The Best Way to Buy Glass on the Internet

Most glass is a mixture of a large amount of silica that comes from fine white sand or pulverized sandstone, combined with smaller amounts of an alkali like soda (sodium bicarbonate) or potash to lower its melting point, and lime (from limestone) to help stabilize the mixture and ultimately make the glass stronger and water-resistant. These elements are naturally occurring throughout North America.

The silica, soda and lime are fused together at extremely high temperatures. By adding other substances during the process, the properties of the glass can be altered, including its color, how reflective it is, how brilliant or sparkling it looks, how well it acts as an insulator and more. Bits of old or broken glass from previous manufacturing, called 'cullet' is usually recycled into the mix. However, it is not just these additives alone that affect the final piece of glass, but also the way in which it is heated, cooled and formed.

First, the silica/soda/lime mixture is heated in melting furnaces to approximately 2,500° Fahrenheit, sometimes for as long as 24 hours. The molten glass is then cooled several hundred degrees, to a temperature at which it can be worked. At this stage the glass is an orange-red color and looks like a thick liquid. The glass has to be worked quickly to form it into the desired shape, because as the glass cools it becomes hard. The glass can now be blown, pressed, drawn or rolled. Finally, when the glass is formed into the desired shape, it is placed in a 'lehr' oven to be 'annealed.' This special process eliminates areas of stress in the glass to strengthen it, by cooling it down at controlled temperatures.

To blow glass, a blob of molten glass is placed at the end of a long, hollow iron blowpipe. Air is blown in and causes the glass to form into a pear-shaped bulb, which is then rolled on an oiled slab, shaped with tools and sometimes re-blown into a mold. To keep the glass from hardening during this process, it is periodically re-heated in small ovens. If the glass is to be engraved, copper wheels are used, and if it is to be etched, hydrofluoric acid does the job.

For glass bottles, a molten glass bubble is employed. It is placed in a mold, and the air pressure in the bubble forces the glass against the side of the mold. Once the glass cools and hardens, the mold is opened and a newly-made glass bottle removed.

Pressed glass, which offers better control of the glass's density than blowing, was the first glass to be manufactured on a large scale with the invention of a glass-pressing machine in the U.S. in the 1920s. This worked by taking the molten glass from the furnace and dividing it up into small sections. These would be placed in molds made of iron or brass. A plunger would press the glass down into the mold, and after a few seconds it was ready.

Because the cold metal of the mold would produce wrinkles in the hot glass, intricate patterns were used to disguise the flaws. It wasn't until twenty years later that a technique was developed to heat the molds to very high temperatures before dropping in the molten glass, which eliminated the wrinkles. The decorative patterns accordingly became simpler.

Drawn glass is the process used to manufacture tube and rod-shaped glass, as well as some sheet glass. To make tubes, the molten glass gets drawn over a hollow cylinder or cone that has air blowing through it, to keep it from collapsing until the glass hardens around it. The tube can be drawn out horizontally, vertically, or at an angle. Conversely, to make glass rods, the air inside the cylinder or cone is eliminated.

Rolled glass is used for some flat glass, and means that the sheet of glass rolls along the assembly line as it is manufactured. Because of the type of metal used in the rollers, they aren't damaged by the heat of the molten glass. However, the glass can wind up with a rough surface. When this process was first developed, molten glass was poured onto large tables and then rolled flat onto plates. After it cooled, the glass was ground and polished.

Then a series of innovations began at the turn of the last century and continued through the first world war, improving the quality and economy of both drawn, sheet glass and rolled glass.

Float glass is the most widely used type of flat glass today. It was first developed after the second world war by a British company but not introduced to the market until they felt it was perfected, in 1959. Its manufacture is unique because the molten glass is formed by floating it on a bed of molten tin kept at high temperature. The glass spreads out and flattens, and is then drawn out into a continuous glass ribbon. The surface of the glass winds up being extremely smooth, with a brilliant finish like sheet glass, plus the optical quality of plate glass.

Cutting and drilling glass is a fine art. If the glass needs to be cut it must first be scored with a glass-cutter wheel. Pressure is then applied across the score to force a break. Another way to cut glass is thermally. This can be done with a focused flame heating a narrow strip of the glass. A water jet is then directed on the heated strip to break the glass. Or, a ring of focused flames are used to heat a particular area of the glass until it becomes soft enough to pull apart. Glass can be drilled, with either a steel drill, a tungsten-carbide drill, or for the best control, quality and speed of production, a diamond core drill.

Glass is categorized by its composition. The most common - ninety percent of manufactured glass - is so-called soda glass, the combination of silica, soda and lime. Although it's the cheapest to make, it is also the least resistant to high temperature, or sudden changes in temperature, or chemicals.

Lead glass will usually have at least twenty percent lead oxide content. It looks brilliant in the light, especially when cut and faceted, and is more expensive that soda glass. But similar to soda glass, lead glass will also not do well with high temperature or sudden changes in temperature.

For better resistance to temperature changes and chemicals, the more expensive borosilicate glass is used. It has a minimum of 5% boric oxide, and is used to make light bulbs, sealed-beam headlights, bakeware and labware. And even more durable than borosilicate glass is aluminosilicate glass.

Fiberglass and foam glass are mostly used for insulation purposes. To make fiberglass, the molten glass is formed into continuous, hair-like glass filaments. Foam glass is made by trapping gas bubbles in the glass, creating an almost spongy consistency.

Colored glass is made by adding chemicals into the mix, with particular chemicals creating specific colors. For example, the amber or brown glass you see used in beer bottles gets its color from iron sulphide. Iron-chromite creates shades of green, while cobalt makes beautiful shades of blue.

A mirror is any smooth surface that reflects back an image of an object. This happens when the light rays - called photons - coming from that object (for example, your face) hit the smooth surface of the mirror and bounce back at the same angle at which they hit the object. When the photons hit a rough surface they scatter and so can't cause a reflection.

But not every smooth surface creates a reflection. That's because some of these surfaces absorb the photons hitting them so they're unable to bounce back. What's more, when we do stand in front of a reflecting surface like a mirror, not all the photons that bounce off us will hit the mirror, because our bodies are rough objects themselves. But enough do bounce back that an image is formed, at exactly the same angle at which the photons hit the mirror. And keep in mind, the mirror image is actually backwards, as you can easily see if you stand in front of it holding a newspaper or magazine.

Mirrors are usually made of clear plate glass that has one side coated with a metal film to create the reflecting surface, called 'silvering'. Where the surface and the glass meet is called the 'mirror line.' The most common type of mirror is the plane mirror, which has a flat surface. The rays of light that hit it are reflected back virtually unchanged, so the position of the image in the mirror is the same distance as the real object in front of it, and its size is the same size. You experience this every time you stand in front of the mirror in the bathroom - the closer you stand to the mirror, the closer your image appears in it.

Convex and concave mirrors - also known as 'spherical mirrors' - alter the look of the original object and the distance it appears to be in the mirror. A concave mirror (think of it as being 'caved in') has the mid-point of the reflecting surface farther away from the object in front of it than the edges. So the image that forms in a concave mirror depends on where it is in relation to the center of the mirror. It may be larger, smaller, or the same size. It may be upside down or right side up, and will be distorted from the real object.

Conversely, a convex mirror has the mid-point of the reflecting surface closer to the object than the edges, so the image that forms is consistently smaller than the real object in front of it, and standing upright. It will also be distorted. For example, drivers will stick a small convex mirror on their car or truck's rear-view mirror to get a 'bigger picture' of what's behind them than a regular, plane-type mirror shows.

Two-way mirrors are a special kind of mirror, usually used for observation and security purposes. They are sometimes also called one-way mirrors or see-through mirrors. You've probably seen them in action on tv shows or in movies where an interrogation is taking place. The perp is questioned in a room that has a mirror, while detectives observe him from behind the mirror in an adjoining room. So how come the criminal sees only his reflection but the cops see clear through the glass?

The answer is actually pretty simple. The two-way mirror in fact has only a very thin layer of reflective material applied to the glass. In fact, it's so thin it's referred to as being 'half-silvered,' as opposed to 'silvered' which is how you'd describe a regular mirror. So only half of the light that hits the mirror gets reflected, while the other half passes through it.

But with only half the reflecting power, you'd think the criminal would be able to see the detectives. The reason why he can't is the way the two rooms are lit. If you watch the interrogation scene, you'll notice the room with the criminal is brightly lit, so there is enough light to hit the mirror and reflect back his image. However, the room with the detectives is dark, so there's not enough light to pass through that side of the glass back into the interrogation room.

Here's another way to think of it: if the perp were to suddenly jump up and switch off the lights in the interrogation room, or if one of the cops accidentally turned on the lights in the observation room, the two-way mirror would act like a window, with everyone being able to see each other perfectly clearly.

Naturally occurring glass, such as obsidian, has been used since the Stone Age. Obsidian is usually black and has impurities, and is created when magma from a volcano cools. It was the raw material for extremely sharp knives in certain Stone Age cultures because, like flint, it can be fractured to produce sharp blades or arrowheads. There is speculation that obsidian may also have been polished to create early mirrors during this period.

For example, the Pre-Columbian Mesoamericans used obsidian for many objects, including tools and decorative pieces where it was extensively carved. They also made a type of sword with obsidian mounted in wood. It combined the sharp cutting edge of a blade with the ragged cut of a serrated knife, creating a formidable weapon.

Manufactured glass was possibly first made by the Phoenicians. Although they used glass as a glaze for pottery as early as 3000 BC, there is also archaeological evidence to suggest that the first glass objects were made in Mesopotamia. Their glass beads, seals, and architectural decorations date from around 2500 B.C.

The earliest known glass beads from Egypt were made during the New Kingdom, about 2000 - 1500 B.C., and came in a variety of colors. They were highly prized as a trading commodity, especially blue ones which supposedly possessed magical powers. Also used in glass since early Egyptian history is manganese - one of the oldest glass additives - that turned the glass purple.

In addition to glass beads, the Egyptians made small glass jars and bottles using the core-formed method. This process entails winding molten glass threads around a bag of sand tied to a rod. The glass has to be kept in motion until the required shape and thickness is achieved, and the glass is continually reheated to fuse the threads together. The final step is to allow the rod to cool, then to puncture the bag, drain the sand and remove the rod.

By the 5th century B.C. this technology had spread as far as Greece, and by the 1st century B.C. there were many glass centers located all around the Mediterranean. It was at this time at the eastern end of the Mediterranean, that glass blowing, both free-blowing and mould-blowing, was discovered.

Along with the birth of the Roman Empire came the development of many more new glass techniques, and as the Empire spread so did the popularity of glass. Whether through wars or trade, the use of glass objects and the techniques used for making glass were spread as far north as Scandinavia and the British Isles.

This spreading out resulted in glass artists congregating in areas such as Alexandria in Egypt where the historically significant "Portland Vase" was created, the Rhine Valley where Bohemian glass was developed, and Byzantium, where glass designs became very ornate and processes such as enameling and gilding came into being.

Window glass was also quite commonly used in ancient times, and examples found in Egypt from the 1st century B.C. were translucent and very thick. After the fall of the Roman Empire, the Emperor Constantine moved to Byzantium where the use of glass continued.

Glass reached China by the "Silk Road" that essentially came into being from the 1st century B.C., following efforts by China to create routes to the Western world, and India. It was most used during the time of the Byzantine Empire at its west end, and from the time of the Three Kingdoms to the Yuan Dynasty at its east end.

Trade among countries also developed on the sea, between Alexandria in Egypt and Guangzhou in China. There was also what has been called a "Silk Route" across the Indian Ocean. It extended, along ports on the coasts of India and Sri Lanka, all the way to Roman-controlled ports in Egypt and territories on the northeastern coast of the Red Sea.

The Kushan Empire, in the northwestern part of India, was located at the center of these exchanges. They fostered multi-cultural interaction, the proof of which is their 2nd century hoards that were discovered, filled with glass and other treasures from Greece, Rome, China and India.

In Europe, the popularity for glass had waned, but revived in the 7th and 8th centuries. Objects found on an island near Venice attest to this, and make a connection between Roman times and the later importance of that city in the production of glass. In fact, from the 10th through the 14th centuries, Venetian glass was highly prized.

The 11th century saw the emergence in Germany of new ways of making sheet glass by blowing spheres, swinging them around to form cylinders, cutting them into sheets while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice.

Also around this same time another important technical breakthrough was made in Northern Europe when soda glass (meaning it is made with soda ash) was replaced by glass made from a much more readily available material, potash (obtained from wood ashes). From this point on, northern glass differed from that made in the Mediterranean area, where soda remained in common use.

Glass in the 11th century also included the emergence of glass mirrors in Islamic Spain. And although stained glass was introduced 100 years prior, it was not widely used until the 12th century.

From the 14th century, the center of glass making was the island of Murano in Venice. The Venetian Republic, fearing fire and destruction to the city's mostly wood buildings, had ordered glassmakers to move their foundries to Murano in 1291. Murano's glass makers were soon the island's most prominent citizens. Many new techniques were developed here and soon Murano's lucrative export trade included dinnerware, mirrors, and other luxury items.

What made the glass significantly different was the local quartz pebbles used to make it. They were almost pure silica and were ground into a fine, clear sand that was combined with soda ash obtained from the Levant, which the Venetians held the sole monopoly on. This resulted in a superior form of glass - and the Venetians having a trade advantage over other glass producers.

The glass makers were allowed to wear swords, were not prosecuted by the government and were able to marry their daughters into Venice's most desirable families. But there was a catch - glass makers weren't allowed to leave the Republic. However, many of them decided to take the risk and set up glass furnaces in surrounding cities and even as far as England and the Netherlands.

Murano's glass makers held onto their monopoly on quality glass for centuries, developing or refining many technologies such as crystalline glass, enameled glass, glass with threads of gold, multicolored glass ("millefiori"), milk glass and imitation gemstones made of glass.

Conversely, the quality of English glass initially left a lot to be desired - in fact, in a court case anything a witness saw through a closed window was not admissible as evidence.

But this changed when, around 1688, a process for casting glass was developed. This led to glass becoming a much improved, and more commonly used, material. France soon perfected the manufacture of plate glass, and by the late 17th century English flint glass with lead oxide became suitable for cut glass designs.

The Crown glass process was used up to the mid-1800s. In this method, the glassblower would spin around 9 pounds of molten glass at the end of a rod until it flattened into a disk approximately 5 feet in diameter. The disk would then be cut into panes. And with the invention of the glass pressing machine in 1827, the mass production of inexpensive glass articles commenced.

In the US, the first glass factory had been established in 1608. The cylinder method of creating flat glass was first used in the 1820s, to commercially produce windows. This and other types of hand-blown sheet glass were replaced in the 20th century by rolled plate glass.

Safety vs. tempered glass Although both are considered 'safer' because they have been manufactured for strength and, if damaged, to not shatter into sharp shards, there is a difference both in the manufacture and performance of safety and tempered glass.

Safety glass was first invented early in the 20th century by a French scientist. Today the process used is essentially the same, where two pieces of sheet or plate glass are bonded together with a layer in between them, which is usually plastic. This lamination keeps the glass from shattering and is why it is used for car windshields.

Conversely, tempered glass is found in the passenger and rear windows of a car, as well as many other applications. In the tempering process, the annealed glass is heated in a furnace past its 'annealing point,' then immediately cooled down with air jets. This forces the surface to quickly become cool and stiff, while the core of the glass, because it is still much hotter, takes longer to cool down and finally harden.

And that is what improves the strength of the tempered glass. If the surface receives any stress, it does not carry through to the core of the glass, so a crack is unable to form. If the glass does receive damage, it shatters into small cubes rather than sharp shards.