(Excerpt from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Glazing means to fit with glass or glasslike material. There are several types of glass and plastic available to glaze pictures, and the following discusses these materials.
There are two weights of glass commonly used in picture framing, picture glass and single-strength glass. Picture glass is lighter and not as strong as single strength. Picture glass was popular some years ago because it is thinner and therefore occupies less space; the popular mouldings of that time were not as deep and space was a consideration. The lightweight characteristic of picture glass had an advantage in larger frames, but at a certain point, its reduced strength became more important than the weight factor. Today, quality picture glass is expensive and hard to find. All of this has discouraged its use. Single-strength glass comes in two grades, A and B. B is the same as A except that it is not inspected before it is packed. Single-strength is the glass most commonly used today. Double-strength glass, which is approximately 20 percent stronger, is rarely used even for larger pictures because of its heavy weight and slight, but noticeable, green tint. Glass transmits about 80 to 83 percent of light, and the greenish tint varies in degree depending on the thickness of the glass.
Nonglare glass, which is glass whose surface has been etched to diffuse the reflection, is most effective when the artwork is placed up against the glass (which violates one of the basic rules about paper artwork). The greater the space between the artwork and the nonglare glass, the fuzzier the image becomes. In most cases the space created by a single mat will not make the image sufficiently fuzzy to disturb most people, and there are circumstances where reflection is a serious problem. In these situations nonglare glass may be considered. I prefer a crystal clear image with reflection to paying more and getting a fuzzy image with a diffused reflection.
A new product called Denglas, made by Denton Vacuum, Inc., which has a metalized coating, would be the ideal solution, except for its extremely high price tag. Regular glass has an approximately 8 percent reflection. Denglas has a 1 percent reflection and can also absorb some ultraviolet light. Each sheet of glass is apparently made with a coating of metallic particles applied in a vacuum chamber and it is this coating of the surface of the glass that produces this effect. Denglas was originally developed for scientific apparatus. Its effectiveness is particularly startling when used with very dark images. Because the cost of using this glass could double the price of framing, it is primarily reserved for use with very valuable artwork, or where light reflection is an overwhelming problem.
There are three types of plastic sheeting used most frequently in the picture framing industry: polystyrene, polycarbonate, and acrylic. (Plexiglas is the trademark of Rohm & Haas for clear acrylic sheeting.) Polystyrene is a relatively soft plastic used in the making of such mass-produced moulded plastic products as plastic cups, plastic toys, and inexpensive box frames. This product is not ideal for valuable artwork because it yellows as it ages. It is also more brittle than polycarbonate and acrylic.
Polycarbonate, because it is virtually unbreakable, is used in protecting valuable artifacts or artworks that are on public display. It is rarely used in conventional framing because of the high cost and because it must be cut on a table saw using a special blade manufactured for this purpose. A ventilation system is necessary to protect against toxic vapors that may arise in the heat produced during cutting. In general, using polycarbonate in conventional framing is considered overkill.
Clear acrylic (polymethyl methacrylate) sheets in 1/8 inch, and 3/16 inches for works over 40″ x60″, are the most common replacement for glass in picture framing when the possibility of breakage outweighs other concerns. The advantages of acrylic are that it is lighter weight than glass, it can be scored and broken to size without expensive equipment, it is far less breakable in a picture frame than glass, and it is clearer and transmits more light (90 percent) than glass. The disadvantage is that acrylic cannot be used with chalk, charcoal, or pastel artwork due to the static charge it develops, which can lift a significant amount of material off the surface of the artwork and adhere it to the surface of the acrylic sheet. Exposure to strong ultraviolet light can yellow acrylic, but this is of more serious concern to institutions and industries that are using such special lighting as mercury vapor lamps. Acrylic also scratches easily and is a fire hazard. When plastics such as these are heated or burned without sufficient oxygen (which would be the case in most ordinary room fires), they give off large quantities of toxic and suffocating vapors.
Regular acrylic sheeting, like glass, provides little protection of artwork from the effects of ultraviolet light, one of the major contributors to the fading or darkening of artists colors. Virtually all artists’ colors, in varying degrees, are sensitive to the visual light spectrum and to the invisible ultraviolet spectrum. Ultraviolet light is of greater concern because of its higher energy, which, besides fading or darkening a color, can produce chemical changes that include making artwork more brittle and structurally weaker. These changes can also occur from the higher energy levels of the visible light spectrum such as violet and blue, but not to the same degree as ultraviolet. The major sources of ultra-violet light are direct sunlight and fluorescent light.
In most cases, it is not difficult to avoid exposure to direct or even a large amount of reflected sunlight. It is usually just a matter of placement. Exposure to ultraviolet light from fluorescent light fixtures, which is a common form of lighting in institutions, is a problem. To solve this problem, especially for dealing with artwork or artifacts that are particularly sensitive, ultraviolet-filtering clear acrylics were developed. (The only form of acrylic that will filter both ultraviolet light and visible light comes only in black. So, as long as you wish to see your artwork you will have to accept the effects of most of the visible light spectrum.)
There are two basic types of clear acrylic that will filter ultraviolet light. One type can filter all ultraviolet light as well as part of the visible violet light range. (Visible violet light is part of the visible light spectrum, which can also have a considerable effect.) This type of acrylic can reduce the damage caused by ultra-violet light by 95 percent; nevertheless, it has a faint, somewhat distracting, visible yellow tint. The other type of acrylic has no effect on the visible light range and will protect against most, but not all, the ultraviolet light spectrum, thus reducing possible damage from this part of the spectrum by 90 percent. Since ultraviolet light is invisible, so is the protection and therefore there is no objectionable yellow tint.
Ultraviolet filtering acrylic is currently used only in special situations where exposure to this type of light will cause severe or long-term damage. The high cost of this material prevents its indiscriminate use. Rohm & Haas produces the two types of products for filtering ultraviolet light, Plexiglas OF-1, which has the yellow tint and provides the most protection, and Plexiglas UF-3, which provides a reasonable amount of protection without the yellow tint.