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GLASS

LEARNING OBJECTIVE:

Upon completing this section, you should be able to identify the different types of glass, glazing materials, and describe procedures for cutting, glazing, and installing glass.

It is surprising how many types of glass and glass-like materials are used in construction. Each has its own characteristics, advantages, and best uses. In this section, we’ll cover the various types of glass and materials, and the methods used in assembling glass features ("glazing").

TYPES

The "Glass and Glazing" section of construction specifications contains a wide range of materials. These may include sheet glass, plate glass, heat- and glare-reducing glass, insulating glass, tempered glass, laminated glass, and various transparent or translucent plastics. Also included may be ceramic-coated, corrugated, figured, and silvered and other decorative glass. Additional materials may include glazier’s points, setting pads, glazing compounds, and other installation materials.

Sheet/Window

Sheet or window glass is manufactured by the flat or vertically drawn process. Because of the manufacturing process, a wave or draw distortion runs in one direction through the sheet. The degree of distortion controls the usefulness of this type of glass. For best appearance, window glass should be drawn horizontally or parallel with the ground. To ensure this, the width dimension is given first when you are ordering.

Plate

Plate glass is similar to window and heavy-sheet glass. The surface, rather than the composition or thickness, is the distinguishing feature. Plate glass is manufactured in a continuous ribbon and then cut into large sheets. Both sides of the sheet are ground and polished to a perfectly flat plane. Polished plate glass is furnished in thicknesses or from 1/8 inch to 1 1/4 inches. Thicknesses 5/16 inch and over are termed "heavy polished plate." Regular polished plate is available in three qualities: silvering, mirror glazing, and glazing. The glazing quality is generally used where ordinary glazing is required. Heavy polished plate is generally available in commercial quality only.

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 to soften the light or provide privacy. It may be obtained with a pattern on one or both sides.

Patterned

Patterned glass has the same composition as window and plate glass. It is semitransparent with distinctive geometric or linear designs on one or both sides. The pattern can be impressed during the rolling process or sandblasted or etched later. Some patterns are also available as wired glass. pattern glass allows entry of light while maintaining privacy. It is also used for decorative screens and windows. Patterned glass must be installed with the smooth side to the face of the putty.

Laminated

Laminated glass is composed of two or more layers of either sheet or polished plate glass with one or more layers of transparent or pigmented plastic sandwiched between the layers. A vinyl plastic, such as plasticized polyvinyl resin butyl 0.015 to 0.025 inch thick, is generally used. Only the highest quality sheet or polished plate glass is used in making laminated glass. When this type of glass breaks, the plastic holds the pieces of glass and prevents the sharp fragments from shattering. When four or more layers of glass are laminated with three or more layers of plastic, the product is known as bullet-resisting glass. Safety glass has only two layers of glass and one of plastic.

Safety

Safety glass is available with clear or pigmented plastic, and either clear or heat-absorbing and glare-reducing glass. Safety glass is used where strong impact may be encountered and the hazard of flying glass must be avoided. Exterior doors with a pane area greater than 6 square feet and shower tubs and enclosures are typical applications.

Glazing compounds must be compatible with the layers of laminated plastic. Some compounds cause deterioration of the plastic in safety glass.

Mirrors

Mimers are made with polished plate, window, sheet, and picture glass. The reflecting surface is a thin coat of metal, generally silver, gold, copper, bronze, or chromium, applied to one side of the glass. For special mirrors, lead, aluminum, platinum, rhodium, or other metals may be used. The metal film can be semi-transparent or opaque and can be left unprotected or protected with a coat of shellac, varnish, paint, or metal (usually copper). Mirrors used in building construction are usually either polished plate glass or tempered plate glass.

Proper installation requires that the weight of the mirror be supported at the bottom. Mastic installation is not recommended because it may cause silver spoilage.

Plexiglas®

Sheets made of thermoplastic acrylic resin (Plexiglas® and Luciteo, both trade names) are available in flat and corrugated sheets. This material is readily formed into curved shapes and, therefore, is often used in place of glass. Compared with glass, its surface is more readily scratched; hence, it should be installed in out-of-reach locations. This acrylic plastic is obtainable in transparent, translucent, or opaque sheets and in a wide variety of colors.

GLAZING MATERIALS

In this section, we’ll discuss the various types of sealers you’ll need to install, hold fast, and seal a window in its setting.

Wood-Sash Putty

Wood-sash putty is a cement composed of fine powdered chalk (whiting) or lead oxide (white lead) mixed with boiled or raw linseed oil. Putty may contain other drying oils, such as soybean or perilla. As the oil oxides, the putty hardens. Litharge (an oxide of lead) or special driers may be added if rapid hardening is required. Putty is used in glazing to set sheets of glass into frames. Special putty mixtures are available for interior and exterior glazing of aluminum and steel window sash.

A good grade of wood-sash putty resists sticking to the putty knife or glazier’s hands, yet it should not be too dry to apply to the sash. In wood sash, apply a suitable primer, such as priming paints or boiled linseed oil.

Putty should not be painted until it has thoroughly set. Painting forms an airtight film, which slows the drying. This may cause the surface of the paint to crack All putty should be painted for proper protection.

Metal-Sash Putty

Metal-sash putty differs from wood putty in that it is formulated to adhere to nonporous surfaces. It is used for glazing aluminum and steel sash either inside or outside. It should be applied as recommended by the manufacturer. Metal-sash putty should be painted within 2 weeks after application, but should be thoroughly set and hard before painting begins.

There are two grades of metal-sash putty: one for interior and one for exterior glazing. Both wood-sash putty and metal-sash putty are known as oleoresinous caulking compounds. The advantage of these materials is their low cost; their disadvantages include high shrinkage, little adhesion, and an exposed life expectancy of less than 5 years.

Elastic Compounds

Elastic glazing compounds are specially formulated from selected processed oils and pigments, which remain plastic and resilient over a longer period than the common hard putties. Butyl and acrylic compounds are the most common elastics. Butyl compounds tend to stain masonry and have a high shrinkage factor. Acrylic-based materials require heating to 110°F before application. Some shrinkage occurs during curing. At high temperatures, these materials sag considerably in vertical joints. At low temperatures, acrylic-based materials become hard and brittle. The y are available in a wide range of colors and have good adhesion qualities.

Polybutane Tape

Polybutane tape is a nondrying mastic, which is available in extruded ribbon shapes. It has good adhesion qualities, but should not be used as a substitute or replacement for spacers. It can be used as a continuous bed material in conjunction with a polysulfide sealer compound. This tape must be pressure applied for proper adhesion.

Polysulfide Compounds

Polysulfide-base products are two-part synthetic rubber compounds based on a polysulfide polymer. The consistency of these compounds after mixing is similar to that of a caulking compound. The activator must be thoroughly mixed with the base compound at the job. The mixed compound is applied with either a caulking gun or spatula. The sealing surfaces must be extremely clean. Surrounding areas of glass should be protected before glazing. Excess and spilled material must be removed and the surfaces cleaned promptly. Once polysulfide elastomer glazing compound has cured, it is very difficult to remove. Any excess material left on the surfaces after glazing should be cleaned during the working time of the material (2 to 3 hours). Toluene and xylene are good solvents for this purpose.

Rubber Materials

Rubber compression materials are molded in various shapes. They are used as continuous gaskets and as intermittent spacer shims. A weathertight joint requires that the gasket be compressed at least 15 percent. Preformed materials reduce costs because careful cleaning of the glass is not necessary, and there is no waste of material.

MEASURING AND CUTTING GLASS

Always measure the length and width of the opening in which the glass is to fit at more than one place. Windows are often not absolutely square. If there is a difference between two measurements, use the smaller and then deduct 1/8 inch from the width and length to allow for expansion and contraction. Otherwise, the glass may crack with changes of temperature. This is especially true with steel casement windows.

Cutting glass is a matter of confidence-and experience. You can gain both by practicing on scrap glass before trying to cut window glass to size. Equipment required for glass cutting consists of a glass cutter, a flat, solid table, a tape measure, and a wood or metal T-square or straightedge. Look at figure 4-28. You should lightly oil the cutting wheel (view A) with a thin machine oil or lubricating fluid. Hold the cutter by resting your index finger on the flat part of the handle, as shown in view B.

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Figure 4-28.-Glass cutting.

To cut a piece of glass, lay a straightedge along the proposed cut, as shown in view C. Hold it down firmly with one hand and with the glass cutter in the other, make one continuous smooth stroke along the surface of the glass with the side of the cutter pressed against the straightedge (view D). The objective is to score the glass, not cut through it. You should be able to hear the cutter bite into the glass as it moves along. Make sure the cut is continuous and that you have not skipped any section. Going over a cut is a poor practice as the glass is sure to break away at that point. Snap the glass immediately after cutting by placing a pencil or long dowel under the score line and pressing with your hands on each side of the cut (view E). Frosted or patterned glass should be cut on the smooth side. Wire-reinforced glass can be cut the same as ordinary glass, except that you will have to separate the wires by flexing the two pieces up and down until the wire breaks or by cutting the wires with side-cutting pliers.

To cut a narrow strip from a large piece of glass, score a line and then tap gently underneath the score line with the cutter to open up an inch or so of the score line (view F). Next, grasp the glass on each side of the line and gently snap off the waste piece (view G). Press downward away from the score mark. If the strip does not break off cleanly, nibble it off with the pliers (view H) or the notches in the cutter. Slivers less than 1/2 inch wide are cut off by scoring the line and then nibbling off the waste. Do not nibble without first scoring a line. You can smooth off the edges of glass intended for shelving or tabletops with an oilstone dipped in water, as shown in view I. Rub the stone back and forth from end to end with the stone at a 45° angle to the glass. Rub the stone side to side only, not up and down.

No attempt should be made to change the size of heat-strengthened, tempered, or doubled-glazed units, since any such effort will result in permanent damage.

All heat-absobing glass must be clean cut. Nibbling to remove flares or to reduce oversized dimensions of heat-absorbing glass is not permitted.

SHEET GLASS SIZES AND GRADES

Sheet glass is produced in a number of thicknesses, but only 3/32- and 1/8-inch sheets are commonly used as a window glass. These thicknesses are designated, respectively, as single strength (SS) and double strength (DS). Thick sheet glass, manufactured by the same method as window glass, is used in openings that exceed window-glass-size recommendations. Table 4-1 lists the thicknesses, weights, and recommended maximum sizes. Sheet glass comes in six grades (table 4-2).

 

Table 4-1.-Weight and Maximum Sizes of Sheet Glass

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Table 4-2.-Grades of Sheet Glass

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The maximum size glass that may be used in a particular location is governed to a great extent by wind load. Wind velocities, and consequently wind pressures, increase with height above the ground. Various building codes or project specifications determine the maximum allowable glass area for wind load.

SASH PREPARATION

Attach the sash so that it will withstand the design load and comply with the specifications. Adjust, plumb, and square the sash to within 1/8 inch of nominal dimensions on shop drawings. Remove all rivets, screws, bolts, nail heads, welding fillets, and other projections from specified clearances. Seal all sash corners and fabrication intersections to make the sash watertight. Put a coat of primer paint on all sealing surfaces of wood sash and carbon steel sash. Use appropriate solvents to remove grease, lacquers, and other organic-protecting finishes from sealing surfaces of aluminum sash.

Wood

On old wood sashes, you must clean all putty runs of broken glass fragments and glazier’s points— triangular pieces of zinc or galvanized steel driven into the rabbet. Remove loose paint and putty by scraping. Wipe the surface clean with a cloth saturated in mineral spirits or turpentine; prime the putty runs and allow them to dry.

On new wood sashes, you should remove the dust, prime the putty runs, and allow them to dry. All new wood sashes should be pressure treated for decay protection.

Metal

On old metal sashes, you must remove loose paint or putty by scraping. Use steel wool or sandpaper to remove rust. Clean the surfaces thoroughly with a cloth saturated in mineral spirits or turpentine. Prime bare metal and allow it to dry thoroughly.

On new metal sashes, you should wipe the sash thoroughly with a cloth saturated in mineral spirits or turpentine to remove dust, dirt, oil, or grease. Remove any rust with steel wool or sandpaper. If the sash is not already factory-primed, prime it with rust-inhibitive paint and allow it to dry thoroughly.

GLAZING

"Glazing" refers to the installation of glass in prepared openings of windows, doors, partitions, and curtain walls. Glass may be held in place with glazier’s points, spring clips, or flexible glazing beads. Glass is kept from contact with the frame with various types of shims. Putty, sealants, or various types of caulking compounds are applied to make a weathertight joint between the glass and the frame.

Wood Sash

Most wood sash is face-glazed. The glass is installed in rabbets, consisting of L-shaped recesses cut into the sash or frame to receive and support panes of glass. The glass is held tightly against the frame by glazier’s points. The rabbet is then filled with putty. The putty is pressed firmly against the glass and beveled back against the wood frame with a putty knife. A priming paint is essential in glazing wood sash. The priming seals the pores of the wood, preventing the loss of oil from the putty. Wood frames are usually glazed from the outside (fig. 4-29).

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Figure 4-29.-Types of wood-sash glazing.

As we noted earlier, wood-sash putty is generally made with linseed oil and a pigment. Some putties contain soybean oil as a drying agent. Putty should not be painted until it is thoroughly set. A bead of putty or glazing compound is applied between the glass and the frame as a bedding. The bedding is usually applied to the frame before the glass is set. Back puttying is then used to force putty into spaces that may have been left between the frame and the glass.

Metal Windows and Doors

Glass set in metal frames must be prevented from making contact with metal. This may be accomplished by first applying a setting bed of metal-sash putty or glazing compound. Metal-sash putty differs from wood-sash putty in that it is formulated to adhere to a nonporous surface. Figure 4-30 shows examples of the types of metal-sash putty. Elastic glazing compounds may be used in place of putty. These compounds are produced from processed oils and pigments and will remain plastic and resilient over a longer period than will putty. A skin quickly forms over the outside of the compound after it is placed, while the interior remains soft. This type of glazing compound is used in windows or doors subject to twisting or vibration. It may be painted as soon as the surface has formed.

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Figure 4-30.-Types of metal-sash glazing.

For large panes of glass, setting blocks may be placed between the glass edges and the frame to maintain proper spacing of the glass in the openings. The blocks may be of wood, lead, neoprene, or some flexible material. For large openings, flexible shims must be set between the face of the glass and the glazing channel to allow for movement. Plastics and heat-absorbing or reflective glass require more clearance to allow for greater expansion. The shims may be in the form of a continuous tape of a butyl-rubber-based compound, which has been extruded into soft, tacky, ready-to-use tape that adheres to any clean, dry surface. The tape is applied to the frame and the glass-holding stop before the glass is placed in a frame. Under compression, the tape also serves as a sealant.

Glass may be held in place in the frame by spring clips inserted in holes in the metal frame or by continuous angles or stops attached to the frame with screws or snap-on spring clips. The frames of metal windows are shaped either for outside or inside glazing.

SETTING GLASS IN WOOD AND METAL SASHES

Do not glaze or reglaze exterior sash when the temperature is 40°F or lower unless absolutely necessary. Sash and door members must be thoroughly cleaned of dust with a brush or cloth dampened with turpentine or mineral spirits. Lay a continuous 1/6-inch-thick bed of putty or compound in the putty run (fig. 4-31). The glazed face of the sash can be recognized as the size on which the glass was cut. If the glass has a bowed surface, it should be set with the concave side in. Wire glass is set with the twist vertical. Press the glass firmly into place so that the bed putty will fill all irregularities.

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Figure 4-31.—Setting glass with glazier’s points and putty.

When glazing wood sash, insert two glazier’s points per side for small lights and about 8 inches apart on all sides for large lights. When glazing metal sash, use wire clips or metal glazing beads.

After the glass has been bedded, lay a continuous bead of putty against the perimeter of the glass-face putty run. Press the putty with a putty knife or glazing tool with sufficient pressure to ensure its complete adhesion to the glass and sash. Finish with full, smooth, accurately formed bevels with clean-cut miters. Trim up the bed putty on the reverse side of the glass. When glazing or reglazing interior sash and transoms and interior doors, you should use wood or metal glazing beads. Exterior doors and hinged transoms should have glass secured in place with inside wood or metal glazing beads bedded in putty. In setting wired glass for security purposes, set wood or metal glazing beads, and secure with screws on the side facing the area to be protected.

Weed-sash putty should be painted as soon as it has surface-hardened. Do not wait longer than 2 months after glazing. When painting the glazing compound, overlap the glass 1/16 inch as a seal against moisture.

For metal sashes, use type 1 metal sash elastic compound. Metal-sash putty should be painted immediately after a firm skin has formed on the surface.

Depending on weather conditions, the time for skinning over may be 2 to 10 days. Type II metal-sash putty can usually be painted within 2 weeks after placing. This putty should not be painted before it has hardened because early painting may retard the set.

Clean the glass on both sides after painting. A cloth moistened with mineral spirits will remove putty stains. When scrapers are used, care should be exercised to avoid breaking the paint seal at the putty edge.

After installing large glass units in buildings under construction, it is considered good practice to place a large "X" on the glass. Use masking tape or washable paint. This will alert workers so they will not walk into the glass or damage it with tools and materials.

 

David L. Heiserman, Editor

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Revised: June 06, 2015