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 . 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 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
Single-layer
Single-layer anti-reflective coatings aim to achieve the refractive index of 1.25 (halfway between air and glass), and can be made 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%.
Multi-layer
The lowest reflection can be achieved with multi-layer anti-reflective coatings, which can be applied by magnetron sputtering, evaporation, or sol-gel process (or other processes that can control the uniformity of deposition on the nanometer scale), and can reduce the light reflection to lower than 0.25% per side (0.5% total).
Features of anti-reflective coatings
Light reflection – the main goal of anti-reflective coatings is to reduce the light reflection that causes the glare. Therefore, the lower the light reflection, the less glare reaches the viewer. The best anti-reflective products available for the picture framing market have light reflection of 0.5%.http://www.flabeg.com/files/glas/downloads/PDFs/ENG/FLABEG_ARTControl_EN.pdf http://www.groglass.com/images/pdfs/artglass_us_web.pdf The seemingly small differences in light reflection are actually very important due to the logarithmic response of human eyes to signal intensity (Weber's law). In other words, under normal lighting conditions, the human eye perception of the intensity of a reflected light source in a 1%-reflecting glass surface will be perceived as more than twice of the same light source in 0.5%-reflecting glass. Light absorption – light absorption of glazing is the light that is neither transmitted nor reflected by the glazing. Since light is not necessarily absorbed uniformly, some wavelengths may be transmitted more than others, causing the transmitted color to be distorted. A good way to detect light absorption of glazing is the white paper test. This test, used to detect the transmission color of glazing, involves placing a piece of glazing on white paper, and comparing the paper's color with and without the glass. A slight greenish tint will indicate the presence of iron oxide in the raw materials used to produce clear float glass. Additional transmitted colors may result from the absorption of any applied coatings. Light transmission – the lower the light reflection and light absorption, the higher the light transmission, and therefore, the visibility of the objects displayed behind the glazing. Reflected color – Uncoated glass reflects light uniformly and does not cause reflected light to be distorted (a white light source reflected in a non-coated glass pane will still appear white). However, anti-reflective coatings typically cause some wavelengths of light to be reflected more than others, causing a shift in the reflected color. This way, a white light source reflected in an anti-reflected glass surface may appear green or blue or red, depending on the wavelengths that are favored by a particular anti-reflective coating design. Intensity of reflected color – the intensity of a reflected color can be measured by its relative distance from the color neutral zone (i.e. white). Due to the variability of industrial processes, some producers design their anti-reflective coatings to have more intense colors so that the statistical deviation of outcomes falls within a specific color (green or blue, etc.). The tighter a manufacturer's control of its processes, the closer the design can be to the color neutral zone, without crossing over from a designated color. Reflected color under an angle – as a reflected light source is reflected from the glazing under a shallow angle, some anti-reflective coatings may cause the reflected color to shift. Therefore, in picture framing, a stable color under a wide viewing angle is desirable. Cleaning – since anti-reflective coatings render the glass surface virtually invisible, the dirt or soiling of the surface is much more visible on an anti-reflective surface. This enhanced visibility of surface smudges presents difficulties when cleaning anti-reflective-coated glass. Therefore, some anti-reflective coatings have special surface treatments to improve cleanability, while others give special cleaning instructions to avoid damage to their coatings. Handling – Some coatings are more durable than others. A scratch through an anti-reflective coating is also much more visible than a scratch through the surface of uncoated glass due to the difference in the reflectivity of the scratched surface (for glass, about 8%) and the reflectivity of the anti-reflective surface around the scratch (about 0.5%). Therefore, anti-reflective coatings with more scratch-resistance are preferred in art glazing. Magnetron-sputtered and sol-gel anti-reflective coatings are typically metal oxides with superior hardness compared to other application methods.
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. The following technologies are used to reduce the amount of UV from reaching the artwork: