Glazing with Glass or Plastic

(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.

Glass

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.

Plastics

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.