Picture-framing glass usually refers to a variety of types flat glass or acrylic ("plexi") used for framing artwork and for presenting art objects in a display box (also, "conservation framing"). The different types include the loosely defined "conservation glass", "anti-reflective glass", and "museum glass". These aim to reduce one or both of distracting reflections from light sources, and damaging ultra-violet radiation, above all from sunlight. Typically, the types of art displayed and often stored framed under glass are watercolours, drawings and other works on paper. Oil paintings are most often not framed under glass.
Purpose 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. 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. If the framed object or artwork is UV-resistant, then 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's 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 by light reflection or light absorption of the glazing material. The total light transferred through the glazing material (light transmission) is reduced by reflection and 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 1.8 to 2.5 millimetres (0.071 to 0.098 in). Clear glass has light transmission of approximately 90%, absorption of approximately 2%, and reflection of approximately 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment. This is also known in the trade as "green glass" as the edge of sheet appears green.
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-millimetre (0.079 in) thicknesses for picture-framing applications. Because low-iron glass light absorption can be as low as 0.5%, compared to about 2% for clear glass, the light transmission will be better than clear glass. Low-iron glass has light transmission of approximately 91.5% and reflection of 8%.
Laminated Glass Laminated glass offers shatter resistance and protection from malicious breakage. The most commonly used configuration is polyvinyl butyral (PVB) foil sandwiched between two layers of 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.
Acrylic Some types of acrylic glass can have the high light transmission and optical quality of glass. Acrylic is also lightweight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, oversized works of art. In general, acrylic sheets scratch easily and retain 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 about 1) into glass or acrylic (refractive index of about 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 that 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. 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.
Anti-reflective coatings
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