WORKS OF art can be permanently damaged, drastically reducing their esthetic and investment value, by a sloppy or unscrupulous framing job. The best way to protect your collection is to choose your picture framer with care. Try hard to find a conscientious craftsman who can be trusted to use reliable materials.
Acidic materials are the big bugaboo. Art also must be protected from light (especially sunlight), moisture, heat (never hang a painting over a fireplace or radiator), air pollution, insects, and handling and transport.
Many kinds of art need framing, including works on paper (watercolors, pastels, fine photographs, etchings, lithographs, etc), textiles, and oil or acrylic paintings on canvas. Framing works done on paper is the most complicated, and therefor most perilous, job to undertake. Mats and backings, glass and moldings (the frame itself) are all involved.
Mats (carboard borders) are needed to keep the picture from touching the glass, which may contain injurious impurities. But primarily the air space created protects the picture from moisture condensation which can lead to waterspotting, mold and rot. Photographs might stick to the glass without a mat.
Two kinds of boards can be used for mats: regular mat board and rag board, a fact not realized by many people, causing much of the misunderstanding and trouble. Regular mat board is available in a great variety of enticing colors and textures but it is very acidic and can do lasting harm to an artwork.
Acid causes discoloration, brittlenes and deterioration of paper and textiles, sometimes in a few months or sometimes in a few years, depending on climatic conditions.
On the other hand, rag board, sometimes known as museum board, can be virtually free of acid. More expensive than regular mat, it comes in a number of neutral colors from whites to tan. It should always be used in contact with an original work of art. If a strong color or a texture is desired, the framer can use mat board over a layer of rag. The layer will keep the acid away from the picture.
The difference between mat and rag can be seen in the bevel of the board. Rag board is exactly the same color all the way through its thickness. Mat board is a different color in the center. If you have asked for rag and you have notice this difference in the bevel, ask the framer to reframe your picture -- you don't have rag board. Mat board can be used, however, on anything which is not expected to have a long life -- reproductions, prints, some photographs, etc.
The backing (the stiff board which holds the limp paper) should also, of course, be acid-free. It's always advisable to have a work on paper mounted on rag board using two acid-free hinges at the top. The hinges hold the paper in place and the acid-free mat is attached over this making a nice healthy environmental for the art work.
Over the mat goes the glass. The frame holds this sandwich together. For further protection at the back, Fome-Cor or chipboard can be used. A good framer seals the picture with a final layer of tough paper to keep out the dust. Never accept a picture from a framer which has been backed with corrugated cardboard (you can tell by running your fingers over the back). It contains a maximum amount of acid which will gradually turn up on the front of the pictures as brown stripes.
Light, especially sunlight, can also be damaging to an art work, so the kind of glass used can be important. Good frames carry a choice. Grade b picture glass is suitable for most framing. It has almost no green cast and causes less distortion than regular glass.
If light is a real problem as with museum quality work, the framer can use ultraviolet plexiglass. It is very light in weight, looks like clear glass and is expensive, but offers the greatest protection because it filters out harmful rays which cause fading and discoloration. Ordinary plexiglass, also light in weight, doesn't have the filtering power of UV but can be used for very large pictures, especially those to be transported.
Both kinds of plexi have the disadvantage of attracting dust and being easy to scratch. Special cleaners are necessary. Non-glare glass, which was a fad a while back, is expensive but doesn't offer any extra protection and tends to obscure the image. In addition to the above, some framers carry museum glass (trade name Denglas) which is nearly invisible, has a chemically neutral coating, and is very expensive.
You can dry mount art. The picture is sealed onto a backing by heating it in a press with an adhesive tissue between. The backing can be Fome-Cor (warps the least and is the lightest), chipboard, Upson board or rag board. The process is suitable mainly for reprinted posters and reproductions. Many framers simply refuse to dry mount any original work of art. They claim that the process damages the value of the art because it is not reversible, i.e. the picture can never be removed from the backing again. Actually a paper conservator may be able to remove the backing but it's a difficult an costly job.
Moldings are available in an enormous variety of styles, widths, shapes and price ranges. Samples will be hanging around the walls of a framer's shop making it easier for you to choose. Most good framers believe that the frame should enhance the picture, be chosen "from the picture out" rather than "from the room (where it is to be hung) out."
Paintings on canvas are less complicated to frame. The paint surface has usually been protected by the artist with a coating of picture varnish or one of the newer and more satisfactory synthetic coatings so they rarely require glass. The paintings should be fastened into the frame with metal strips and screws rather than brads or nails, and should be backed with Fome-Cor or chipboard for protection against damage, dust and foreign objects. You should ask your framer not to fit your painting too tightly into the frame; the wooden stretcher supporting the canvas may need to be enlarged slightly eventually and space should be left for this.