Types of glasses
Chemically Strengthened Glass:
Chemically strengthened glass is produced by an ion exchange process that takes place when annealed glass is immersed in a molten salt bath. As smaller sodium ions in the glass are replaced with larger potassium ions from the salt solution, the faces of the glass are put into compression relative to the core, and the glass is
pre stressed in a manner similar to the one that occurs with heat treating. However, because the temperatures involved in chemical strengthening are lower, chemically strengthened glass does not experience the optical distortions or warping that are common with heat-treated glass.
Depending on the particulars of the treatment process, the strength and toughness of chemically strengthened glass can exceed those of tempered glass. Unlike tempered glass, chemically strengthened glass can be cut after strengthening, although its strength is diminished along the cut edges. When chemically strengthened glass breaks, it produces large, hazardous shards. So, like heat-strengthened glass, it cannot be used where safety glazing is required unless it is laminated. Chemical strengthening is used for pieces of glass that are not easily heat treated, such as those that are small, thin, or oddly shaped. It is also used in some fire-rated glass products (discussed below) and in laminated form for security glass, blast-resistant glass, and wind borne debris-resistant glass.
Fire-rated glass in fire doors, fire windows, and fire resistance rated walls must maintain its integrity as a barrier to the passage of smoke and flames even after it has been exposed to heat for a period of time. Some tempered or laminated glass products can achieve test ratings of up to 20 minutes of fire resistance. Wired
glass is produced by rolling a mesh of small wires into a sheet of hot glass. When wired glass breaks from thermal stress, the wires hold the sheets of glass in place so that the glass continues to act as a fire barrier. It carries a fire resistance rating of 45 minutes. Optical-quality ceramic is more stable against thermal breakage than any type of glass. It looks and feels like ratings ranging from 20 minutes to 3 hours.
A number of producers are equipped to imprint the surface of glass with silk screened patterns of ceramic-based paints. The paints consist primarily of pigmented glass particles called frit. After the frit has been printed on the glass, the glass is dried and then Þ red in a tempering furnace, transforming the frit into a hard, permanent ceramic coating. Many colors are possible in both translucent and opaque finishes. Typical patterns for fritted or silk screened glass are various dot and stripe motifs (Figure 17.10), but custom- designed patterns and even text are easily reproduced. Fritted glass is often used to control the penetration of solar light and heat into a space.
Frits are used to create special opaque glasses for covering spandrel areas (the bands of wall around the edges of floors) in glass curtain wall construction. A uniform coating of frit is applied to what will be the interior surface of the glass. Some spandrel glasses are made as similar as possible in exterior appearance to the glass that will be used for the windows on a specific project. It is very difficult, however, even with reflective coated glass, to make the spandrels indistinguishable from the windows under all lighting conditions. Most spandrel glasses are made to contrast with the windows of the building. Many suppliers can apply thermal insulation on the interior of the glass, complete with vapor retarder. Spandrel glass is usually tempered or heat strengthened to resist the thermal stresses that can be caused by accumulation of solar heat behind the spandrel.
Tinted and Reflective Coated Glass:
Solar heat buildup can be problematic in buildings with large areas of glass, especially during the warm part of the year. Fixed sun-shading devices outside the windows are the best ways of blocking unwanted sunlight, but glass manufacturers have also developed tinted and reflective glasses that reduce glare and cut down on solar heat gain.
The transparency of glass to visible light is called its visible light transmittance (VT). It is measured as the ratio of visible light that passes through the glass relative to the amount of light striking the glass. Clear glasses have visible light transmittance in the range of 0.80 to 0.90, meaning that 80 to 90 percent of the visible light
striking the glass passes through to the building interior.
The remaining 10 to 20 percent is either reflected or absorbed by the glass and converted to heat. By tinting glass, its visible light transmittance is reduced. Tinted glass is made by adding small amounts of selected chemical elements to the molten glass mixture to produce the desired hue and intensity of color in grays, bronzes, blues, greens, and golds. The visible light transmittance of commercially available tinted glasses
ranges from about 0.75 in the lightest tints to 0.10 for dark gray. The overall reduction in solar heat gain is often significantly less, however, because the solar radiation absorbed.
Reflective Coated Glass:
Thin, durable films of metal or metal oxide can be deposited on a surface of either clear or tinted glass sheets under closely controlled conditions to make reflective coated glass, also called solar control glass. Depending on its composition, the film may be applied to either the inside of the glass or the outside. In double glazing, it may also be applied to either of the surfaces that face the space between the layers of glass. While remaining thin enough to see through, the film reflects a substantial portion of the incident visible light.
Visible light transmittance and solar heat gain coefficients (SHGC) for reflective coated glasses vary significantly, depending on the density of the metallic coating and the tinting of the glass to which it is applied. Reflective coated glasses appear as mirrors from the outside on a bright day and are often chosen by architects for this property alone. At night, with lights on inside the building, they appear as dark but transparent glass.