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Glass basics

How Glass Is Made

Until the middle of the 20th century, glass panes were produced by drawing or moulding molten glass, and then polishing out the imperfections. In 1959, the float glass process was invented, eliminating the need to polish and therefore reducing the labour required. The float glass process is how Guardian's 25 plants around the world make their glass.

The float glass process begins with a mixture of raw materials, including about 60% quartz, 20% soda and sulphate, and 20% limestone and dolomite. Huge agitators crush and then process these materials into a mixture. A blend comprising of roughly 80% of this mixture and 20% of recycled scrap glass is fed into the furnace and melted at about 1600°C. The result is a chalk-natron-silicate glass. After refining the molten mixture, the molten glass is fed into the conditioning basin and left to cool to approx. 1200°C before flowing over a refractory spout into a liquid tin bath.

Molten glass spreads evenly over the surface of this liquid tin bath, “floating” on top of the liquid tin with its inherent surface tension and density, which is heavier than the density of liquid glass. As a result, the molten glass smoothly and evenly moulds itself to the surface shape of the liquid tin. Reducing the temperature in the tin bath from approx. 1000°C to approx. 600°C turns a viscous mass of molten glass into a solid glass sheet that can be lifted right off the surface of the tin bath at the end of the floating process.

Properties of Glass

Normal float glass has a slightly greenish tint. This colouring can mainly be seen along the edge of the glass and is caused by the naturally existing ferric oxide in the raw materials. By selecting extremely ferric oxide-poor raw materials, or by undergoing a chemical bleaching process, the melt can be turned into an absolutely colour-neutral, extra white glass. Guardian produces this type of glass, called Guardian UltraClear ™.

Tinted glass can be produced using coloured mass. Chemical additives in the mixture allow green, grey, blue, reddish and bronze coloured glass to be produced during certain production floating line periods. Changing glass colour in the vat naturally means a considerable amount of work and increased cost due to scrap and loss in productivity. Thus, it is only produced for special campaigns.

Most of today’s glass production is float glass, with thicknesses usually ranging from 2 – 25 mm and a standard size of 3.21 x 6 m that is used for further processing.

The Need for Glass Coatings

The physical definitions of light, energy and heat are described by defined wavelengths of the electromagnetic spectrum. When wavelengths hit an object, certain wavelengths are reflected, absorbed and transmitted through the object. Guardian Glass applies coatings to float glass in order to manipulate which wavelengths are reflected, absorbed and transmitted to serve functional and aesthetic requirements.

For example, designing glass coatings that reduces solar heat requires controlling the radiation emitted by the sun that strikes the earth. Guardian Glass coatings are designed to reflect the infrared wavelengths (solar heat) while at the same time transmitting the visible light wavelengths (daylight) through the glass.

Applying Coatings to Glass

Float glass coatings are produced in huge quantities, primarily in 2 techniques. One is the chemical pyrolysis process, also called hardcoating, which occurs online during glass production on the float line. Metal oxides are permanently baked onto the surface, and are very hard (hardcoatings) and resistant, but their properties are very limited due to their simple structure.

Guardian Glass focuses on the second coating process called vacuum deposition process or magnetron-sputtering. Magnetron sputtering deposits metal oxides onto glass perfectly smoothly in a sequence designed to achieve outstanding optical and thermal properties. Glass is sent to into a coater with vacuum chambers each designed to apply a layer that material to the glass.

The material (which is a metal plate) that is going to be deposited on the glass surface is mounted on an electrode that has a high electrical potential. Electrode and material are electrically isolated from the wall of the vacuum chamber. The strong electrical field (fast electrons) ionize the sputter gas argon. The accelerated argon ions are able to break off material from the target by colliding with it, which then comes into contact with the glass, where it is deposited onto the surface evenly.

Thermal Insulation

Architects and consumers are demanding greater thermal insulation of their building envelopes to satisfy economic, ecological and comfort requirements. Guardian is working tirelessly to enhance building performance without sacrificing the natural light people desire. The motivations behind greater thermal insulation are:

Economic
Building owners and managers are willing to invest in solutions that will reduce their energy costs. Technological advances of the last three decades have produced systems and equipment that can coat high-tech insulating glass with razor-thin, neutral coatings using low-cost processes. This technology is now applied to millions of square meters of glazed areas of windows and facades.

Ecological
Consumer and industry awareness for environmental sustainability have grown rapidly within the building industry. Due to its natural ingredients and superior energy-balancing properties, glass is a key element to reduce heating and cooling costs and to achieve the goals of globally recognized certification programs for building sustainable and environmentally friendly buildings.

Comfort
Better thermal insulation reduces unpleasant drafts from glazing areas, without sacrificing the benefits of daylight that make people feel better and more productive.

Insulating Glass

To achieve thermal insulation properties, two or more float glass panes are combined with a spacer to form an insulating glass unit (IGU). This reduces the loss of conditioned air—reducing heating expenses in colder months and air conditioning costs in warmer ones.

Until very recently, the majority of IGUs has been produced using aluminium spacers. Increased requirements have created thermo-technically improved alternatives that are gaining ground in insulating glass production. Extremely thin stainless steel profiles possessing considerably reduced heat conductivity as compared with aluminium are the most frequent alternative. They are similar to aluminium, however, in terms of their mechanical stability and diffusion capability.

Insulating glass can increase its thermal insulation performance using three additional techniques:

Low-E coatings
Sputter coatings designed to increase thermal insulation, such as many Guardian ClimaGuard ® and SunGuard ® coatings, can be applied to surface #2 or #3 (depending on climate). ClimaGuard IS-20 is a durable coating that can be applied to the inside glass surface to further insulate IGUs.

Gas in the interspace
The resulting hermetically sealed interspace is often filled with especially high thermal insulating inert gas. The width of the pane interspace depends on the inert gas that is used. Argon is used most often, krypton more rarely. To reach its optimum thermal insulation efficiency, argon needs an interspace of 15 - 18 mm; krypton needs only 10 - 12 mm for better insulating results. The interspace is usually filled to 90% capacity. Krypton is many times more expensive than argon since it is more rare.

Triple-glazing
Especially for projects where sustainability certification is a priority, triple-glazing (using 3 glass lites) enhances thermal insulation performance even further.

Solar Protection Glass

Modern architecture continues to use more and more glass, largely thanks to advances in solar protection glass. Solar protection glass reduces the greenhouse effect that mainly occurs in summer due to that fact that rooms can heat up to the point that they become unpleasant to be in, while dramatically reducing a building’s operating cost throughout the year. Solar protection glass offers:

Financial savings
Solar protection glass minimizes the amount of heat energy that penetrates a building, limiting the extreme costs of air conditioning, since it costs much more to cool the interior of a building than to heat it. Large window and facade surfaces allow a great deal of light deep into a building’s interior, thereby avoiding excessive use of artificial lightning.

Ecological benefits
Energy is saved by reducing the amount of cooling power use or reducing the phases of artificial light – helping to reduce human impact on the environment. ln this context, it is a logical consequence to certify these types of sun protection glass products according to worldwide-approved certification systems for sustainable constructions , such as to LEED, Breeam and Passive House.

Comfort
Solar energy can be leading cause of overheated room, which make occupants uncomfortable. Today’s architecture – which strives to create living and working areas that are close to nature and are open and spacious – has shifted away from this opaque way of construction towards transparency. Therefore it is essential to master the essential parameters of the sun protection using glass to create a functional and comfortable interior.

Design Options

For centuries, generations have used glass for filling “light holes” in massive outer walls. This has drastically changed in the last three decades. Today glass itself forms and shapes the space and creates room enclosures, thus creating transparent architecture that allows people to feel close to nature. A glass’s finish on a façade always influences its reflective properties, which can range from being produced so that the glass is very reflective, reflects an overall color, or has a weak reflection.

Screen-Print technique
The screen-print which is mainly dedicated for painting partial areas and used for specific design colour which joins firmly with the glass surface and coating in the following tempering process. This technology is adopted for larger quantities and is ideal for parapet glasses components is less suitable for larger areas and homogeneous painting.

Design glasses
Transparent elements can become more and more visual and functional decoration façets. From etching and shot blasting over the ceramic screenprint up to laminated glass with inside foils, the design can be a decorative ornament or symbol or also an all over illustration or matting.

Screen-print directly onto the glass
The one colour screen-print directly onto the glass has a long tradition. The enamel or ceramic paint which is a mixture of small milled glass and joining colour pigments is pressed with a scraper through the open parts of the sieve onto the glass.

Transfer colour print on glass
The transfer print offers an alternative to achieve a multi colour print instead of the single colour screen-print. Also enamel and ceramic colours can be transferred via digital print on transfer foils and can thus reproduce multi colour motives. These printed foils will then be fixed on glasses that are going to be tempered. During the tempering process these transfer foils will burn residue-free and the painted colours will join as previously described.

Coloured foils in laminated glass
Within the same lamination process exists a large pallet of different colour foils, which can be combined of to achieve each imaginable color in laminated glass.

Bent architectural glass
Architects and designers love to interrupt straightness, corners and curves with soft curves. In the applications of the building envelope, glass is generally bent through a thermal gravity process. A glass pane is laid over a bending form and in the bending oven heated up to 550 - 620℃. After having reached the softening point the pane descends (through gravity) slowly into the bending mould and adopts its shape. The following cooling down defines the shape of the glass. Slow cooling, free from residual stress, produces a glass which can be further processed, whereas fast cooling creates a partial or fully tempered glass which is not applicable for further processing.

This is just a beginning

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