCC gratefully acknowledges the work of Margaret R. Brown in illustrating this technical leaflet.