Heat  Absorbing Heat-absorbing  glass  contains  controlled  quantities of a ferrous iron admixture that absorbs much of the energy of the sun. Heat-absorbing glass is available in plate,  heavy  plate,  sheet,  patterned,  tempered,  wired, and  laminated  types.  Heat-absorbing  glass  dissipates much of the heat it absorbs, but some of the heat is retained.  Thus,  heat-absorbing  glass  may  become  much hotter than ordinary plate glass. Because  of  its  higher  rate  of  expansion,  heat- absorbing glass requires careful cutting, handling, and glazing. Sudden heating or cooling may induce edge stresses,  which  can  result  in  failure  if  edges  are improperly  cut  or  damaged.  Large  lights  made  of heat-absorbing glass that are partially shaded or heavily draped  are  subject  to  higher  working  stresses  and require  special  design  consideration. Glare  Reducing Glare-reducing glass is available in two types. The first type is transparent with a neutral gray or other color tint, which lowers light transmission but preserves true color vision. The second type is translucent, usually white, which gives wide light diffusion and reduces glare.  Both  types  absorb  some  of  the  sun’s  radiant energy and therefore have heat-absorbing qualities. The physical characteristics of glare-reducing glass are quite similar to those of plate glass. Although glare-reducing glass  absorbs  heat,  it  does  not  require  the  special precautions  that  heat-absorbing  glass  does. Insulating Insulating glass units consist of two or more sheets of glass separated by either 3/16-, 7/32-, or 1/4-inch air space.  These  units  are  factory-sealed.  The  captive  air  is dehydrated  at  atmospheric  pressure.  The  edge  seal  can be made either by fusing the edges together or with metal spacing strips. A mastic seal and metal edge support the glass. Insulating  glass  requires  special  installation  pre- cautions.  Openings  into  which  insulating  glass  is installed must be plumb and square. Glazing must be free of paint and paper because they can cause a heat trap that may result in breakage. There must be no direct contact between insulating glass and the frame into which it is installed. The glazing compound must be a nonhardening  type  that  does  not  contain  any  materials that will attack the metal-to-glass seal of the insulating glass. Never use putty. Resilient setting blocks and spacers  should  be  provided  for  uniform  clearances  on all units set with face stops, Use metal glazing strips for 1/2-inch-thick  sash  without  face  stops.  Use  a  full  bed  of glazing compound in the edge clearance on the bottom of the sash and enough at the sides and top to make a weathertight seal. It is essential that the metal channel at the perimeter of each unit be covered by at least 1/8 inch of compound. This ensures a lasting seal. Tempered Tempered glass is plate or patterned glass that has been reheated to just below its melting point and then cooled very quickly by subjecting both sides to jets of air. This leaves the outside surfaces, which cool faster, in a state of compression. The inner portions of the glass are in tension. As a result, fully tempered glass has three to five times the strength against impact forces and temperature  changes  than  untempered  glass  has. Tempered  glass  chipped  or  punctured  on  any  edge  or surface  will  shatter  and  disintegrate  into  small  blunt pieces. Because of this, it cannot be cut or drilled. Heat Strengthened Heat-strengthened  glass  is  plate  glass  or  patterned glass with a ceramic glaze fused to one side. Preheating the glass to apply the ceramic glaze strengthens the glass considerably,   giving   it   characteristics   similar   to tempered  glass.  Heat-strengthened  glass  is  about  twice as strong as plate glass. Like tempered glass, it cannot be cut or drilled. Heat-strengthened  glass  is  available  in  thicknesses of 1/4 and 5/16 inch and in limited standard sizes. It is opaque and is most often used for spandrel glazing in curtain wall systems. Framing members must be sturdy and  rigid  enough  to  support  the  perimeter  of  the tempered  glass  panels.  Each  panel  should  rest  on resilient  setting  blocks.  When  used  in  operating  doors and windows, it must not be handled or opened until the glazing compound has set. Wired Wired glass is produced by feeding wire mesh into the center of molten glass as it is passed through a pair of rollers. A hexagonal, diamond-shaped square, or rectangular pattern weld or twisted wire mesh may be used. To be given afire rating, the mesh must be at least 25 gauge, with openings no larger than 1 1/8 inches. Also, the glass must be no less than 1/4 inch thick. Wired glass may be etched or sandblasted on one or both sides 4-20