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Monday, March 24, 2014

By Kolja Klok   Posted at  2:43 AM   laminated glass No comments

Crown glass (window)
One of the earliest methods of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force shaped the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and trimmed to form a rectangular window to fit into a frame.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain, hence the name "bullseye." Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diaper latticed windows was in part because three regular diamond-shaped panes could be conveniently cut from a piece of Crown glass, with minimum waste and with minimum distortion.
This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet, and rolled plate processes, but it is still used in traditional construction and restoration.

Laminated glass

Broken tempered laminated glass "wet blanket effect"
Laminated glass is manufactured by bonding two or more layers of glass together with layers of PVB, under heat and pressure, to create a single sheet of glass. When broken, the PVB interlayer keeps the layers of glass bonded and prevents it from breaking apart. The interlayer can also give the glass a higher sound insulation rating.
There are several types of architectural  glasses manufactured using different types of glass and interlayers which produce different results when broken.
Laminated glass that is made up of annealed glass is normally used when safety is a concern, but tempering is not an option. Windshields are typically laminated glasses. When broken, the PVB layer prevents the glass from breaking apart, creating a "spider web" cracking pattern.
Tempered laminated glass is designed to shatter into small pieces, preventing possible injury. When both pieces of glass are broken it produces a "wet blanket" effect and it will fall out of its opening.
Heat strengthened laminated glass is stronger than annealed, but not as strong as tempered. It is often used where security is a concern. It has a larger break pattern than tempered, but because it holds its shape (unlike the "wet blanket" effect of tempered laminated glass) it remains in the opening and can withstand more force for a longer period of time, making it much more difficult to get through.
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.

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