Crown glass: The earliest style of glass window:- Window glass blown into a crown or hollow globe that is flattened and cut before use. This is produced by reheating and spinning out a bowl-shaped piece of glass (bullion) that causes the glass to extend into a flat disk by centrifugal force. The glass is then cut into the size required 2. One of the two principal types of optical glass used in the production of compound lenses. The Crown glass, which is an alkali-lime silicate optical glass, has a low index of refraction and low dispersion (its Abbe v-value is larger than 50 or 55, depending on its index).

 

Glass block :- Glass brick, also known as glass block, is often used as an architectural element in underground parking garages, washrooms, municipal swimming baths, and other areas where privacy or visual obscuration is desired, while admitting light. Glass block was originally developed in the early 1900s to provide natural light in industrial factories.

 

Cylinder glass:- A technique for producing sheet glass dating from the 11th century. By blowing a hollow glass sphere and swinging it vertically, gravity pulls the glass into a cylindrical "pod" measuring up to 3 metres long, with a width of up to 45 cm. While still hot, the ends of the pod are cut off and the resulting cylinder cut lengthways and laid flat.

Sheet glass:- See the definitions for the following processes, listed in order from oldest to most recent: "crown glass" (definition 1), "cylinder glass", "drawn glass", "Fourcault process", "Libbey-Owens process", "Pittsburgh process", "float process". The float process is now the standard method of producing sheet glass world-wide.

Rolled plate glass:- Rolled (or cast) glass is a translucent glass with 50-80% light transmission, depending on its thickness and type of surface. It is used where transparency of the glass sheet is not important or not desired. To produce rolled glass, molten glass pours from the melting tank over a refractory barrier (the "weir") and onto the machine slab where it flows under a refractory gate (the "tweel"), which regulates the volume of glass, and then between two water-cooled rollers. The distance between the rollers determines the thickness of the glass.

 

Polished plate glass:- The plate glass process starts with sheet or rolled plate glass. This glass is dimensionally inaccurate and often created visual distortions. These rough panes were ground flat and then polished clear. This was a fairly expensive process.

 

Before the float process, mirrors were plate glass as sheet glass had visual distortions that were akin to those seen in amusement park or fun-fair mirrors.

 

Float glass:- 90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.

 

A very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. The tin side of glass is also softer and easier to scratch.

 

Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

 

Annealed glass:- Annealed glass is glass without internal stresses caused by heat treatment (ie by rapid cooling, or by toughening or heat strengthening). Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Float glass is annealed during the process of manufacture. However, most toughened glass is made from float glass that has been specially heat-treated.

 

Annealed glass breaks into large, jagged shards that can cause serious injury, and thus annealed glass is considered a hazard in architectural applications. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools or domestic houses.

 

Figure rolled glass:- Rolled (or cast) glass is a translucent glass with 50-80% light transmission, depending on its thickness and type of surface. It is used where transparency of the glass sheet is not important or not desired. To produce rolled glass, molten glass pours from the melting tank over a refractory barrier (the "weir") and onto the machine slab where it flows under a refractory gate (the "tweel"), which regulates the volume of glass, and then between two water-cooled rollers. The distance between the rollers determines the thickness of the glass.

 

Laminated glass:- Laminated (or compound) glass consists of two or more sheets of glass with one or more viscous plastic layers "sandwiched" between the glass panes. The solid joining of the glasses takes place in a pressurised vessel called an autoclave. In the autoclave, under simultaneous heating of the already processed layers of glass and special plastic, lamination occurs. When laminated safety glass breaks, the pieces remain attached to the internal plastic layer and the glass remains transparent.

 

Toughened glass (tempered glass):- Toughened glass (also known as tempered glass) is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations.

 

At home you are likely to find toughened glass in shower and sliding glass patio doors. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and any glass where there is a danger of human impact.

 

Toughened glass is typically four to six times the strength of annealed glass. Although toughened glass is most susceptible to breakage via edge damage, breakage can also occur from impacts in the centre of the glass pane.

Heat-strengthened glass:- Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.

Chemically strengthened glass:- Chemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required.

 

Chemically strengthened glass is typically six to eight times the strength of annealed glass.

 

The glass is chemically strengthened by submerging the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 °C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.

 

These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. The surface compression of chemically strengthened glass may reach up to 690 MPa.

 

There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article.

 

Chemical strengthening results in a strengthening similar to toughened glass, however the process does not use extreme variations of temperature and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can often be significantly bowed.

 

Also unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength.

 

Chemically strengthened glass was used on some fighter aircraft canopies.

 

Low-emissivity glass:- Commonly known as "low-E" glass and often used in double and triple glazing units, this window glass has a special thin-film metallic or oxide coating which allows the passage of short-wave solar energy into a building but prevents long-wave energy produced by heating systems and lighting from escaping outside. Low-E glass thus allows light to enter while also providing thermal insulation.

 

Self-cleaning glass:- A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A 50 nanometre coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photo-catalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.

 

Insulated glazing:- Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. This type of glazing has functions of thermal insulation and noise reduction.

 

Evacuated glazing:- Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan and China. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing.

 

An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6 mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gaseous conduction in a vacuum.

 

Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is outgassing; that is, heating it to liberate any gases adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heat-strengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for outgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infrared radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however.

 

Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20 mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1 m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centred around the spacers, where the glass is slightly colder than average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer.

 

The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing’s overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.