Glass

Float Glass / Low Iron / Tinted / Coated / Reflective / LowE / Toughened / Heat Soaked Toughened / Heat Strengthened / Thermal Shock-Stress / Ceramic / Laminated / Security / IGU / Surface Numbering / Curved / Decorative / Glass Performance Terminology / Switchable Privacy Glass 

Introduction
Glass has been described as the 'greatest building material known to man' and is a building material with outstanding durability and it plays a crucial part in creating the visual appearance of all buildings.

Understanding glass: its relationship to light, transparency, reflection, insulation, solar gain and sound control and what these qualities can achieve in a building; together with the impact on indoor environment quality, resultant productivity and the built aesthetic is critical when designing with glass.

Woods Glass (NZ) Ltd (Auckland, Wellington and Christchurch are subsidiaries of Thermosash Commercial and were purchased in order to provide greater vertical integration to the business and enable us to work closely in sourcing specific building glass solutions - including performance glass and green façade solutions, as well as undertake the majority of Thermosash's commercial contract glazing around New Zealand.

Woods Glass are also considered the market specialists in Total Vision Solutions - on which Thermosash work closely with the engineering and where applicable, integration into curtainwall systems.

Whilst all float glass is imported into New Zealand, local processing is available for some products so for specific design projects there is the opportunity to get exactly the glass performance you need for your building glazing solution without compromise, from a range of local and international processors.

New Zealand glass standards require that safety glass is used in many situations: such as doors, side panels, low level glazing, internal partitions, shopfronts, educational buildings, bathrooms, overhead and sloped glazing, structural glazing and balustrades. There are many recorded instances of the wrong specification being applied to projects - working with a company that understands the needs is important. 

• Float Glass (annealed) - The float glass process is the most common process of production. The process involves the melting of the raw ingredients in a furnace and floating it on a large bed of molten tin. The mixture slowly solidifies over the tin as it enters the annealing oven where it is slowly control cooled. The glass exits the cooling process as one large sheet when it is cut to meet the customer's needs. The speed at which the glass is drawn over the tin bath controls the thickness which ranges from 3-19mm.

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• Low Iron (extra clear) Float Glass - The main ingredient of glass is silica which has naturally occurring iron oxides and it is these oxides that cause the greenish tint you can see in most butt jointed structurally glazed situations such as total vision systems, shop windows, showers, glass cabinets etc. Low iron glass is made from a high quality silica which is almost iron oxide free, offering a clearer glass allowing much higher light transmission. All other finishing processes are the same as standard glass. 

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• Tinted Glass - Manufactured by the same process as float glass, however manufacturers can introduce oxides to tint the glass in a range of shades, typically green, blue, grey and bronze. They aid in reducing UV, visible light transmission and solar gain. When a glass has a tint it can get hot and create thermal stress based on the colour and darkness of the tint the glass may require heat treatment. Tinted glass can provide significant benefits when used in IGU's and in conjunction with a Low E coated glass.

• Coated Glass - Float glass can be coated on the float line with pyrolytic coatings (hard Coatings or off line with sputtered coatings (soft coatings). The coatings are transparent and can be reflective for solar control and or Low E for insulation.

• Reflective Glass - Designed to reflect solar heat and light and reduce glare and solar gain in a building. There is a wide range of reflective glass types and colours and properties vary widely.

• Low E - Low emissivity glass (Loe E) is a special coating that is designed to increase the thermal performance of windows and reduce heat transfer. In simple terms it is like a 'transparent tin foil'. The pyrolytic coatings can be used in single glazing, but Low E coatings are best applied to surface 2 or 3 of an IGU and used in conjunction with a tinted glass to assist in reducing solar gain.

• Toughened - Also know as tempered glass, is a grade A safety glass that is easily identifiable in its broken state by its characteristic small dice sized pieces of glass which significantly reduce the risk of injury. Toughened glass must have all its finishing work e.g., holes, chamfers and polishing undertaken prior to the process of toughening being undertaken. The toughening process involves heating of the glass to a very high temperature and then, rapid cooling by blasting air on the surface to create high compression stress and tensile stress in the centre. The additional stress imposed in the glass creates thermal and mechanical strength. Toughened glass is susceptible to NSI (Nickle Sulphide Inclusions) which may cause spontaneous fracture of the glass. Therefore any safety glass installed overhead or as a barrier should be heat soaked and or laminated so it remains in the frame and has a second layer of glass as a contingency.

• Heat Soaked Toughened Glass - Heat soaking is an additional destructive testing process that follows the toughening process and is aimed at detecting impurities in the glass like Nickle Sulphide Inclusions (NSI's). The heat soak process heats the glass slowly up to a moderately high temperature and slowly cools it again - should the glass have an inclusion or other fault in it thisprocess generally causes the glass to fail. It is a safe guard process to ensure that any toughened glass that is being used as a life safety barrier or could be installed overhead does not fail in service. NB. The process does still not offer a 100% defect free certification.

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• Heat Strengthened Glass - Similar to the toughening process, although the glass is heated to slightly lower temperatures and has less air quench meaning the glass has lesser surface compression stress. It does not fracture in the same fashion as toughened glass and it is not a safety glass, however as it will generally break from side to side the glass will remain in the frame after breakage. Due to the heat strengthening process this glass is still very stable to thermal stress and is most commonly used in tinted glass and coated glass where it may be partially exposed to large temperature differentials in facade vision or spandrel panels which are susceptible to large internal heat gains. There is a significantly reduced risk of this type of glass having NSI failures. This is a common form of glass processing for curtainwalls for both the vision and spandrel area.

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• Thermal Shock/Stress -  Is a phrase often used where the glass is heated unevenly and, if not heat treated or toughened, could be susceptible to differential heat absorption (uneven expansion or contraction) - often due to shading across a glass pane, causing fracturing.

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• Ceramic Frit - Ceramic Enamel glass is often known as 'fritted' glass as it uses crushed glass frit mixed with special enamel paint and colour oxides that are fired onto the glass surfaces during the heat treatment process. The ceramic enamel is applied by rolling, spraying, screen printing or digital printing and is very stable in colour and durable. The pattern or design can be solely for aesthetic purposes or can aid in privacy screening providing solar shading or reducing glare. Direct on glass digital printing allows for virtually any image in multiple colour to be printed on glass for interior and external glass applications.

• Laminated Glass - Is a Grade A safety glass and provides increased safety because it holds together of broken reducing the likelihood of serious injury whether in a vertical or overhead situation. It can also offer increased security as the glass will generally remain in the frame if impacted. Laminated glass consists of two or more layers of glass, factory bonded together with plastic interlayer. Laminated glass is available in a range of interlayers and the most common being;

Polyvinyl Butyle (PVB).

This the most common form of interlayer and is available in 0.38, 0.76, 1.1 and 1.52mm thicknesses but can be layered thicker.

Cast in Place (CIP) resin.

There are two types of resin interlayer used, Safety and Acoustic, which provides better sound control. The resins are UV cured between glass layers which are separated by a clear edge tape.

Ethylene Vinyl Acetated (EVA)

This is a newer interlayer which is available in 0.4, 0.8, 1.2, 1.6mm thicknesses and can be layered thicker. It is ideal for laminating a range of special products including inserts in the laminate.

SentryGlas Plus (SGP).  

This is a special high strength interlayer for barriers and structural glazing application, which is more rigid than other interlayers.

Laminated glass can also assist in a reduction of noise transfer through the glass andlowered UV transmission. Laminated glass can be incorporated with other glass types in an IGU to provide a further increase in thermal performance. Laminated glass has size limitations depending on the process but Woods Glass has access to some of the largest lamination manufacturing sizes available. PVB is available in various tints and colours including translucent and EVA can be laminated with coloured PET inserts and or a translucent interlayer.

• Security Glass - Laminated glass can be made to resist severe purposeful or accidental impact and is often constructed from combinations of multiple layers of glass and thick interlayer.  It can be made to resist flying debris (cyclone) smash and grab (anti-bandit), intruders (intruder resistant), bullets (bullet resistant) and various combinations are available depending on the level of resistance or gun class. The glass must match the frame design in order to equally resist the force imposed. 

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• Insulating Glass unit (IGU) - An IGU has two or more panes of glass factory sealed with an air-gap between and is generally known as Double Glazing. Since single glass has poor thermal resistance - by adding the air-gap the insulation value increases significantly by eliminating heat transfer. An IGU works to insulate in both winter and summer conditions and reduce the cost of heating and cooling in buildings. The measurement of thermal performance is known as the U-Value, and this can range from 3 to 1 W/m2k depending on the air space and glass type. The glass layers can be high performance Low E glass which reduces the U value. Because the IGU is sealed it can also be filled with inert gas such as argon to increase thermal performances. IGU's provide many advantages to a building envelope: reduced heat loss, heat gain, condensation, reduced noise and potentially reduced mechanical plant - and above all, increase occupant comfort.  

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• Surface Numbering with Glass 

  

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• Curved Glass - This is a specialist process where glass is heated and curved to the designer's specifications - a radius as little as 450mm dia is achievable, as well as laminated and toughening processes.

  

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• Decorative Glass - Glass can be made in a range of decorative forms, from patterned glass to coloured glass and from opaque to translucent. It can be printed and painted and etched and all combination of the above by laminating, so the options are unlimited.

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• Glass Performance 

There are many factors that drive the selection of glass - below is the common terminology often refereed to in regards to performance with a brief explanation of what each means; 

• VLT (Visible Light Transmission) - Measured as a percentage of visible light that passes through the glass. A high percentage indicates a glass that may have high clarity but may create glare. Balancing the VLT with other critical factors is important. 
• VLR-e (Visible Light Reflectance from the Exterior - Measured as a percentage of visible light that is directly reflected from the exterior of the glass surface. A low reflectance value could be around 10% as clear float glass is 9% with specialist shop front or car showrooms as low as 1%. Higher reflectance glasses help to reduce VLT and solar gain as they tend to reflect both heat and light.
• SC (Solar Shading) - The Shading Coefficient (SC) is a measure of the heat gain through glass from solar radiation. Specifically, the Shading Coefficient is the ratio between the solar heat gain for a particular type of glass and that of double-strength clear glass. e.g. referenced against a sheet of 3mm clear float glass as 1.00, compared to a high performing glass that could be as low as 0.40. 
• SHGC (Solar Heat Gain Co-efficient) -  Expressed as a number between 0 and 1. The SHGC is the combination of incident solar radiation directly transmitted and absorbed and subsequently released inward. The lower a window's SHGC the less solar heat it transmits. Solar heat gain can provide free heat in the winter but can also lead to overheating in the summer. How to best balance solar heat gain with an appropriate SHGC depends upon the climate, orientation, shading conditions and other factors. Where your project is subject to a long air-conditioning season, it is most important to reduce solar gain and therefore reduce air-conditioning loads. 
• SC (Shading Coefficient) - This measure is the ratio between the solar heat gain (SHGC) for a particular type of solar control glass and that of 3mm clear float which has a SC of 1.00. A very high performance glass can have a SC as low as 0.20.
• U Value (Insulation) - This is a measure of heat transfer through the glass due to thermal conductance and the differential between interior and exterior environments and temperatures. Single clear glass has a U value of about 6 and IGU about 3, a Low E  IGU about 2 and a very high performance IGU about 1.
• VLR-OUT (Visible Light Reflectance from the Exterior) - Measured as a percentage this value indicates the visible light that is directly reflected from the exterior of the glass surface. A low reflectance value could be around 7% with specialist shop fronts or car showrooms as low as 1%, however a higher reflectance will often mean a lower VLT.

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• Switchable Glass - is a laminated glass with a liquid crystal inter-layer that carries an electrical charge. When current runs through the glass inter-layer the crystals align which makes the glass transparent.  However when the power is turned off, the crystals randomly overlap causing an opaque privacy glass.   Refer to the individual Switchable Glass category under Materials for further information.