Finishing techniques of float glass

Angela van der Burght

If stricter demands are made on ordinary clear float glass, other than just transparency, firmness against the wind or, for instance, insulation, something must be changed within or on the surface of the glass. This is especially true for glass used in building constructions (building glass, structural glass or glazing glass) and automotive glass (glass for self-propelling vehicles such as cars, ships, trains and airplanes), which can modified for - among others - heat and sound insulation, light-inhibiting, breaking security or resistance to sudden loads like vandalism and fire, smoke and heat resistance can be influenced. By refining on-line (originating techniques during the production) and off-line coating and finishing (working and processing techniques of ready-made flat glass), the physical, optical and decorative qualities of flat glass change. On- and off-line finishing techniques are used at glass companies and big holdings like Pilkington, Saint Roch/Saint Gobain Glass A.G., Glaverbel, Scheuten Glasgroep, Tamglass, Euroglas Benelux or Glas Trösch. These companies display a variety of brand names for their standard products. Generally, technical flat glass like borosilicate, optic and quartz glass has smaller sizes than usual for building construction.

To understand the various positions of a glass plane, you must position yourself within a room and look at the glass window. Position 1 is the backside of the glass that faces outdoors. Position 2 is the side turned towards us. Position 3 is the backside of the second pane (facing outside again) and number 4 is facing the inside. For safety and insulation some windows are made of 3 or even more glass planes and position 5 faces outside again etc.

SUN CONTROL
According to the thickness of glass and the angle of incidence, 75 to 90 percent of sunbeams shines through ordinary glass. Ray transmission can be decreased by using multi-laminate compounds, using glass with an increased light and infrared absorption or by dispersion and by float glass with altered surface qualities. Special coating techniques can change the capacity of reflection and absorption of the glass surface, that optimises energy saving and climate control.
- Body coloured float glass with an increased absorption rate has a high percentage of iron oxide which makes the glass greener and blocks the infrared from sunlight. The glass will absorb half of the sun’s heat then, the rest dissipates upon contact with the cooler air outside. Automotive glass makes extra use of this fast cooling down factor through air circulation. These pot metal or coloured-in-the-mass windows used in high-rise buildings have to be toughened because of the intense thermal tension that occurs in the irregular warming and cooling from sunlit or shaded parts. Like sun glasses for your eyes, body coloured float glass is besides the green produced in yellow, grey, blue and bronze for dimming light. The yellows particularly filter ultra-violet and short wave blue out of the light, and prevent the loss of colour and bleaching of paper, textiles and paintings. NB: The opposite: Extra clear float glass is called white glass.
- With laminated safety glass, a special UV reflecting foil (German: Folienverbundglas) can be put between the laminates and the glass plates which then is melted together in an autoclave, a special vacuum kiln. Automotive windows can be made with special graduated coloured laminates.
- Dichroic glass resolves the light within different spectrum colours by dichroic layers (10 or even 40 layers are possible) and is achieved by atomization of cathodes. Depending on the angle of light passing through, only a part of the radiation passes through, while another part is reflected. Different colours are obtained by reflection and transmission.
- Enamel print: Screen-printing, simple patterns and colours in transparent, opaque, translucent and interference glass paints are applied to stop or disperse the light. After applying the enamel, the glass can be toughened immediately after the staining. Cold enamels are provided with a pyrolytic coating for weather resistance. Fully enamelled glass is specially used for parapets and wall coverings.
- Pyrolysing is a process in which semi-conducting metal oxides are directly applied on float glass during production on line. The glass is also called K-glass. For these hard coatings thin oxide can also be used to reduce the emission grades.
- The plunging procedure is a method in which a glass plate is plunged in a metal oxide solution. This coating is fired afterwards.
- With the chemical procedure a metal oxide solution will be sprinkled on the still hot glass.
- The vacuum method consists of three techniques: the high vacuum method or thermal evaporation, the cathode atomization or sputtering, whereby in an electric field evaporated or loosened metal ions then precipitate on the glass surface and the magnetic enforced sputter procedure, in which a magnetic field is used for the cathode atomization. These soft and semi-hard coatings must be protected against the weather and are placed with Position 2 and 3 with these surfaces facing the hollow side of double window units. These reflective coatings mostly consist of several layers: the adhesive, interference, working and protective layers. Together with the interference layer, selective coatings can be applied for improving colours and emission grades.
- Sun-reflecting foils of different colours and patterns are self-adhesive and put on the glass. These are vulnerable to scratches and damp, from the danger of coming unstuck and are only used for sun screening in existing situations.
- Thermotrope glass reacts to heat: at a certain temperature, clear glass turns milk white. The basic material consists of water and a synthetic material (hydrogel) or two synthetic materials (polymer mixture). The polymer chains, stored between two glass laminates, are long and transparent when the glass is cold. When it is warm the polymer chains scroll, so that a milk white impression arises and lets through less heat. The process is irreversible and not frost-resistant.
- Thermochrome glass is a type of glass that reacts to heat and has Low-E-coatings: especially within the infrared area, metal oxides can acquire other transmission qualities. Vanadium oxide becomes a semi-conductive or dielectric at a certain temperature with a low absorption in the infrared area. When the temperature increases, the material becomes more metallic-like and reflects the infrared.
- Electrochrome glass changes under the influence of tiny electric charges from clear to milk-white glass (in building construction) or from clear transparent to dark (automotive glass). Upon a reversal of tensions within the glass it becomes clear again.
-Phototropic glass is a self-regulating system in which the quantity of the light passing through it changes with the quantity of incidence of ultraviolet rays. The glass darkens as the sun’s intensity increases. Self-colouring eyeglasses work with this principle.
- Liquid Crystal glass creates molecules that are enclosed between two glasses laminates pass through polarized light unaltered, via electricity.
- Prisms, diffraction glass, reflex glass, refraction glass or prism glass and holographic films can all be used to restrict direct sunlight.
- Photovoltaic systems turn sunlight into energy. Solar cell PV modules can be assimilated between glasses laminates and become solar panels this way.

HEAT
Thermal insulation is required when heat passes through by convection, conduction and radiation.
- Insulating glazing, double glazing units, sealed double-glazed windows or double-sheet glazing units with special coatings are the most important heat insulator and consist of two or three glass panes, that are soldered on special metal frames so that an inner space or cavity has been enclosed. To prevent condensation of moisture, a drying aid (granule) is put in these holders. The enclosed air or gas-filled cavity is a bad conductor of warmth, cold or noise. The thicknesses of these two windowpanes must always be different to avoid polarization rings and sound resonance. A common combination is an outer solar control glass and an inner Low Emission glass. The insulation glazing system needs to be built in very carefully to prevent extension (pressure from outside) or crushing (overpressure from outside) of the panes.
- The principle of Low-Emission coating is based on a physical rule in which a material has the same warmth emission as it can absorb. Glass absorbs only a small part of the short-wave radiation (light) and transmits almost all long-wave heat radiation outside. In double-glazing system position 3 is provided with a metal-based Low-E coating: when light rays pass through, heat rays will be reflected. The glass is coated by sputtering, pyrolysis or a synthetic foil. Also, by enlarging the inner space as well as by filling it with translucent insulation material such as acrylic foam, polymethyl methacrylic, polycarbonate or semi-homogenous substances such as aerogel, the isolation grade can increase.
- With special coated Electric Heated glass the glass panes are warmed up to prevent the loss of energy, condensation or freezing of the glass.

STRENGTH
In pre-stressed glass, heat strengthened, strengthened glass, or thermal hardening, the toughened or hardened and the curve-pulled tension is changed: when glass is curved, a tensile force develops on the long, straight side and a compressive tension on the short side. When the tensile force becomes too much, the glass breaks. When the compressed outside zone of the glass plate is enlarged, it becomes stronger. Application of the compressive zone is called toughening or hardening and can be done chemically or thermally.
- Toughened glass is five to eight times stronger than ordinary glass, therefore, the most appropriate product when glass has drill holes or cutouts in it. These fully tempered glass panels become completely disorientated and rupture into small fragments: such pieces are crumbly, not sharp nor heavy as shard fragments. Consequently, only weight can cause injuries. If the weight of these falling glass fragments is a problem, then this type of glass can be laminated into safety glass. All mechanical treatments such as drilling and polishing must be done before the toughening or hardening. The heat soak test examines the glass for nickel sulfide pollution, which could break the toughened glass spontaneously, sometime later. Touch control techniques in domestic appliances are hardened for security reasons. Anisotropy is the effect you see when looking through Polaroid sunglasses on hardened glass: in tension zones light beams reflect two-fold and appear like cloudy spectrum colours.
- During heat strengthening, half-hardening or thermal hardening glass is heated up to 650 degrees Celsius, the point at which glass will not weaken. After this, air on its surfaces forcibly cools it down, quickly shrinking the outer-side. The core of the plate, still hot, cools down, but will be obstructed by the outer zones. So, a pushing zone develops, enclosed within the glass, while the central part is charged with a drawing force. If glass has only to resist thermal forces, it will be half-toughened by cooling down less fast. This heat strengthened or pre-stressed glass is about twice as strong as annealed glass, but breaks like this normal float glass. Fractures then create fragments or shards.
- In 1891, glass chemically hardened or strengthened by ion exchange for heat resistance, glass cookery dishes and laboratory glass was invented at Schott Glassworks. High extending glass has been placed over a low extending layer after a chemical hardening of the glass, then, finally it is strong. Chemically hardened glass has been plunged into a salt solution with potassium ions. The potassium ions replace the sodium ions in the glass. By their larger volume they generate a pushing force in the outer glass laminate. This glass is especially used in the boat and aircraft industry, for washing machines and clothes dryers, lamps and for hardening eyeglasses.

SECURITY
Glass, a strong but easily broken material requires extra precautions in order to increase safety: falling through safety or break-proofing at parapets and balustrades, fireproofing, protection against vandalism, 'invisible' doors, burglar-proofing, security against explosions, resistance to bullets, avalanches and sports centres’ activities, glass for roofs, domes, ceiling lights and even protection against electromagnetic rays. To obtain these qualities, all kinds of glass are used: toughened or heat strengthened these glass panes are made using safety and wire glass, often in combination with insulation glass.
- Laminated safety glass with PVB-foil consists of two or more glass plates, sometimes laminated with special synthetic laminates, PVB foils or inter layers in between by rolling or packing the glass and laminates stacked into a vacuum wrapping that will be well pressed and melted into a whole in an autoclave kiln. Depending on the demands, the thickness, the quantity of plates and foils is determined. The laminate can be transparent, coloured, decorated or opaque. Other materials and screen prints can be enclosed for art purposes.
- Laminated glass with polycarbonate is used for burglar, sound and bullet proofing. One of the plates is replaced by a polycarbonate one. Another advantage is that it extinguishes fire. Even security helmets and spectacles are given a laminated glass with a synthetic layer.
- Cast-in-place (CIP) techniques laminate glass and polycarbonate by casting a multi-component resin between two glass panes. The resin can be mass-coloured.
- Electromagnetic shielding is a buffer against electromagnetic rays of machines and installations used to protect networks and computer information from unwanted radiation. Glass with a metal coating connected with a frame forms a protective cage.
- Radar dimming is needed in the vicinity of airports to prevent technical malfunctions. Special coatings can absorb or reflect radar beams.
- Protection glass is used for TV-screens, machines and instruments.
- Armoured glass consists of four laminated plates of glass and synthetic materials of 25 mm thickness.
- Alarm glass is laminated glass with contact wires, connected to an alarm system.
- Wire-laminated glass is provided with a steel network.

FIRE
Glass does not burn, but normal float glass is not fireproof and breaks at high temperatures and lets through flames, heat and smoke gases. Monolithic systems must be completely avoided. Fire resistant insulation glass is called fireproof glass. Useful glass sorts - depending on being fire resistant for 30 or more minutes - can be laminated armoured glass, toughened borosilicate (for example Pyran) and glass ceramics, as well as specially finished hardened multi-laminated safety glass and glass with foaming laminates and aqua-gels.

SOUND INSULATION
Sound-proofing properties can be increased by: better sealing the edges of window frames, a thicker glass plate, by enlarging the cavity between two insulating plates or by filling them with special gas. The use of laminated glass with polymethylbutyral or methacrylic casting resins obstructs noise by its toughness or viscosity. These resins can be coulered in the mass. The thickness of the glass plates must be different.

DURABILITY
Glass can be corroded by pollution: either by rust (for example near train stations), concrete and acid rain. However, float glass is very resistant to pollution when cleaned regularly, not constantly exposed to steam and damp and when prevented from steeping in lye. If oxides are added to the glass mass, durability is increased. Later, special coatings like Clearshield, that smoothes the surface can be added to the glass surface. The glass becomes hydrophobic (water rejecting), apolar and in this state it needs less maintenance. Because of this special finish, Position 1 cannot be re-coated and suction cups will not have any grip anymore on the surface. Glass must be stored in a dry environment. Separation and packing material must have a low pH to prevent steeping in lye.

CURVING
Curved glass is manufactured by heating and bending one of the basic products in a bending kiln. The glass can be subsequently annealed or toughened. Curved glass can be laminated and built into insulating units provided that the glass has been matched in bending. Special movable moulds curve the glass gided by gravity, thereby obtaining the right arch. For automotive glass sometimes two glass plates are curved simultaneously to laminate them afterwards.


Ton Slits: Bijlmermeer
In the south-east of Amsterdam lies the Bijlmer, short for Bijlmermeer, one of the polders surrounding Amsterdam. A complex community centre - built between 1963 and 1973 - was developed here to provide in proximity to offices and companies, luxurious accommodation for a hundred thousand new house hunters in 1950 in the post-war booming economy. The development’s main objective was to join living, working and travel needs. This then ‘modern’ concept was developed by the urban development bureau of the City of Amsterdam with the architects Kees Rijnboutt, Kromhout & Groet, Offenhof, Geerts and Pi de Bruin. Specially designed concrete factories were built to cast the individual elements for this system of building.
The honeycomb city plan arranged high-rise buildings of 9 floors on top of a street level base, its storerooms facing a park like environment with traffic-free pedestrian zones, while the metro, being some 9 meters high, and fast highways connected with the city of Amsterdam.

In 1992, in the heart of this city plan, an El Al Boeing 747 crashed into two flats. Many people were killed and buildings demolished. In the middle of the park like landscape a monument was erected, designed by architect Herman Hertzberger and his daughter, around "the tree that saw everything". Children who lost family members and many little friends created individual concrete tiles with glass mosaics to lay on the floor of this commemorative site.

Herzberger also designed a water garden with a ground shape mimicing the demolished buildings. Now, for the last few years, the Bijlmer has been the focus of full renovation. The flats were not the cause of disappointment to its inhabitants but blame was focused on the lack of more luxury like elevators, shorter corridors and solutions to overly large public spaces lacking social control. Advised to tear down the whole complex in 1994, this has now modified as demolishing old buildings like the parking garage and replacing the spaces with houses, renovating the old high-rise buildings while building new ones. The park and lanes will be redesigned as well. Jazzing things up, the working group Kunst en Vernieuwing (Art and Renewal) wanted to make some larger murals. Together with the employers of the housing association New Amsterdam, in conjunction with the landscape designers of the GK-area (between Ganzenhoef and Kraaiennest) and residents associations, a plan has been developed to guarantee quality, diversity and the right connection with nature and the city of Amsterdam. In the form of a contest, out of 250 designs responding to this competition, 12 artists were chosen to design the murals for one gable end of every high-rise building renovated. From those twelve, six were chosen and guided by the Amsterdams Fonds voor de Kunst (Amsterdam Fund for the Arts) to actually realize their work. Technique and autonomous visions were free.

Ton Slits calls his design A Sign At Large. With a height of 2800 cm and a width of 1439 cm he used one gable end of a high-rise building as his canvas - like thin sheets of paper - layered with meanings: the blue ground representing an "all covering space or thinking layer”, the universal. The first coat used in this system, StoSilco Garant, contains a medium of silicone resin emulsion with cobalt coloured pigments. The second layer symbolises by dark green lines the influences from outside, orange dots are energy. Finally, black discs represent by their reflection or mirroring the concrete, the actual, and the environment. "That environment, a melting pot of several cultures with as many different experiences and interpretations of form and colour, forced me to search for a universal expression. With respect to the architecture, I thought that I had to ‘clinch’ my work, so to speak, to the façade of the building. Also I wanted to stay loyal to my vision on art in public spaces: to strive for a space-total-work and stratification in realization and imagination. Autonomous, but with respect for, and making use of, all conditions and elements of the site and architecture."

The black discs are made from toughened safety glass of 1 cm, ø 700 mm, and backside enamelled with SGG Emailit. Three holes are drilled into the glass for attaching the glass discs to the wall. Fenestra Ateliers supervised the glass design with Geert Driessen as the subcontractor who was responsible for the realization of the glass and the construction. The paint was developed and advised by Iston BV, while the company Hemink BV actually painted the wall with their experts after Ton Slits had measured out the drawing on the stone.
Ton Slits > www.tonslits.com