Why Glass Is Important for Framing Your Art Work?
The primary purpose of glazing in art framing is to physically protect the work of art from damaging factors such as humidity, heat, and soiling. Laminated glass and some acrylic may be used to protect against physical damage from glass breakage and to offer protection from a malicious attack. Regular glass as well as some glass surface treatments can also filter some of the damaging ultra-violet radiation (UV) and heat (NIR). 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.In the case if the framed object or artwork is UV resistant, UV protection can still serve the purpose of preserving the integrity and colors of non-conservation grade framing materials susceptible to UV damage, such as mat board (passe partout).
Although protection is a primary purpose of glazing, displaying an artwork is the primary purpose of framing it. Therefore, the least visible glazing best displays the artwork behind it. Visible light transmission is the primary measure of glass' invisibility, since the viewer actually sees the light, reflected from the artwork. Light transmission of glass is especially important in art framing, since light passes through the glass twice – once to illuminate the artwork, and then again, reflected from the artwork, as colors - before reaching the viewer.
Light transmission (for this article, the perceivable visible spectrum between 390 nm and 750 nm is considered) through glass is diminished either bylight reflection or light absorption of the glazing material. The total light transferred through the glazing material (light transmission) is reduced by reflection and/or absorption. In art framing, light reflection causes glare, while light absorption also may cause the transmitted colors to be dulled or distorted. While type of the glass substrate will affect the light absorption of the glazing, the surface treatment can affect light scattering, light reflection and in some cases, light absorption. There are various glazing options to achieve this goal as explained in the following sections on Types of Picture Framing Glass.
Types of Picture Framing Glass
Regular (or "Clear")
Due to widespread availability and low cost, Soda Lime Glass is most commonly used for picture framing glass. Glass thicknesses typically range from thin 2.0 mm, to 2.5 mm. Clear glass has light transmission of ~ 90%, absorption of ~ 2%, and reflection of ~ 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment.
Low-iron (or "Extra-Clear," "Water White," etc)
Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 mm thicknesses for picture framing applications. Because low iron glass light absorption can be as low as 0.5% vs. ~ 2% for clear glass, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
Laminated glass offers shatter-resistance and protection from malicious breakage to art glazing. The most commonly used configuration is Glass + PVB Foil + Glass. Some variations of foils and glass thicknesses can offer shatter- and breakage-resistance or even bullet-resistance. The absorption of laminated glass depends on the glass substrates and foils used in the laminating process. Reflection of laminated glass is similar to monolithic glass, unless surface treatments are applied to reduce reflection.
Some types of acrylic glass can have the high light transmission and optical quality of glass. Acrylic is also light weight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, over-sized works of art. 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.
Glass Surface Treatments and Coatings
Due to the change in the refractive index, as a light beam travels from air (refractive index of ~1) into glass or acrylic (refractive index of ~1.5) and then back into air, these transitions cause part of the light to be reflected. While "anti-glare" (a.k.a. "non-glare" or matte finish) glass treatments focus on scattering the light, "anti-reflective" coatings actually reduce the amount of light, which is reflected from each glazing surface, which has the benefit of increasing the amount of light transmitted through the glazing.
Matte (Etched, "Non-Glare," or "Anti-Glare")
The main purpose of matte glass is to transform the specular reflection into reflection haze. So-called "scattering" of the reflected light renders reflected images blurry, so that distinct reflected shapes and sources of light do not distract from the art viewing experience. Scattering the light does not reduce the reflection or absorption, which remain at the level of the glass substrate. There are several ways of making the glass surface matte – from pressing the pattern when the glass is still soft to fine etching of the glass surface by acid. The quality of matte glass is usually determined by its gloss factor or haze factor.
Single-layer anti-reflective coatings aim to achieve the refractive index of 1.25 (half-way between air and glass), and can be made either by single-layer micro-porous structures achieved by etching,hybrid materials and other processes suitable for producing large-area coatings for art framing purposes. Single layer coatings have been used as a lower-cost alternative to multi-layer anti-reflective coatings. Single-layer anti-reflective coatings can reduce light reflection to as low as 1.5%.
The lowest reflection can be achieved with multi-layer anti-reflective coatings, which can be applied by either magnetron sputtering, evaporation or sol-gel process (or other processes, which can control the uniformity of deposition on nanometer-scale), and can reduce the light reflection to lower than 0.25% per side (0.5% total).
UV Filtering Coatings
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.
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. However, by including more art damaging factors than UV radiation between 300 nm and 380 nm, these methods provide a more holistic relative ranking tool. For example, comparing a 99% and 92% UV Blocking glazing, would translate to 44% and 41%, respectively, under the KDF.
How much UV filtering should a glazing have
The discussion on how much UV filtering is necessary in art framing is complex and controversial, driven by conflicting corporate interests. 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). On the other, by the fact that not only UV, but also visible light damages an artwork. According to the National Fenestration Rating Council, only 40% of artwork fading is caused by UV radiation. The remaining damage comes from the visible light, heat, humidity and material chemistry. This means that increasing visible light transmission by an anti-reflective coating actually increases the amount of damaging radiation on an artwork.
One of the most thorough and independent studies was conducted by the US Library of Congress in an effort to display and preserve the US Declaration of Independence. 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. Sealing the display by a chemically inert gas such as nitrogen, argon or helium also aided its preservation. 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 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,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.