Category Chemistry

Mirrors are made by coating the back of a sheet of glass with an alloy of mercury and another metal. This means that light does not pass through the glass, but is bounced back to give a reflection.

The nature of modern mirrors is not fundamentally different from a pool of water. When light strikes any surface, some of it will be reflected. Mirrors are simply smooth surfaces with shiny, dark backgrounds that reflect very well. Water reflects well, glass reflects poorly, and polished metal reflects extremely well. The degree of reflectivity—how much light bounces off of a surface—and the diffusivity of a surface—what direction light bounces off of a surface—may be altered. These alterations are merely refinements, however. In general, all reflective surfaces, and hence, all mirrors, are really the same in character.

Man-made mirrors have been in existence since ancient times. The first mirrors were often sheets of polished metal and were used almost exclusively by the ruling classes. Appearance often reflected, and in some cases determined, position and power in society, so the demand for looking glasses was high, as was the demand for the improvement of mirror-making techniques. Silvering—the process of coating the back of a glass sheet with melted silver—became the most popular method for making mirrors in the 1600s. The glass used in these early mirrors was often warped, creating a ripple in the image. In some severe cases, the images these mirrors reflected were similar to those we’d see in a fun-house mirror today. Modern glassmaking and metallurgical techniques make it easy to produce sheets of glass that are very flat and uniformly coated on the back, improving image clarity tremendously. Still, the quality of a mirror depends on the time and materials expended to make it. A handheld purse mirror may reflect a distorted image, while a good bathroom mirror will probably have no noticeable distortions. Scientific mirrors are designed with virtually no imperfections or distorting qualities whatsoever.

Materials technology drastically affects the quality of a mirror. Light reflects best from surfaces that are non-diffusive, that is, smooth and opaque, rather than transparent. Any flaw in this arrangement will detract from the effectiveness of the mirror. Innovations in mirror making have been directed towards flattening the glass used and applying metal coatings of uniform thickness, because light traveling through different thicknesses of glass over different parts of a mirror results in a distorted image. It is due to these irregularities that some mirrors make you look thinner and some fatter than normal. If the metal backing on a mirror is scratched or thin in spots, the brightness of the reflection will also be uneven. If the coating is very thin, it may be possible to see through the mirror. This is how one-way mirrors are made. Non-opaque coating is layered over the thin, metal backing and only one side of the mirror (the reflecting side) is lit. This allows a viewer on the other side, in a darkened room, to see through.

Glass, the main component of mirrors, is a poor reflector. It reflects only about 4 percent of the light which strikes it. It does, however, possess the property of uniformity, particularly when polished. This means that the glass contains very few pits after polishing and will form an effective base for a reflective layer of metal. When the metal layer is deposited, the surface is very even, with no bumps or wells. Glass is also considered a good material for mirrors because it can be molded into various shapes for specialty mirrors. Glass sheets are made from silica, which can be mined or refined from sand. Glass made from natural crystals of silica is known as fused quartz. There are also synthetic glasses, which are referred to as synthetic fused silica. The silica, or quartz, is melted to high temperatures, and poured or rolled out into sheets.

A few other types of glass are used for high-quality scientific grade mirrors. These usually contain some other chemical component to strengthen the glass or make it resistant to certain environmental extremes. Pyrex, for example, is a borosilicate glass—a glass composed of silica and boron—that is used when mirrors must withstand high temperatures.

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Since medieval times, glorious decorative windows have been made by joining small pieces of coloured and painted glass together with lead strips. The lead is soft and easy to bend but strong enough to hold the glass.

The creation of beautiful stained glass windows requires artistic vision and great skill in working with glass and metal. It’s an art form that requires tremendous dedication and many years of training. So how did stained glass artists create these works of art so many years ago?

Stained glass windows began with a design. An artist would first develop full-size sketches (called “cartoons”) that portrayed the overall composition of the windows, including the shapes of individual pieces of glass, the colors of glass to be used, and the details to be painted onto the glass pieces.

Once the design was complete, individual pieces of glass could be cut from larger pieces of colored glass. Special tools, such as dividing irons and grazing irons, were used to cut and shape pieces of colored glass to fit the shapes called for in the design.

With pieces of colored glass cut and shaped, artists would then paint the individual pieces of glass to achieve the exact colors with the precise details they desired. Stained glass artisans used vitreous paints, which contained powdered glass particles suspended in liquid.

After they were painted, the individual pieces of glass would be fired in a wood-fired oven known as a kiln. The powdered glass particles in the vitreous paint would melt, causing the paint to fuse with the glass permanently.

To assemble the individual glass pieces into a completed panel, the pieces would be held together with narrow, flexible strips of lead, which would be joined together by a lead and tin alloy called solder. Finally, semi-liquid cement was applied to secure the glass pieces within the lead strips and make the finished window waterproof.

While this might sound like a simple, easy process, it was not! There were many variations to the process, and talented artists could spend thousands of hours working on creating large stained glass windows for mosques and cathedrals.

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Glassblowers dip a long tube into molten glass, and then blow air into it as it cools, causing the glass to form a bubble. While it is still very warm, this bubble can be shaped, cut with shears, or added to other glass shapes. A slightly different method is used when glassware is made by machine. Then, lumps of hot glass are placed in a mould and air is blown in to force the glass to the sides of the mould. With both methods, the glass can be engraved, or sandblasted to give it a rough texture, after it has cooled.

Glass blowing is a glass forming technique that humans have used to shape glass since the 1st century B.C. The technique consists of inflating molten glass with a blowpipe to form a sort of glass bubble that can be molded into glassware for practical or artistic purposes.

Thanks to the glass blowing process, glass has been one of the most useful materials in human society for centuries. We’ve laid the exact process, step by step, to give you a better understanding of how exactly we’ve made the best use of this wonderful material.

Before starting the glass blowing process, the glass is placed in a furnace that heats it to a temperature of 2000 degrees, making it malleable. Next, the glass is gathered by inserting one end of the blowpipe into the furnace, and rolling it over the molten glass until a “gob” of glass attaches to it.

The next step is to roll the molten glass on a flat metal slab called a marver. The marver acts as a means to control the shape and temperature of the glass. The glass is taken back and forth from the marver to the glory hole, a hot chamber used to reheat the glass in order to make it malleable again.

To give the glass color and design, it’s dipped in crushed colored glass, which fuses to the main glass piece almost immediately due to the hot temperature. Once the main glass piece has been fused with crushed colored glass, it is taken back to the marvel where it is rolled again.

The final step is to remove the glass from the glass pipe. To do this, steel tweezers called jacks are used to separate the bottom part of the blown glass while rotating the blowpipe. Thanks to the separation with the jacks, the glass can be removed from the blowpipe with one solid tap.

The last step is to take the blown glass to an annealing oven using heat resistant gloves. This allows the glass to cool slowly over several hours, as it is highly perceptive to breaking when exposed to rapid temperature changes.

Hardened metal blades can cut glass but are easily blunted. More often, glass is cut with the hardest natural substance known — a diamond. If a furrow is made in glass with a diamond, it will usually break cleanly when pressure is applied to it.

A glass cutter is a tool used to make a shallow score in one surface of a piece of glass that is to be broken in two pieces. The scoring makes a split in the surface of the glass which encourages the glass to break along the score. Regular, annealed glass can be broken apart this way but not tempered glass as the latter tends to shatter rather than breaking cleanly into two pieces.

A glass cutter may use a diamond to create the split, but more commonly a small cutting wheel made of hardened steel or tungsten carbide 4–6 mm in diameter with a V-shaped profile called a “hone angle” is used. The greater the hone angles of the wheel, the sharper the angle of the V and the thicker the piece of glass it is designed to cut. The hone angle on most hand-held glass cutters is 120°, though wheels are made as sharp as 154° for cutting glass as thick as 0.5 inches (13 mm). Their main drawback is that wheels with sharper hone angles will become dull more quickly than their more obtuse counterparts. The effective cutting of glass also requires a small amount of oil (kerosene is often used) and some glass cutters contain a reservoir of this oil which both lubricates the wheel and prevents it from becoming too hot: as the wheel scores, friction between it and the glass surface briefly generates intense heat, and oil dissipates this efficiently. When properly lubricated a steel wheel can give a long period of satisfactory service. However, tungsten carbide wheels have been proven to have a significantly longer life than steel wheels and offer greater and more reproducible penetration in scoring as well as easier opening of the scored glass.

In the middle Ages, glass was cut with a heated and sharply pointed iron rod. The red hot point was drawn along the moistened surface of the glass causing it to snap apart. Fractures created in this way were not very accurate and the rough pieces had to be chipped or “grozed” down to more exact shapes with a hooked tool called a grozing iron. Between the 14th and 16th centuries, starting in Italy, a diamond-tipped cutter became prevalent which allowed for more precise cutting. Then in 1869 the wheel cutter was developed by Samuel Monce of Bristol, Connecticut, which remains the current standard tool for most glass cutting.

Large sheets of glass are usually cut with a computer-assisted semi-automatic glass cutting table. These sheets are then broken out by hand into the individual sheets of glass (also known as “lites” in the glass industry).

Process

Glass cutters are manufactured with wheels of varying diameters. One of the most popular has a diameter of 5.5 mm. The ratio between the arc of the wheel and the pressure applied with the tool has an important bearing on the degree of penetration. Average hand pressure with this size wheel often gives good results. For a duller wheel on soft glass a larger wheel (e.g., 6 mm) will require no change in hand pressure. A smaller wheel (3 mm) is appropriate for cutting patterns and curves since a smaller wheel can follow curved lines without dragging.

The sheet of glass is typically lubricated along the cutting line with light oil. The cutter is then pressed firmly against the surface of glass and a line is briskly scribed to form a “score” or “cut”. The glass is now weakened along this line and the panel is ready to be split. Running pliers may then be used to “run” or “open” to the split.

General purpose glass is mostly made by the float glass process and is obtainable in thicknesses from 1.5 to 25 mm. Thin float glass tends to cut easily with a sharp cutter. Thicker glass such as 10 mm float glass is significantly more difficult to cut and break; glass with textured or patterned surfaces may demand specialized methods for scoring and opening the cuts.