Glass for buildings

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Introduction

Glass is a material made from liquid sand. It is the name given to any amorphous (non-crystalline) solid that displays a glass transition near its melting point which is around 1,700°C (3,090°F). This means that materials transform from a hard and brittle state into a molten state, or vice versa depending on whether the glass transition temperature is the melting or solidifying point. An amorphous solid has some of the crystalline order of a solid and some of the random molecular structure of a liquid.

Silicate glass is the most common form, which consists mainly of silica or silicon dioxide, SiO2. Impurities or additional elements and compounds added to the silicate to change the color and other properties of the glass.

Glass is a very commonly used material because, whilst still molten, it can be manipulated into forms suitable for a very wide range of different uses, from packaging and household objects to car windscreens, windows, and so on.

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History of glass

Archaeological evidence has been found of man-made glass dating back to 4000 BC in the form of decorative glazes. In prehistoric times, weapons were made using obsidian and fulgurite, naturally occurring glasses found in volcanic regions and after lightning strikes respectively. Around 1500 BC glass was first used as a material for making hollow containers.

The Romans excelled at glassmaking and were responsible for introducing it to Britain. Although very guarded over the secrets of glassmaking, when the Roman Empire fell, the skills proliferated throughout Europe and the Middle East.

Glass was popularised as a technical building material by Joseph Paxton’s Crystal Palace at the Great Exhibition of 1851. Since that period, glass manufacturing techniques have developed considerably with advancements in science and technology.

In 1958, Pilkington and Bickerstaff introduced a commercial float glass process, a method that would revolutionize glass manufacturing. Their method gave glass sheets uniform thickness and very flat surfaces, precisely the characteristics that mean nearly all modern windows are made from float glass.

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Pilkington manufacturing process

The Pilkington process for manufacturing glass involves the following basic stages:

• Raw materials (clear sand, calcium oxide, sodium carbonate) are weighed and mixed proportionally along with the addition of admixtures to provide specific technical or visual properties.
• The mixture is heated in a gas-fired furnace or electric smelter, pot furnace or kiln.
• Molten glass is formed at around 1700°C (3090°F) and is floated on molten tin to form glass of the desired thickness.
• The glass is cooled, the precise process determining its overall strength.

The composition of glass and the cooling rate can be varied to give a range of properties depending on the end use required:

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Composition of glass

There are many different types of glass, each with different chemical and physical properties dependent upon their application. The main types of glass include:

Made of 70-80% silica, 7-13% boric oxide, smaller amounts of alkalis and aluminium oxide. Widely used by the chemical and pharmaceutical industries as well as for common household items, under the trade name Pyrex.

This is the most common type of glass, made primarily from sand. As it is normally colourless, this glass is popularly used for windows.

The composition of glass fibre varies according to its application. For building insulation, the glass used is normally soda lime, whereas for textiles, an alumino-borosilicate glass with very low sodium oxide content is preferred because of its good chemical durability.

This is used to make predominatly decorative glass objects. Calcium oxide is replaced by lead oxide, and potassium oxide replaces all or a high proportion of the sodium oxide. Lead glass sparkles brightly and has a relatively soft surface, both properties that make it so suitable for decorative uses.

This is named after the modern process used to create large, thin, flat panels from molten glass which is floated onto a pool of molten metal such as tin. This process produces a very smooth sheet of glass with a highly consistent thickness.

For more information see: Float glass.

This is a piece of float glass that has been cooled in a slow and controlled manner. The internal stresses within the sheet of glass are reduced by this process making the resulting glass stronger and less likely to break than it would otherwise be. There can be safety concerns using annealed glass as it can break into large jagged shards.

This is made from a sheet of annealed glass reheated beyond its annealing point of around 1,200ºF and then cooled slowly. Heat strengthened glass may be twice as strong as annealed glass, but may still need to be laminated for use in buildings.

Tempering is the process by which annealed glass is heated in the same way as heat strengthened glass. The glass is cooled more rapidly which allows the internal portion of the glass to remain fluid for longer than the outer surfaces. This means that an equal amount of tensile and compressive stresses are formed across the glass which allows it to become in the order of four times as strong as annealed glass. This is used as a safety glass, as it shatters into small granular pieces rather than sharp shards, reducing the risk of injury.

This is used as a means of testing glass panes that are to be used in safety critical situations, such as a glass railing. Tempered glass panes are heated to a temperature of around 550ºF for a few hours. This causes any unstable nickel sulfide inclusions (imperfections that may cause spontaneous breakage of the pane) to expand disproportionately to the glass, making the glass break.

Laminated glass is used where glazing must remain intact if it is broken, either for safety or security reasons. It is made by fusing two or more layers of glass with inter-layers of polyvinyl butyral (PVB) through the use of heat and pressure. If it is made using heat strengthened glass, the pane will break into large pieces but will be held in the frame by the PVB inter-layer. If it is made from tempered glass, the sheet may fall out of the frame but will mostly stay together due to the inter-layer.

For more information see: Laminated glass.

This is most often used as a fire resistant glass because the wire holds the glass in place if high temperature causes it to break. The wire mesh is better at holding glass in place than the PVB films used in laminated glass.

For more information see: Wired glass

The term ‘low-e glass’ is used to describe glass that has a coating added to one or more of its surfaces to reduce its emissivity so that it reflects, rather than absorbs, long-wave infra-red radiation.

In cooler climates this means that long-wave infra-red radiation that builds up inside a building is reflected by the glass back into the space, rather than being absorbed by the glass and then partially re-radiated to the outside. This reduces heat loss and so the need for artificial heating.

In hotter climates, a low-e coating means that long-wave infra-red radiation outside the building is reflected back out of the building, rather than being absorbed by the glass and then partially re-radiated to the inside. This reduces the heat build-up inside the building and so the need for cooling. In hotter climates, a low-e coating might be used in conjunction with solar-control glass to reduce the amount of short-wave solar radiation entering the building.

The two main types of low-e coating are tin and silver. Tin oxide is applied to the glass at high temperatures to create a very hard and durable low-e coating. Silver coating must be enclosed within the glazing unit so that oxidation doesn’t cause the degradation of the silver over time.

A transparent coating can be applied to glass during the manufacturing which reacts with the sun’s UV rays to break down dirt and grime which forms on the outside of the windows, and when it rains, the decomposed dirt natural rinses away. The coating has hydrophilic properties which mean it attracts water over its entire surface, avoiding unsightly uneven water marks.

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Others

See also:

See also: Types of glass.

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Related articles on Designing

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External references

Glass is a non-crystalline amorphous solid that is often transparent; it has wide applications in construction. This article examines some of the different types of glass used for construction.

Image Credit: Vladitto/Shutterstock.com

Glazing material is typically made from soda-lime sheet glass. This is floated or rolled into sheets, then cut to size by scoring and snapping or laser cutting with diamond-tipped cutting tools or with water jets.

Glass can be strengthened with thermal or chemical tampering, and it can be bent and curved while it is being heated. Some manufacturers add surface coatings for specific functions like scratch resistance or UV light blocking.

Structural glazing is ubiquitous now and is a key aesthetic element behind modern architecture. Around the world, skylines in modern cities are dominated by large, glass-faced skyscrapers. Stainless steel fittings are countersunk into recesses in the glass panels to create the modern skyscraper look.

Glass used for construction contains a mixture of raw materials: silica, sodium potassium carbonate, lime, lead oxide, and manganese oxide.

In construction, different types of glass perform different tasks or functions. Below is an alphabetical list of the main types of glass used for construction.

Chromatic Glass

Chromatic glass is used whenever interiors need protection from sunlight. ICUs, meeting rooms, and airports use this type of glass to keep building occupants safe from glare.

Chromatic glass can be produced with electric lamination methods, making electrochromic glass. Thermochromic glass is made with heat-sensitive lamination, and photochromic glass is made with light-sensitive lamination.

Energy-Efficient Glass

Energy-efficient glass types have been developed to help buildings keep energy costs down, especially in cooler climates. This type is made by adding a thin coating to one side of a sheet of float glass.

The coating allows energy from the sun – light and heat – to pass through the glass panels in only one direction. This means windows can act as radiators when the sun is shining on them, and they lose less heat in the shade.

Float Glass

Float glass or soda lime glass is the main type of glass panel used in construction; most other glass types are made by adding coatings to float glass panels or modifying the standard float glass composition or method.

Float glass is made with calcium silicate and sodium silicate. The name comes from the production process: molten glass is poured onto a bed of molten tin, where it floats.

The liquids repel one another so that the floating glass spreads to cover the level surface on top of the melted metal. When the glass cools, it sets hard without bonding to the tin. Then, it can be removed and processed.

Float glass can be made in thicknesses ranging between 2 mm and 20 mm, and generally weighs between 6 kg and 36 kg / m2. Untreated float glass can be cut with hand glass cutting tools easily.

Image Credit: Davinsi/Shutterstock.com

Glass Blocks

Glass blocks are made out of two halves pressed together while the connecting sides are still molten, leaving a hollow center. They are used in decorative applications to allow light through while obscuring a view, such as in bathrooms and street-level windows.

Insulated Glazing

Insulated glazing provides extra thermal insulation by trapping layers of air between panes of glass. Double glazing is the most widely used type, but triple glazing units are becoming more common with growing concerns over energy efficiency in buildings.

Laminated Glass

Laminated glass is made with multiple layers of glass stacked together with a transparent, flexible bonding material tying the layers together.

It is usually resistant to UV and is soundproof, and is used for tough applications like glass canopies and walkways.

Shatterproof Glass

Shatterproof glass is made with plastic polyvinyl butyral resin that stops it from forming into sharp pieces if it breaks.

Sheet Or Flat Glass

Sheet glass is less strong than float glass but cheaper and faster to produce. It is made by passing molten glass through a roller to create a relatively flat finish.

Sheet glass is only suitable for greenhouses or decorative applications due to its reduced strength.

Tinted Glass

Glass tinting is achieved by adding one of many coloring agents to the silica mixture to color the final glass. Iron oxide, for example, makes a green coloring, while sulfur makes glass look blue.

Toughened Glass

Toughened glass or tempered glass is used throughout the construction industry. It is tempered to make it less likely to break. If it does break, it breaks into small pieces with rounded edges. Toughened glass is generally also shatterproof.

It is used in any setting where there is a risk of breaking the panel, such as shower screens, guard rails, and kitchens.

Wired Glass

Wired glass is toughened glass with a wire mesh embedded inside the glass. This works to keep the glass panel in place in the event of a break or a crack.

More from AZoBuild: A to Z of Insulation Materials

References and Further Reading

Savage, P. (2017). The Rise of Glass Buildings. [Online] Glass Times. Available at: https://glasstimes.co.uk/featured-articles/the-rise-of-glass-buildings/ 

Ten Types of Glasses: Engineering Properties, and Applications in Construction. [Online] The Constructor. Available at: https://theconstructor.org/building/types-glass-properties-applications-construction/14755/ 

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