Bark Frameworks

Materials and Methods of Preservation Framing - Glazing

Glazing samples on matboard show the tints of different glazing materials.

  • In 1991 Jared Bark wrote a paper, Glazing for Framing and Case Making, in which he listed the properties of the ideal glazing material. What he outlined in that paper about the properties of ideal glazing material remain relevant today. In the years since, improvements have been made in frame glazing, and there are now many choices of glazing with a wide set of characteristics.

Outlined below are the properties of the ideal glazing material, with added information about products on the market that meet these criteria. We also indicate the different aspects of these materials and the circumstances for which they are recommended.:

Transmits all the light in the visible spectrum and is therefore un-tinted

·        Acrylic glazing (Acrylite and Plexiglas are two well-known brands) is virtually un-tinted, though in its UV blocking version has a slight green, blue or gray cast.

·        Glass may appear slightly green due to iron oxide impurities in the glass. “Water white” or “low iron” glass transmits virtually the whole spectrum and has almost no tint, but also allows transmission of more UV than conventional glass.

Blocks ultraviolet light

·        Acrylic glazing has been available with UV blocking agents for decades.

·        Coated and laminated glass sheets are available that block UV as effectively as UV blocking acrylic.

Is non-reflective

·        Several varieties of coated “anti-reflective” glass are available from framers.  All eliminate about 99% of reflection. Windows and lights are reflected though, and the reflection is tinted.

·        There is one coated acrylic, Optium, which is similar to the coated glass materials.

Carries no static charge

·        Glass carries almost no electrostatic charge.

·        Acrylic sheet is subject to high static charge, especially when humidity is low. Anti-static acrylic cleaning solutions will reduce the charge temporarily. Loose media, such as charcoal, chalk or pastel, and thin or lightly sized papers (such as Japanese papers) will be attracted to static charged acrylic glazing.

·        Optium acrylic’s coating, applied to reduce reflection, also virtually eliminates static charge.

Does not expand and contract in response to temperature and humidity

·        Glass does not expand or contract in response to changes in humidity, and very little to changes in temperature—even less than aluminum or steel.

·        Acrylic expands and contracts about ten times as much as glass in response to temperature change; and it responds to changes in humidity, though to a lesser degree and more slowly than it responds to temperature change.

Is close to unbreakable—and if broken, does not produce sharp splinters

·        Glass shatters. If a frame glazed with glass must be shipped, the glass should be masked with an adhesive film (such as Glass Skin). Glasses coated to reduce reflection are sometimes damaged by such adhesives, so manufacturer’s guidelines should be followed. Laminated glass, if broken, adheres to its inner plastic layer, limiting or eliminating loose shards of sharp glass in the frame.

·        The impact resistance of acrylic glazing is several times that of picture glass; this property is hardly affected by changes in temperature. It rarely cracks or breaks in shipping, and if it does, the broken edges are not as sharp as glass. For these reasons, and because tape removal is very difficult, it should not be taped. If needed, impact resistant acrylic is also available.

·        Polycarbonate is more resistant than acrylic, but its other properties make it less attractive than acrylic for framing.

Is abrasion resistant

·       Glass is abrasion resistant.

·       Coated glass must be treated carefully. The surface can be scratched and once scratched is irreparable. 

·      Acrylic can be manufactured with an abrasion resistant surface. This surface is less tough, however, than the surface of glass.

Is light weight

·        Glass is about twice as heavy as acrylic, and this can become a major issue for large frames. For example, a 48” x 96” sheet of 4.5 mm (about 3/16”) acrylic weighs about 35 pounds and a sheet of glass that size weighs about 70 pounds. But for large frames, thicker laminated glass is generally chosen and a sheet this size would require a thickness of 6 mm and would weigh about 100 pounds.

Is rigid and free of distortion

·         Glass is rigid and free of distortion

·         Acrylic sheet is flexible, and subject to deflection which distorts reflections.  A concern for framers is that glazing may bow in, touching a framed work of art. This may be caused by the expansion of acrylic that is too tight in the frame, exhibition lights that warm the interior of the frame, or differences in relative humidity inside and outside the frame, with the acrylic bowing to the more humid side. The thinner the sheet, the more likely the bowing. Acrylic glazed frames should not be shipped lying flat since horizontal bowing is substantial and the glazing could contact the surface of the framed artwork.

Is available in large size

· Maximum for laminated glass is about 6’ x 10’.

· Maximum for acrylic sheet is about 9’ x 14’.

·  Maximum  for Optium acrylic sheet is 6’ x 10’.

Shows no effects of aging

·        Glass is ageless as far as framing is concerned.

·        Acrylic is an organic material and all organic materials age. But the process is glacially slow, and imperceptible. It is often assumed that acrylic yellows with age. If it does, the human eye can’t perceive it.

·        Whether UV blocking acrylic loses its effectiveness over time has been an unanswered question until recently.  

In early 2011 we tested ten sheets of UF3 (UV blocking) acrylic that we had installed in frames during the 1980’s. We found that they were still excellent UV blockers. All these roughly 25 year old sheets of acrylic blocked over 99% of UV. See full article.

In choosing which glazing sheet to use, some of the questions to consider are:

·        Is the work susceptible to UV damage?

·        Under what conditions—such as lighting, climate control, public access—is the work likely to be displayed?

·        Is it intended that the work will be shipped?

·        Is the media loose (charcoal, pastel or similar)?

The answers to these questions will inform decisions about glazing choice. Another major consideration is cost. Costs for the more than twenty five glazing materials we now use vary widely, with the most expensive costing about fifty times more than the least expensive.