Experts believe that the ancient Syrians discovered glassmaking, probably by accident, about 3,000 B.C. A manufactured green glass rod found in ancient Babylonia (modern Iraq), dates to 2,600 B.C. Syrian glass was a simple melted mixture of soda ash, lime and sand. Glassmakers formed it into final shapes by sculpting it while still hot.

When Egypt conquered Syria in 1,400 B.C., the captured Syrian glass workers were sent back to Egypt. They refined glass making into a higher art. Glassworkers produced vessels, vases and eating utensils by pouring several thin layers of molten glass into sand molds in the shape of the object. The glassware was decorated by adding molten colored glass drips to the final layer.

The Egyptians discovered that they could blow a gob of molten glass from the end of a hollow metal tube into the mold. This technique evolved into what we now know as glass blowing. Glass blowing remains a useful technique for creating many types of glassware.

Window glass originated in Rome, but it was very thick and translucent. That is, it let light in but people couldn't see out. In 1291, on the Italian island of Murano, workers developed a clear, almost transparent glass called "cristallo." This is where the word "crystal" comes from. Murano glassware became popular throughout Europe, and Italy built up a thriving export trade.

In the Middle Ages, glass making was still a hand-made process. Window glass was made by blowing the molten glass into a flat disc which was then spun so that centrifugal force caused the glass to thin out and flatten. These discs were cut into small panes of glass, usually limited to 18 square inches. Glass workers searched for improvements.

Cylinder glass was one such improvement. The molten glass was blown into a cylinder which was cut apart, then reheated and flattened. In the 16th and 17th Centuries, the English discovered that using coal instead of wood in their furnaces produced a much clearer glass. Although the panes were wavy and full of bubbles, and sometimes light amethyst or amber in color, people could actually see through their windows.

Louis Lucas de Nehou, a Frenchman, developed a manual process for making plate glass in 1688. The method was cumbersome-it took 16 days from start to finish and produced glass so expensive that only the very rich could afford it. For the next two hundred years, improvements were made in this process, primarily in the power sources needed to melt the raw materials into glass, and in methods to increase the amount of glass that could be produced. But the French plate glass method remained the basic technique. Finally, in the 1900's, technological improvements were developed which made possible large scale glass manufacturing as we know it today.

Modern Glass

Today, the glass making industry is very sophisticated. Glaziers use a wide variety of glass, depending upon the functions that the glass must perform. What Is Glass? Basically, glass is sand-a very high quality silica sand, to which other materials are added. The resulting mixture is called a batch. Some of the other materials included in the batch are salt cake, limestone, dolomite, feldspar, soda ash and powdered cullet. Cullet is broken glass. It can be left over 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 more 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.

Types of Glass

Glass comes in many shapes and forms. 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.

Because the process is so highly automated, individual lites of glass are not labeled. Shipments of large custom-cut lites are generally shipped in cases that list size, quantity and quality. Each case weighs from 3,000 to 4,000 pounds. Glass can also be shipped in a loose pack, called a stoce. The stoce is bound together by banding material. Stoce glass weighs from 4,000 to 10,000 pounds.

There are two types of glass made by the float process:
• Clear glass
• Tinted or heat absorbing 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. This characteristic of glass is called its light transmittance.

The specifications written by the American Society for Testing Materials (ASTM) separates tinted and heat absorbing glass into two categories. This is misleading, however, because all tinted glass absorbs heat. Tinted, or heat absorbing glass, is made by adding coloring agents to the batch mix. These agents include bronze, gray, green and blue. What tinting does:
• Tinting glass: Reduces the amount of light that passes through the glass.
• Causes the glass to absorb more of the sun's rays.

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.

Edge conditions are crucial to the effectiveness of tinted glass because a flaw at the edge can cause the glass to fail as it absorbs heat. Two types of failures are:
• Heat breaks
• Pressure breaks

A heat break occurs at an angle of 90 degrees to the surface of the glass. Heat breaks resemble smooth curves. A pressure break occurs along the surface, usually starting at a corner.

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 Glass
• Wired Glass

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. Patterning glass has several purposes:
• It controls light.
• It diffuses the details of objects.
• It is decorative.

Patterned glass is available in colors, but the choice is limited. Some of the patterns, because of their depth, make tempering the glass impossible.

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 in the sash if it shatters. Although manufacturers have unique wire patterns, there are some common ones. A diamond shaped pattern is called misco. A baroque pattern is square. Wired glass can be patterned on one or both sides. If the glass is patterned on both sides, it is usually called rough glass.

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. There are limitations on the square footage allowed in openings. In other applications, the edges of the glass must be sealed to prevent the wires from rusting. 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.

Strengthening Glass

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 Glass
• Tempered Glass

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.

In many modern buildings, the glass must be as strong as possible. Three basic reasons to strengthen glass are to:
• Increase Wind Load
• Increase Impact Resistance
• Combat Thermal Stress

Architects and designers must consider the force of wind on a building or installation when choosing glass. Wind causes glass to deflect. This deflection strains not only the glass itself but the entire glazing system: the framework, gaskets and sealants.

Impact resistance is closely related to wind load because the wind carries such things as hailstones, dust, small stones and other debris. During tornadoes and hurricanes, the wind carries many larger objects.

As glass heats, it expands. The center portion of a lite gets hotter and expands at a greater rate than the edges. The stresses on the edges are usually greater at the center of each edge and decrease toward the corners. The imbalance strains the edges. This is called thermal stress. The edge strength of the lite, therefore, greatly determines its ability to resist breaking. Clean-cut edges offer the greatest edge strength. This is particularly crucial with heat-absorbing glass. A well-designed glazing system also reduces stresses on the glass.

Heat-strengthened glass is made by heating annealed glass uniformly, then cooling it at a slower rate than tempered glass.
Characteristics include:
• Is about twice as strong as regular annealed glass of the same size and thickness.
• Is more resistant to wind loading and impacts than regular annealed glass though less resistant than tempered 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. It is also used in the making of spandrel glass. Spandrel glass is obscure glass that is used in non-vision areas. 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.

Glass gains considerable strength from tempering. A lite of tempered glass is about four times stronger than a lite of annealed glass of the same size and thickness. Characteristics include:

• 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.

The main reasons to use tempered glass are:

• Tempered glass, when broken, is designed to shatter into cube-shaped particles. It therefore qualifies as a safety glazing material.
• Tempered glass offers greater strength against deflection, and thus, better resistance to the force of wind, than heat-strengthened glass. It is more effective if placed within a well-designed, overall glazing system.
• 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.
• Tempering increases a lite's edge strength. Thus tempered glass is specified when designers anticipate high thermal stresses.

Tempered glass is made by heating annealed glass uniformly. The glass can be from 1/8" to 3/4" thick. 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. Two processes are used to temper glass:
• Vertical tempering
• Horizontal tempering

In vertical tempering tongs are used to suspend the glass from its top edge. It moves vertically through the furnace in this manner. In horizontal tempering the glass moves through the furnace on stainless steel or ceramic rollers. Of the two processes, horizontal tempering is the more common. Tempered glass is identified by a permanent label, called the bug, which is placed into the corner of each tempered lite. Tempered glass cannot be cut, drilled or edged. These processes must be performed on the glass before tempering.

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. Characteristics include:
• 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). For example, laminated glass with a .030 PVB layer sandwiched between two pieces of two-millimeter annealed glass meets the minimum requirement for safety glazing.

Applications-In addition to safety glazing, laminated glass has many specialty applications, including sound reduction and security.

Reflective, Low Emissivity, and Insulating Glass

Modern glass is called upon to perform many tasks. An important one is to control the amount of heat and light that passes through the glass. Three types of glass designed for this purpose are:
• Reflective Glass
• Low Emissivity Glass
• Insulating 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 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.

Three different processes are used to deposit the coating on the glass:
• Wet Chemical Deposition
• Vacuum Deposition
• Pyrolitic Deposition

Wet Chemical Deposition-In this process the glass is submerged in a tank containing a chemical solution. The metallic oxide is transferred to the surface of the glass by a chemical reaction.
The coating is very fragile and must be protected immediately. The usual method of protection is to use the glass in an insulating glass unit or in a laminated glass product.

Vacuum Deposition-In this process the glass is placed in a vacuum chamber containing a special atmosphere. When electrical energy is added to the chamber, a complex magnetic reaction takes place that causes the metal atoms to strike the surface of the glass at high speed. The atoms coat the surface of the glass uniformly. This process is commonly called sputter coating. Sputter coated reflective glass has a few disadvantages:
• It cannot be heat strengthened or tempered because the heat would destroy the coating. And, since the coating increases the amount of the sun's rays the glass absorbs, it may be necessary to heat treat the glass before coating.
• The soft coating can be damaged easily before installation.
• The glass has limited compatibility with sealants.

Pyrolytic Deposition-The word pyrolytic is used to describe a change brought about by heat. In pyrolytic deposition the metallic oxide is added to the glass while the glass is hot. This can be done in an oven or during the process of making the float glass. Generally, pyrolytic coated glass is installed with the coated side facing outdoors. The coating itself reflects most of the sun's rays before they reach the glass. In many cases, this eliminates need for heat strengthening or tempering that might be required with other types of reflective glass. Characteristics include:
• Pyrolytic coatings are more durable than wet chemical or sputter coatings.
• Annealed glass with a pyrolitic coating can be heat treated or tempered without affecting the coating.

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. Characteristics include:
• Low E glass:
• Reduces heat loss through windows.
• Re-radiates the heat absorbed from sunlight back inside the room.
• Allows sunlight into a room without letting heat escape outdoors.
• Resists ultraviolet light, which results in less damage to carpets, draperies and other furnishings.
• Reduces glare.

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. They reduce glare and reflect the sun's heat away from the structure.

Insulating glass units, commonly called ig units, are made from two or more lites of glass separated by a sealed air space. The metal tube around the perimeter of the insulated unit which seperates 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 dessicant. The perimeter of the entire unit is sealed with a high grade sealant. Characteristics include:
• IG Units:
• Reduce the tendency of condensation to form on the room side of the glass.
• Reduce cold transmittance at windows and helps maintain a uniform temperature. In the winter, ig units reduce heat loss and in the summer they reduce heat gain.
• Reduce the level of noise from the outside.

There are two types of ig units commonly manufactured:
• Single Seal Units
• 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.

Every ig unit is fabricated according to a set process:
1. The glass is cleaned.
2. The spacers are cut to size.
3. A corner key is inserted into one end of the spacer.
4. The spacers are filled with dessicant.
5. The spacer frames are assembled.
6. If the unit is to be double sealed, a ribbon of polyisobutylene (PIB) tape is applied to one side of the spacer frame. This tape forms the primary seal.
7. The frame is set on the first lite.
8. If the unit is to be double sealed, a ribbon of PIB tape is applied to the facing side to the spacer frame.
9. The second lite is set. The lites are usually clamped together or secured by a weight.
10. The perimeter is sealed.

Single-sealed units can use several types of sealants:
• Hot melt butyl
• Polysulfide
• Silicone
• 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.

Each surface of an insulating glass unit is designated by number:
Number 1 surface - faces the exterior.
Number 2 surface - inside of the first lite.
Number 3 surface - faces the number 2 surface.
Number 4 surface - faces the interior.

When reflective glass is used in an insulating glass unit, the surface on which the reflective coating is placed makes a great deal of difference. For example, a bronze coating placed on the number 1 surface creates a mirror effect. The same coating placed on the number 2 or 3 surface creates a bronze tinted effect.

How Low E, Reflective & IG Contributes to Energy Efficiency
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:
• U value
• 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.

Specialty Glass

There are almost as many types of glass as there are possible uses for them. As you gain experience, you will become familiar with many types of specialty glass. A few of these include:
• Mirrors
• Spandrel Glass
• Laminated Glass
• Art Glass

Mirrors are made from 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 miror depends upon the thickness of the silver layer, glass thickness, and glass color. High quality mirrors can have a copper backing. The copper is deposited over the layer of silver, and offers the greatest amount of protection. Mirrors deteriorate rapidly when exposed to air, so the metal films must be protected immediately by a coating. The glass is usually preheated to a range of 120 - 140 degrees Fahrenheit, before the coating is applied. This paint type coating can be applied in one coat, or consist of two coats of dissimilar, but compatible material.

If the metal backing of a mirror deteriorates, the silver turns black. This condition is known as black edge. Not much definite is known about the causes of black edge. Some experts believe black edge is caused by chloride in water. Others believe it is caused by industrial and household solutions that come into contact with the mirror. Whatever the cause, most agree that sealing the edges of mirrors helps to guard against black edge.

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.
Spandrel glass is made using several methods. One method is to fuse a colored ceramic material, called a frit, to one surface of the glass. The glass must be either heat strengthened or fully tempered because the frit causes the glass to absorb heat. Spandrel glass may or may not be insulated. Some codes require an open-weave glass fiber cloth or special tape to be attached to the back of the spandrel panel to ensure that the panel stays in the opening if it breaks. If reflective glass is used as the spandrel application, the back surface must be obscured in some way so that the building structure does not become visible under certain light conditions.

As mentioned above, laminated glass is used in safety glazing applications. However, it has many other applications:
• Burglar resistance
• Bullet resistance
• Sound reduction
• Sloped glazing
• 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.

A new process, resin laminating, is being used to laminate curved glass and other specialized applications. In this process, two lites are spaced .030" to .060" apart. Three sides of the perimeter are dammed. With the assembly vertical, a liquid chemical mixture is poured into the space and allowed to cure at room temperature for two to ten hours.

Art glass goes by many names: opalescent, cathedral or stained glass. Art glass is usually produced in small batch operations. Thicknesses vary within each sheet and from sheet to sheet. Generally, colors do not match from sheet to sheet. Art glass is available in a maximum thickness of inches. It cannot be tempered.

Source: Basic Guide to Glass and Glazing.