All About Glass
People have known about glass for thousands of years, but few dare to handle it every day. Those who do are glaziers (not glaciers); members of a select, little-known professional trade specializing in glass installation and restoration. As science and research continue to develop new glass products, the glazing profession becomes more complex. People with mechanical ability, detailed thinking and hands-on work skill, who love a new challenge every day, are in high demand nationwide to join the glazing profession. Glass Doctor franchise owners recruit the best glaziers to be glass specialists, especially those who are certified by the National Glass Association (NGA). Glass Doctor specialists are qualified to cut and handle all sizes and types of glass. Because the glass industry is changing quickly, Glass Doctor specialists continue to develop their skills through Web-based distance learning tools such as MyGlassClass.com. Some Glass Doctor specialists are cross-trained to work on home, auto and commercial glass, while others specialize in one type of glass; similar to general practitioners and specialists in the medical field. While practicing glass repair and replacement is not as complex as practicing medicine, the same care must be taken because the glass is important to the person - the customer - who needs help.
Generally, people know what glass looks like, but they may not know what glass is. Glass is defined as “a hard, brittle, amorphous, usually transparent or translucent material consisting typically of silica.” (The Merriam-Webster Dictionary). The “material” actually is a liquid of very great viscosity or “resistance to flow in a fluid.” Glass is known as “the fourth state of matter” because it has no solid or gaseous state. Basically, glass is sand; a high quality silica sand, to which other materials are added. The resulting mixture is called a batch. Other materials in the batch are salt cake, limestone, dolomite, feldspar, soda ash and powdered cullet. Cullet is broken glass. It remains from a previous batch or from the edges that remain after a batch of glass has been formed and cut to size. Adding cullet helps the batch melt easily. Glass is made by melting and cooling the batch. As the batch cools, it becomes solid without forming crystals. Crystals are three-dimensional building blocks that make a substance internally rigid. The lack of crystals makes glass technically a liquid, not a solid. It also makes glass transparent.
At one time most of the glass manufactured in the United States was plate glass. Plate glass was made by a process of grinding and polishing. No longer made in this country, plate glass has been replaced by float glass.Float glass is a term that refers to a process of making glass that was perfected in 1959 by Pilkington Brothers, Ltd. of England. Float glass is made by pouring the molten glass from a furnace into a chamber that contains a bed of molten tin. The atmosphere inside the chamber is carefully controlled. The glass floats on the tin and forms itself in the shape of the container. It spreads 90 to 140 inches wide at a thickness determined at the time of manufacture. The length of the glass from the furnace to the cutter is about a mile. The upper surface of the glass is called the air side or score side. It is polished with fire. The lower surface is called the tin side. It is not fire-polished. From the chamber, the glass enters an oven, called a lehr. There it is slowly cooled at a specific rate. This process, called annealing, relieves the glass of internal stresses. The rate of cooling is crucial to the success of the final product. The glass emerges from the lehr at room temperature as a continuous ribbon. It is flat, fire-finished on the top, and has smooth, parallel surfaces. Automatic cutters trim the edges and cut the glass to length. There are two types of glass made by the float process, clear glass and tinted glass. Most of the flat glass made by the float process is clear glass. As its name implies, clear glass is transparent and colorless. Depending upon its thickness, clear glass allows about 75 to 92 percent of the visible light to pass through. Tinted glass (also called heat absorbing glass), is made by adding coloring agents to the batch mix. These agents include bronze, gray, green and blue. As the glass gets thicker, the density of the color also increases. This causes the glass to transmit less visible light. The light transmittance of tinted glass varies from 14 to 83 percent depending upon its color and thickness.
Rolled glass is manufactured by pouring glass from the furnace into a series of rollers. It is then shaped to the desired thickness, annealed and cut to size. The two basic types of rolled glass are patterned and wired. Patterned glass is also called figured glass, obscure glass, and decorative glass. It is available in thicknesses from 1/8" to 3/8". Patterned glass is made by passing it through rollers that have patterns on them. The pattern is transferred to one or both sides of the glass. Each manufacturer of patterned glass has unique patterns. Wired glass is made by feeding a welded wire net of a particular design into the molten glass just before it enters the rollers. The wire does not add to the strength of the glass but it does hold the lite (a cut piece of glass) in the window sash if it shatters. Wired glass is used in fire-rated windows and doors because it meets most fire codes. For these applications, all the wires must be embedded in the glass. However, even though it meets fire codes, wired glass is not a safety glass. In fact, it has only one half the strength of annealed glass of the same thickness. In addition, wired glass cannot be tempered.
The rate of cooling directly affects the strength of glass. The regular process of cooling - or annealing - float glass results in a slow rate. Stronger glass can be produced by changing the rate of cooling. Two types of stronger glass are heat-strengthened and tempered. Heat-strengthened glass is cooled at a rate faster than regular annealed glass. Tempered glass, in turn, is cooled at a faster rate than heat strengthened glass. Another way to strengthen glass is to use more than one lite of glass in the application. Laminated glass consists of two or more lites of glass, joined by a layer of plastic.Heat-strengthened glass is made by heating annealed glass uniformly, then cooling it at a slow rate. Heat-strengthened glass is about twice as strong as regular annealed glass of the same size and thickness, which makes it more resistant to wind loading and impacts. When it breaks, heat-strengthened glass fractures into large, jagged pieces similar to annealed glass. Heat-strengthened glass is generally used in high-rise buildings to help the glass resist thermal stress. Because heat-strengthened glass fractures into large jagged pieces, it does not qualify as a safety glazing material. All building codes require safety glazing for shower doors, commercial doors and store fronts for safety purposes. Tempered glass is made by heating annealed glass uniformly. The annealed glass is then cooled rapidly by blowing air uniformly onto both surfaces at the same time. This is known as air quenching. Rapid cooling increases the compression forces on the surface and the tension forces inside the glass. Tempered glass can be from 1/8" to 3/4" thick. Tempered glass is about four times stronger than a lite of annealed glass of the same size and thickness. The only characteristic of the annealed glass affected by tempering is its bending or tensile strength. Tempering increases the tensile strength of glass. This makes tempered glass better able to resist the forces caused by heat, wind and impact. Tempering does not change:
- The color, chemical composition, or light transmission characteristics of the annealed glass
- Its compression strength (the ability of the glass to resist crushing forces)
- The rate at which the glass conducts and transmits heat
- The rate at which the glass expands when heated
- The stiffness of the glass
Tempered glass offers greater strength against deflection and better wind resistance than heat-strengthened glass. Tempering increases the ability of glass to survive the impact of objects that may strike the building. When tempered glass does break, it shatters into small cubes, reducing the likelihood of serious injury on impact. Therefore, it qualifies as a safety glazing material. Tempering increases a lite’s edge strength, so it is specified when designers anticipate high thermal stresses. Many large storefront windows are tempered glass, which can be identified by a “bug,” a permanent label in the corner of the window. Tempered glass cannot be cut, drilled or edged. Laminated glass, sometimes called “lami,” is made by placing a layer of polyvinyl butyral (PVB) between two or more glass lites. The PVB can be clear or tinted and commonly varies in thickness from .015" to .090", but it can be as thick as .120" for special applications. The entire unit is then fused under heat and pressure in a special oven called an autoclave. The laminating process can be performed on clear, tinted, reflective, heat-strengthened or tempered glass. When laminated glass breaks, the glass particles adhere to the PVB and do not fly or fall. Certain combinations of glass and PVB thicknesses qualify as safety glazing materials under the health and safety standards set by the American National Standards Institute (ANSI). A vehicle windshield is made of a layer of laminate sandwiched between two layers of glass. The blue tint across the top of a windshield is actually the edge of the laminate layer. Laminate gives windshields their extra strength to help a vehicle maintain its structural integrity during a crash. When the Ford Model T automobile was first introduced, the glass windscreen was an optional extra feature. Today, most automotive safety experts rank the windshield as the third most important safety component in today’s cars and trucks, according to the Car Care Council, which sponsors National Car Care Month. Seatbelts and airbags rank first and second. All three components are part of what automotive engineers refer to as a vehicle’s safety restraint system (SRS). Bullet-resistant glass is made of several layers of laminate and glass. In between the glass is a polycarbonate material that absorbs the energy of what has been fired at it. The thicker the glass, the higher impact it can withstand. There is even one-way bulletproof glass that enables the target victim to shoot back, but not to be struck. Laminated glass is used in safety glazing applications. However, it has many other applications, such as burglar resistance, sound reduction, sloped glazing and space enclosures. Laminated glass is used as the inboard lite in skylights. By varying the thickness and color of the PVB, laminated glass can be used to reduce the transmission of solar energy, control glare, and screen out ultraviolet radiation, similar to reflective glass.
Reflective glass is clear or tinted glass that has a very thin layer of metal or metallic oxide on the surface. The reflective coating reduces heat gain and glare from the outside while allowing visible light to enter. Characteristics of reflective glass include:
- Appearance: Reflective glass gives a building a mirror-like appearance. The coatings are available in silver, copper, gold and earthtone. They can be combined with tinted glass to give a building a beautiful exterior.
- Energy savings: Because it reflects and absorbs the sun's rays, reflective glass reduces the amount of solar radiation that enters the building. This can save money in heating and air-conditioning costs.
- Comfort: Reflective glass reduces variations in the interior temperature of a building.
Low-Emissivity glass, commonly called low-E glass, is a type of reflective glass that is gaining in popularity, especially in residential and office applications. Low-E coatings are very thin metallic coatings that reduce visible light transmission by about 10 percent compared to uncoated glass. They are applied using either the vacuum (sputter) or pyrolytic process. Low-E glass reduces heat loss through windows and re-radiates the heat absorbed from sunlight back inside the room. Low-E glass allows sunlight into a room without letting heat escape outdoors. Because it resists ultraviolet light, low-E glass prevents sunlight exposure from damaging carpets, draperies and other furnishings. Plus, low-E glass reduces sunlight glare in a room. The main reason low-E glass has these advantages is that it reflects sensible heat. The heat generated by hot water or steam radiators, or the heat from hot air ducts, are examples of sensible heat. Low-E glass retains more of this heat indoors than other types of reflective glass. In northern areas, low-E coatings let in the heat from the winter sun while retaining the heat generated from inside the building. In southern areas, low-E coatings are usually applied to bronze, green or gray tinted glass. The coatings reduce glare and reflect the sun’s heat away from the structure.
Insulating Glass Units
Insulating glass units, commonly called IG units, became options for homes in the 1960s. Considering today’s rising energy costs, IG units now are a viable way to lower energy bills while maintaining a consistent, comfortable temperature in homes and offices. IG units are windows made from two or more lites of glass separated by a sealed air space. The metal tube around the perimeter of the insulated unit that separates the two lites of glass is called the spacer. This spacer comes in thicknesses of 3/16" and larger. It is filled with a special moisture absorbing material called a desiccant. The perimeter of the entire unit is sealed with a high grade sealant. IG units reduce the tendency of condensation to form on the room side of the glass. IG units also reduce cold transmittance at windows and helps maintain a uniform temperature in the home or office. In the winter, IG units reduce heat loss and in the summer they reduce heat gain. Although soundproofing is not their intended purpose, IG units can help reduce the level of noise from the outside, especially if the lites are laminated glass. There are two types of IG units commonly manufactured, Single Seal Units and Double Seal Units. The difference between the two, as their names suggest, is the presence of a single or double seal between the spacer and the glass. Single-sealed units can use several types of sealants: hot melt butyl, polysulfide, silicone, or urethane. Double-sealed units can use PIB tape for the primary seal and hot melt butyl (one part silicone, or two-part polysulfide) for the secondary seal. IG units need not use the same type of glass. Tempered and annealed glass can be used in the same unit. Patterned glass can be used but the pattern should face the outside. If one of the lites is reflective or tinted glass, it must face the exterior. If reflective glass is to face the interior, it may be necessary to temper one or both lites to guard against thermal breaking. A sandblasted finish is not recommended for an insulating glass unit because sandblasting reduces the strength of the glass.
Low-E, reflective and IG glass contribute to energy efficiency by increasing the effectiveness of the insulating system. Energy efficiency is measured in two ways, the U-value and the R-value. The U-value is a measure of the heat gain or loss through glass due to the difference between the indoor and outdoor temperatures. The lower the U-value, the less heat is transmitted through the glass. The R-value measures the overall resistance to heat transfer. The R-value is the reciprocal of the U-value. The higher the R-Value, the less heat is transmitted through the glass. For example, a material with an R-value of 19 is a much better insulator than one with an R-value of 6.
A relatively new product, low-maintenance glass has a coating that helps to break down organic material so that water can remove it as it sheets off the glass. Low-maintenance glass is “sprinkled” with metallic titanium dioxide atoms during the glass manufacturing process. The coating forms a microscopic layer on one side of the glass. When the coated side of the glass is exposed to the sun’s ultraviolet (UV) rays, it chemically reacts to cause organic material on the glass to decompose. Even when the sky is overcast, the coating keeps working, because more than 80 percent of UV rays still penetrate the clouds. When glass is coated in titanium dioxide, it makes water move faster. Instead of beading up, the water sheets off and takes the decomposed dirt away. Low-maintenance glass is hydrophilic, which means it pushes water away, making low-maintenance glass a long-term solution compared to aftermarket chemical coatings. These products are hydrophobic, which means they repel water. A repellent product will degrade over time, but titanium dioxide remains for the life of the window. Titanium dioxide coating reduces the need to use chemical cleaning agents to clean the glass of most stains. If windows get “hard water” calcium deposit streaks after washing or watering the lawn, homeowners should spray a mixture of two drops of detergent to 24 ounces of water. Spray the mixture on the hard water stains, let it sit for a minute and then wipe it off with a clean cloth.
Mirrors consist of high quality, annealed float glass designated either mirror quality or mirror select. Mirrors are made by depositing a layer of silver on one surface of the glass. The surface chosen is the score side, because the tin side does not accept silver properly. The reflective quality of the mirror depends upon the thickness of the silver layer, glass thickness, and glass color. Spandrel glass is not specifically a type of glass. Instead, it refers to the use of obscure glass in non-vision areas of a building. One of these areas is the spandrel of a building. The spandrel is the part of the wall between the head of one window and the sill of the window above it. For aesthetic reasons, this area is often covered with glass.