Glass

Glass History

By 10th century A.D. the Venetian Island of Murano had

become major center for glassmaking

•Crown glass and cylinder glass were produced

Crown glass process

Blowing large glass sphere

Iron rod called punty used to hold glass

Reheating sphere resulting in crown 30”+ in

diameter

Cooled and cut

Crown glass

Face Shield

Punty

Disk or crown shaped glass

Hand Shield

Blowpipe on other side of punty removed leaving a hole

Glass History

• Cylinder Glass Process

Blowing large glass sphere

Swinging action resulted in cylinder

Hemispherical ends were cut

Cylinder was slit lengthwise and

reheated to flatten into rectangular sheet

Cut into panes

Swinging action resulting in cylinder

Hemispherical ends are cut

Blowing glass spheres

Cylinder Glass

•Lack of optical quality of crown and cylinder glass

gave way to introduction of plate glass in 17th century

in France

• Molten glass was cast into frames• Spread into sheets by rollers• Cooled and ground flat• Polished and cut

•Cylinder process evolved in 19th Century, drawing

cylinders of molten glass vertically from a crucible.

•In 20th century drawn glass from a container of

molten glass was produced

Glass History

•In 1959 Pilkington Brothers Ltd started producing float glass

which is the basis of all modern day glass

Float glass

• Glass is floated on molten tin and hardens on it

producing parallel surfaces, high optical quality and

brilliant surface finish

• Automatic cutting can produce glass to desired sizes

• Annealing gradually cools the glass to avoid locked-

in stresses

•Glazing is the installation of glass or the transparent material

in a glazed opening

•Installer of glass is called glazier

•Individual pieces are called lights or lites

Glass History

Float Glass

• Glass is made from– Sand (silicone dioxide)– Soda ash (sodium hydroxide or sodium carbonate)– Lime (calcium)– Alumina– Potassium oxide– Various elements to control color

• Glass is super-cooled liquid • When drawn into small fibers, glass is stronger

than steel but not as stiff.• Strength is impacted by imperfections in glass.• It has no fixed melting point.

Glass Ingredients and Thicknesses

• Thicknesses range from approximately 3/32 inch ( 2.5 mm) (single strength) to 1/8 inch (3 mm) (double strength) to 1 inch (25.4 mm)

• Glass thickness for a window is determined by the size of the light and the expected maximum wind load on the glass

• During its manufacture, ordinary window glass is annealed, meaning that it is cooled slowly under controlled conditions to avoid locked-in thermal stresses that might cause it to behave unpredictably in use.

Glass Ingredients and Thicknesses

• Heat treatments such as tempering impact glass

• strength and uses

• Heat-treated glass is produced by reheating annealed glass in an oven to approximately 1150 degrees Fahrenheit (620oC) and then cooling (quenching) both of its surfaces rapidly with blasts of air while its core cools much more slowly.

• This process induces permanent compressive stresses in the edges and faces of the glass and tensile stresses in the core.

Heat treated glass

• The resulting glass is stronger in bending than annealed glass and more resistant to thermal stress and impact.

• These properties make heat-treated glass useful for windows exposed to heavy wind pressures, impact, or intense heat or cold.

• By adjusting the quenching process, greater or lesser degrees of residual stress may be introduced into the glass, producing products referred to as either "tempered" or "heat-strengthened glass"

Heat treated glass

• Heat-Strengthened Glass– Lower-cost heat-strengthened glass may be used instead of tempered

glass.

– The induced compressive stresses in the surface and edges of heat-strengthened glass are about one-third as high as those in fully tempered glass (typically 5000 psi compared to 15,000 psi for tempered glass, or 34 MPa versus 104 MPa).

– Heat-strengthened glass is about twice as strong in bending as annealed glass and is more resistant to thermal stress. It usually has fewer distortions than tempered glass. Its breakage behavior is more like that of annealed glass than tempered glass. For that reason, it cannot be used where safety glazing is required except in laminated form (laminated glass is discussed below).

Heat treated glass

• Tempered glass – has higher residual stresses than heat-strengthened glass and is about four times as strong in

bending as annealed glass.

– If it does break, the sudden release of its internal stresses reduces tempered glass instantaneously to small, square-edged granules rather than long, sharp-edged shards.

– This characteristic, combined with its high strength, qualifies it for use as safety glazing.

– Tempered glass is also used for all-glass doors that have no frame at, for whole walls of squash and handball courts, for hockey rink enclosures, and for basketball backboards.

– Tempered glass is more costly than annealed glass.

– It often has noticeable optical distortions created by the tempering process.

– In addition, all cutting to size, drilling, and edging must be done before the heat treatment of the glass because any such operations after tempering will release the stresses in the glass and cause it to disintegrate.

– Tempered glass is also sometimes referred to as fully tempered glass to distinguish it more clearly from heat strengthened glass.

Heat treated glass

Tempered glass

Laminated Glass

•Laminated glass is made by sandwiching a

transparent polyvinyl butyral (PVB) interlayer

between sheets of glass and bonding the three layers

together under heat and pressure.

•Laminated glass is not as strong as annealed glass of

the same thickness, but when laminated glass breaks,

the soft interlayer holds the shards of glass in place

rather than allowing them to fall out of the frame of

the window.

•This makes laminated glass useful for skylights

and overhead glazing, because it reduces the risk of

injury to people below in case of breakage

Laminated Glass

•The PVB interlayer may be colored or patterned to

produce a wide range of visual effects in laminated

glass.

•Because laminated glass does not create dangerous,

loose shards of glass when it breaks, it also qualifies

as safety glazing.

• Laminated glass is a better barrier to the

transmission of sound than solid glass.

•It is used to glaze windows of residences, classrooms,

hospital rooms, and other rooms that must be kept

quiet in the midst of noisy environments.

•It is especially effective when installed in two or more layers with airspaces

between.

•In comparison to solid glass, laminated glass also reduces the transmission of

ultraviolet (UV) radiation, a component of sunlight that contributes significantly

to fading and the degradation of interior finishes, furnishings, and fabrics.

•Security glass, used for drive-in banking windows and other facilities that need

to be resistant to burglary, is made of multiple layers of glass and PVB, and is

available in a range of thicknesses to stop any desired caliber of bullet.

•Laminated glass is also used in blast-resistant and windborne debris-resistant

glazing systems

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 prestressed 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.

Chemically Strengthened 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

and in laminated form for security glass, blast-resistant glass,

and windborne debris-resistant glass.

Fire-Rated 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 glass and can achieve

fire resistance ratings ranging from 20 minutes to 3 hours.

Fire-Rated Glass

•Two other fire-rated glass types are fire-retardant filled double glazing and

intumescent interlayer laminated glazing.

Fire retardant filled double glazing consists of a clear, heat absorbing

polymer gel contained between two sheets of tempered glass.

Intumescent interlayer laminated glazing is made of thin layers of

transparent intumescent material sandwiched between multiple layers of

annealed glass.

When either glass type is heated by fire, the gel or intumescent

interlayers react to form opaque, insulating layers. As a result, these

products not only resist the passage of flame and smoke, they also limit the

rise in surface temperature of the glass on the side opposite the fire and

prevent the transfer of radiant heat through the glass. These added

protective properties make these glass types suitable for use in larger sizes

and in a broader range of applications than other types of fire-rated glass.

Fire resistance ratings of up to 2 hours can be achieved.

Fritted Glass• A number of producers are equipped to imprint the surface of

glass with silkscreened 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 fired 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 silkscreened glass are various dot and stripe motifs 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.

Spandrel Glass• 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.

Spandrel Glass• 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 GlassGlass manufacturers have also developed tinted and reflective glasses that reduce glare and cut down on solar heat gain.

Tinted Glass

• 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

Tinted and Reflective Coated Glass

Tinted Glass

• 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 gold.

• 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 by the glass and converted to heat must go somewhere, and a substantial portion of it is conducted or reradiated to the interior of the building

Tinted and Reflective Coated Glass

Tinted Glass

• To evaluate the effectiveness of glass in reducing heat gain from solar radiation, a measure called the solar heat gain coefficient(SHGC) is used; it is the ratio of solar heat admitted through a particular glass to the total heat energy striking the glass. SHGC accounts for the solar radiation that passes through glass, as well as for heat that is conducted or radiated into the space due to heating of the glass itself.

• Clear glasses have solar heat gain coefficients ranging from about 0.90 to 0.70, depending on the clarity and thickness of the glass.

• Solar heat gain coefficients for tinted glasses range from about 0.70 to 0.35, meaning that these glasses allow 70 to 35 percent of the solar heat energy striking the glass to pass through.

Tinted and Reflective Coated Glass

Tinted Glass

Tinted and Reflective Coated Glass

Tinted Glass

• Generally speaking, for buildings dominated by a heating load, glass with a high SHGC is desirable to take advantage of passive solar heat gains.

• In buildings dominated by cooling, glass with a low SHGC is preferable to minimize unwanted solar heating. (Shading coefficient, a measure similar to SHGC, is an older measure of reduction in solar heat gain that has been mostly replaced by SHGC.)

Tinted and Reflective Coated Glass

Tinted Glass

• Visible transmittance and solar heat gain coefficient can be combined to determine the light to solar gain (LSG) ratio, a useful measure of the overall energy-conserving potential of glass. The LSG ratio is defined as the visible light transmittance divided by the solar heat gain coefficient. A glass with high LSG admits a relatively large portion of visible light in comparison to the amount of solar heat admitted, combining the greatest daylighting potential with the least solar heating potential.

• Green and blue-tinted glasses tend to have high LSG ratios values, while those of bronze, gold, and gray tints tend to be lower.

Tinted and Reflective Coated Glass

Reflective Coated Glass

Tinted and Reflective Coated Glass

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

Tinted and Reflective Coated Glass

Reflective Coated Glass

• 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.

Tinted and Reflective Coated Glass

Reflective Coated Glass

• The sunlight reflected by a building glazed with reflective coated glass can be helpful in some circumstances by lighting an otherwise dark urban street space. It can also create problems in other situations by bouncing solar heat and glare into neighboring buildings and onto the street.

Insulating Glass• Window glass is a poor thermal insulator. A single sheet of

glass (single glazing) conducts heat about 5 times as fast as 1 inch (25 mm) of polystyrene foam insulation and 20 times as fast as a well-insulated wall.

• A second sheet of glass applied to a window with an airspace between the two sheets (double glazing) cuts this rate of heat loss in half, and a third sheet with its additional airspace (triple glazing) reduces the rate of heat loss to about a third of the rate through a single sheet.

• A triple-glazed window, however, still loses heat about six times as fast as the wall in which it is placed..

Insulating Glass• To prevent moisture condensation

within the airspace of double or triple glazing (also called insulatingglass units or IGUs), the units are usually hermetically sealed at the time of manufacture with dry air inserted in the space between the glass lights.

• Originally, for small lights of double glass, the edges of the two sheets were simply fused together

Insulating Glass• However, this detail is seldom used now because the fused

glass edge is highly conductive of heat.

• Instead, a hollow metal edge spacer (also called a spline) is inserted between the edges of the sheets of glass, and the edges are closed with an organic sealant compound.

• For slightly improved thermal performance, stainless steel, which is less conductive of heat, may be used instead of aluminum for the spacer, and a sealant material may be placed between the glass and the spacer as a thermal break.

Insulating Glass• For even better thermal performance, so-called warm edge

spacers made of thermally broken aluminum or extruded rubber may be used.

• A small amount of a chemical drying agent, or desiccant, is left inside the spacer to remove any residual moisture from the trapped air. The air is always inserted at atmospheric pressure to avoid structural pressures on the glass.

• When insulating glass units do exhibit internal condensation, it is a sign of failure of the edge seal, and the unit must be replaced

Insulating Glass• The thickness of the airspace in insulating glass units is less

critical to the units insulating value than the mere presence of the airspace: From 3_8 inch (9 mm) up to about 1 inch (25 mm) of thickness, the insulating value of the airspace increases somewhat, but above that thickness little additional benefit is gained.

• A standard overall thickness for large lights of double glazing is 1 inch (25.4 mm), which results in an airspace ½ inch (13 mm) thick if 1_4-inch (6-mm) glass is used.

• and their U-Factors are listed

Insulating Glass• The thermal performance of insulated glazing units can also

be improved by introducing gases with greater density and lower thermal conductivity than that of ordinary air between the sheets of glass. Depending on the gas used and the thickness of the space between the glass sheets, improvements in thermal performance of 12 to 18 percent are possible. Argon and krypton are the gases most commonly used.

Insulating Glass• The performance of glazing as a thermal insulator is

quantified as its U-Factor. U-Factor is expressed as BTUs per square foot-hour-degree Fahrenheit (BTU/ft2-hr-oF) or, in metric units, as watts per square meter- degree Kelvin (W/m2-oK). U-Factor is the mathematical reciprocal of R and as such, lower values represent improved thermal performance.

Insulating Glass

• Some examples glazing configurations and their U-Factors are listed

Low-Emissivity Coated Glass• The thermal performance of glazing can be improved

substantially by the use of glass with a low-emissivity (low-e) coating. Low-e coatings are ultrathin, virtually transparent, and almost colorless metallic coatings that selectively reflect solar radiation of different wavelengths. They have a high visible light transmittance and, depending on the particular coating, a low transmittance for some or all types of infrared radiation (heat).

Self-Cleaning Glass• Self-cleaning glass is coated with titanium oxide on its

exterior surface. This coating acts as a catalyst that enables sunlight to convert organic dirt to carbon dioxide and water.

• It also causes rainwater to run down the surface in sheets rather than to bead up. Nonorganic dirt, such as sand, is unaffected by the catalyst, but the sheets of water are more effective at removing such matter than beaded water.

Chromogenic glass• Glass that can change its optical properties is called

chromogenic glass.

• Thermochromic glass becomes darker when it is warmed by the sun. Photochromic glass becomes darker when exposed to bright light. Both Thermochromic glass and Photochromic glass types are potentially valuable as passive devices to reduce cooling loads in buildings

Chromogenic Glass

• Electrochromic glass changes its transparency in response to the passage of electric current. Also called switchable glass, it can be actively controlled by building occupants or automated systems, allowing, in comparison to passive technologies, more precise response to requirements for control of solar heat gain, daylighting, or occupant privacy.

• Gasochromic glass is another developing switchable technology for altering the light transmittance of glass, in which the transparency of a reactive coating on the number 2 surface of an insulated glass unit is altered by the pumping of gas into or out of the interstitial space of the unit.

Other types of Glass

• Antireflective glass minimizes residual reflections that normally occur when light levels differ significantly on the opposite sides of glass.

• Mirrors are made of mirror glass, which has a thin silver based coating on its back side. A thin layer of copper applied over the silver prevents corrosion, and a second layer of backing paint provides additional protection.

• Patterned glass, hot glass rolled into sheets with many different surface patterns and textures, is used where light transmission is desired but vision must be obscured for privacy.

• Photovoltaic glass is coated with a thin film of amorphous silicon that generates electricity from sunlight.

• Traditional stained glass and contemporary colored glass, formulated with ingredients that alter the color of the glass, can be used in a wide range of artistic and architectural applications.

Other types of Glass

• Transparent plastic sheet materials are often used instead of glass for specialized glazing applications. The two most common plastic glazing materials are acrylic and polycarbonate. Plastic glazing is most commonly used where glass is inappropriate:

• Plastics can be cut to shapes with inside corners (L-shapes and T-shapes, for example) that are likely to crack if cut from glass. They can be bent easily to fit in curved frames.

Plastic Glazing Sheets

• Aerogel, silicon-based foam that is 99.8 percent air, can be used to fill the cavity in double-glazed glass or plastic products. Aerogel is milky in color, not fully transparent, and has a visible transmittance that varies with its thickness. Aerogel-filled glazing has a good light to solar gain ratio, making it an efficient source of diffuse, low-contrast, natural daylight. Currently available aerogel products can achieve insulation value more than twice that of glass fiber insulation.

Aerogel-Filled Glazing