The primary purpose of glazing in art framing is to clearly exhibit the work while physically protecting it from damaging factors such as light ,humidity, heat, and soiling.
Artworks that require protective glazing are those rendered on paper or fabrics (including photographs), which contain pigments and dyes that absorb UV and are susceptible to discoloration.
Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 millimetres (0.079 in) thicknesses for picture framing applications.
In general, acrylic sheet scratches easily and retains a static charge, which can be problematic when framing pastels or charcoals.
Some manufacturers, add dyes to acrylic glass to filter the UV light transmittance, and its surface can also be treated with both anti-static and anti-reflective coatings.
[5] In order to reduce the amount of damaging light radiation transmitted through glazing, some glass coatings are designed to either reflect or absorb the ultraviolet (UV) spectrum.
At least two other methods exist, which provide a more holistic measurement of radiation damage, from both the UV and visible portions of the spectrum: For picture framing purposes, it is not appropriate to use these methods for absolute ratings, since "Better" ratings are obtained with lower visible light transmission, which is not aesthetically desirable in a framing glazing.
There have so far been no independent organizations, not tied to corporate sponsors, which have presented scientifically verifiable and conclusive evidence to the amount of UV filtering necessary for a glazing to both display and at the same time protect an artwork.
On one hand, the issue is complicated by the varying amount of damaging light actually present in an indoor environment (from low level indirect sources to direct daylight).
At first, it was decided to use special yellow "Plexiglass UF3", which removes both the ultra-violet as well as the blue end of the visible spectrum, with significant, but acceptable interference for viewing.
[25] In 2001, the display of the US Declaration of Independence was revised to include a multi-laminated glazing for shatter-resistance, with sol-gel interference-based multi-layer Anti-Reflective coatings on outer surfaces[26] to improve the visibility of the document.
From the above evidence, it can be concluded that if preservation were the only goal of glazing, then only a climate-controlled, dark space would offer the best possible protection for an artwork, which can be exhibited once in every several years,[27] while no glass at all provides a perfect displaying option.
Therefore, for those artworks, which are chosen to be displayed, the ideal amount of UV blocking should be as much as possible, without affecting the visible light transmission.
Note that regular window glazing filters away a significant portion of the UV light, which originates from the sun.