Heat AbsorbingHeat-absorbing glass contains controlled quantitiesof a ferrous iron admixture that absorbs much of theenergy of the sun. Heat-absorbing glass is available inplate, heavy plate, sheet, patterned, tempered, wired,and laminated types. Heat-absorbing glass dissipatesmuch of the heat it absorbs, but some of the heat isretained. Thus, heat-absorbing glass may become muchhotter than ordinary plate glass.Because of its higher rate of expansion, heat-absorbing glass requires careful cutting, handling, andglazing. Sudden heating or cooling may induce edgestresses, which can result in failure if edges areimproperly cut or damaged. Large lights made ofheat-absorbing glass that are partially shaded or heavilydraped are subject to higher working stresses andrequire special design consideration.Glare ReducingGlare-reducing glass is available in two types. Thefirst type is transparent with a neutral gray or other colortint, which lowers light transmission but preserves truecolor vision. The second type is translucent, usuallywhite, which gives wide light diffusion and reducesglare. Both types absorb some of the sun’s radiantenergy and therefore have heat-absorbing qualities. Thephysical characteristics of glare-reducing glass are quitesimilar to those of plate glass. Although glare-reducingglass absorbs heat, it does not require the specialprecautions that heat-absorbing glass does.InsulatingInsulating glass units consist of two or more sheetsof glass separated by either 3/16-, 7/32-, or 1/4-inch airspace. These units are factory-sealed. The captive air isdehydrated at atmospheric pressure. The edge seal canbe made either by fusing the edges together or withmetal spacing strips. A mastic seal and metal edgesupport the glass.Insulating glass requires special installation pre-cautions. Openings into which insulating glass isinstalled must be plumb and square. Glazing must befree of paint and paper because they can cause a heattrap that may result in breakage. There must be no directcontact between insulating glass and the frame intowhich it is installed. The glazing compound must be anonhardening type that does not contain any materialsthat will attack the metal-to-glass seal of the insulatingglass. Never use putty. Resilient setting blocks andspacers should be provided for uniform clearances onall units set with face stops, Use metal glazing strips for1/2-inch-thick sash without face stops. Use a full bed ofglazing compound in the edge clearance on the bottomof the sash and enough at the sides and top to make aweathertight seal. It is essential that the metal channelat the perimeter of each unit be covered by at least 1/8inch of compound. This ensures a lasting seal.TemperedTempered glass is plate or patterned glass that hasbeen reheated to just below its melting point and thencooled very quickly by subjecting both sides to jets ofair. This leaves the outside surfaces, which cool faster,in a state of compression. The inner portions of the glassare in tension. As a result, fully tempered glass has threeto five times the strength against impact forces andtemperature changes than untempered glass has.Tempered glass chipped or punctured on any edge orsurface will shatter and disintegrate into small bluntpieces. Because of this, it cannot be cut or drilled.Heat StrengthenedHeat-strengthened glass is plate glass or patternedglass with a ceramic glaze fused to one side. Preheatingthe glass to apply the ceramic glaze strengthens the glassconsiderably, giving it characteristics similar totempered glass. Heat-strengthened glass is about twiceas strong as plate glass. Like tempered glass, it cannotbe cut or drilled.Heat-strengthened glass is available in thicknessesof 1/4 and 5/16 inch and in limited standard sizes. It isopaque and is most often used for spandrel glazing incurtain wall systems. Framing members must be sturdyand rigid enough to support the perimeter of thetempered glass panels. Each panel should rest onresilient setting blocks. When used in operating doorsand windows, it must not be handled or opened until theglazing compound has set.WiredWired glass is produced by feeding wire mesh intothe center of molten glass as it is passed through a pairof rollers. A hexagonal, diamond-shaped square, orrectangular pattern weld or twisted wire mesh may beused. To be given afire rating, the mesh must be at least25 gauge, with openings no larger than 1 1/8 inches.Also, the glass must be no less than 1/4 inch thick. Wiredglass may be etched or sandblasted on one or both sides4-20
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