Working with Curved Glass

Technical Information

The following information covers a lot of the frequently asked questions about Curved Glass and Glass in general. The bullet listed items below are linked to the relative information contained within this page for easy navigation. If you require further information, please don’t hesitate in contacting our Customer Experience Team, who will be only too happy to help you out.

Bent Glass Specifications & Tolerances

Bent Glass Dimensional Tolerances

So many variables impact how glass can be curved and bent – please contact us directly to discuss your needs as even complex curves may be achieved. The Bent Glass Specifications & Tolerances guide provides some direction on tolerances for tight curves – generally speaking, increasingly tighter curves require increasingly thinner and smaller panes of glass.

A tolerance of ± half of the thickness of the glass to a maximum of ± 6mm is the accepted industry standard.

Glasshape® bends glass by either heating and rolling the glass while in a plastic state, or by the process of “sag bending”. Glass is a visco-elastic material whose mechanical properties change rapidly when heated between 600º and 700º C becoming a ‘plastic’ substance. The science of glass bending uses this plastic phase to produce shapes which are both complex yet free from wrinkles and other optical aberrations starting from flat float glass.

Optical distortion sets the limit for most shaping. All bending requires precise control of stress levels to ensure the product meets regulatory, environmental, robustness and optical requirements. A degree of distortion, both when looking through and in reflection, is inevitable in bent glass, particularly when viewing a moving object through the glass.

All bent glass should be site inspected from a minimum distance of 3m and viewed at right angles to the glass. It should also be noted that bent glass will split direct sunlight into striped shadow. Some variation in edgework may be discernible on exposed edges where different machine or hand forming is required for manufacturing. Such variation will be kept to a minimum.

Glazing Considerations

Standard glazing techniques for flat glass also apply to bent glass with added consideration of the following:

  • Glass radius and rebate radius are rarely exactly the same. Additionally radius at head may differ from that at the sill. Rebate needs to be of sufficient size to accommodate all variances. Where variances occur wet seal is recommended.
  • For annealed bent glass and annealed laminated glass wet seal is the preferred method of glazing to avoid breakage from over tight wedge.
  • Attention needs to be paid to the frame that the bent glass is being glazed into. In particular attention to the blocking of the frame into the sub-sill in the centre of the bow, as this area may droop over time if not blocked sufficiently.
  • Setting blocks and distance pieces: Same as for flat glass with the exception that a central setting block(s) may be required to avoid tipping, depending on the radius.
  • Bent glass vacuum lifters are recommended for assisting with moving the glass into position.

Note: flat glass vacuum lifters are not recommended for use on bent glass as they may lose their vacuum suddenly.

Industry Specific Glazing Guides

Nickle Sulphide Inclusions

“Spontaneous glass fracture” refers to incidents where toughened glass windows shatter without warning.  These failures are due to the presence of nickel sulphide inclusions. For more information about mitigating this please refer to the Heat Soaked Glass section below.

Glasshape® is able to provide a Heat Soak service that will test panels of toughened (tempered) glass for nickle sulphide inclusions. In almost all cases, this destructive test will cause an affected panel to shatter within the safety of the testing oven. If the panel emerges from the testing oven in-tact we can be almost 100% confident that there are no nickle sulphide inclusions, and the panel will not spontaneously fail as a result.

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IGU Handling, Storage & Protection

  • Insulating Glass Units must be transported and stored so that both panes of glass are equally supported. Therefore units should be stored vertically or stored on a 90° angle rack set at 4° -7° from the vertical as in Figure 1.
  • Units may be moved and stored on site storage trolleys similar to example in Figure 2
  • Insulating Glass Units should not be stacked more than 6 deep. For various size units, stack the largest against the supports. Where size changes occur, suitable interleaving should be used.
  • All Insulating Glass Units must be stored in dry ventilated conditions out of direct sunlight.
  • Wrapping plastic over glass for weather protection is not advised as it can allow condensation to accumulate, resulting in damage to the glass or unit seal.
  • Insulating Glass Units that are transported or installed at heights exceeding 1200 metres above sea level require special precautions and advice must be sought from the manufacturer.
  • Strops or slings which support both panes of glass are recommended for moving Insulating Glass Units.
  • If it is necessary to rotate large Insulating Glass Units, a corner block may be used to protect the edge of the unit (Figure 3), or use a vacuum lifter with a rotating head.
  • Care must be taken if handling large Insulating Glass Units with suckers, as this can put undue stress on the seals.
  • Glass grabs which create a crushing force which can damage the spacer are not recommended.

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Glass Types

Float Glass:

Float Glass is annealed glass processed by combining all ingredients and heating them till molten. This molten glass is then poured over a layer of molten tin. The molten glass then ‘floats’ across the top of the molten tin. As molten tin creates a perfect surface, the finish of float glass is extremely flat.

Float Glass can then be processed as cut-to-size, and then further processed into either toughened or laminated (or both) safety glass. Float glass comes in standard thicknesses of 3, 4, 5, 6, 8, 10, 12, 15, 19 & 25mm.

Standard Clear float glass has an inherent green hue when viewed on its edge. This green colour is the result of the iron content in the ingredients when the glass is manufactured. This colour becomes visible when viewing through the face of the glass as the glass thickness increases. To overcome this colouring a Low Iron form of glass has been developed. It has a pale blue colour when viewed on its edge. Where a tinted glass is used the greenness of standard glass is less of an issue.

There are many colours of tinted glass available on the market produced by the various glass manufacturers. The colours are not standardized across manufacturers and can be slightly different from one batch to the next from the same manufacturer. Getting an exact match in colour when replacing one piece after a number of years can never be guaranteed and requires some careful colour matching.

Float glass or standard glass is very vulnerable to breaking when tension is applied or it is impacted. It will break into large, sharp splinters that can be very hazardous if not handled correctly. Cracked or broken float glass can fall out of its frame or collapse without warning and should be treated very carefully when removing or handling. Due to the danger of broken float glass and its vulnerability to breakage it is only used in fixed glazing or fully framed glazing where there are no special strength requirements such as fixed domestic glazing.

To improve the performance of standard glass in safety, strength and insulation it can be processed further by Laminating, Heat Strengthening, Toughening, incorporating it into a Double Glazed Unit or coating it with special coatings. A number of these processes can be combined to produce glass that can meet very specific requirements.

Heat Strengthened Glass:

Heat Strengthened Glass is commonly referred to as HS glass. This is float glass that has been thermally treated to gain strength. Float annealed glass is heated inside a roller-hearth furnace up to 670°C, and then cooled back to room temperature again within a few minutes.

This process creates stress layers throughout the glass, however not as much as toughened safety glass. HS glass has approximately twice the strength than that of standard Float Glass and normally would have a residual surface compression of no less than 50MPa.

HS glass is NOT a safety glass unless it is laminated to another panel of glass. When HS glass breaks, it cracks from edge to edge in larger pieces than that of toughened glass. Due to the HS glass process being less harsh in phases of heating and cooling to that of toughened glass, it is far less susceptible to roller wave and therefore is visually far superior to that of toughened safety glass.

HS glass can be laminated to the other types of glass to produce a Grade A safety glass or a glass with specific requirements. HS Glass is commonly used for Super Yacht marine windscreens where strength and optical perfection are required.

Toughened Safety Glass (Tempered Glass):

Toughened Safety Glass is commonly referred to as TSG (it’s also known as tempered glass). This is float glass that has been thermally treated to gain strength. Glass is heated inside a roller-hearth furnace up to 700°C, and then rapidly cooled back to room temperature, at a controlled rate.

This process creates stress layers throughout the glass, hence giving TSG up to 5-8 times more strength than standard Float Glass. TSG will typically have a polished edge to reduce failure during the toughening process. Toughened glass cannot be cut. All cutting and processing such as edge work, polishing, beveling and holes must be completed to the annealed glass before the toughening process makes it tempered glass.

When TSG breaks, it shatters into small particles due to the tension in the glass. These pieces are cube shaped rather than splinters.

All TSG has some element of roller wave. Roller wave is an optical disturbance caused by the heating process required to soften the glass before being toughened. Visually roller wave produces distorted reflections on the surface of the glass. This feature is more prevalent in thinner glass than thicker glass. Expect roller wave to be present in glasses 4mm – 6mm. Glasses from 8mm – 12mm have slight roller wave, whilst glass thicker than 12mm have minimal roller wave. Please note that if 2 layers of TSG are to be laminated that the roller wave effect could double.

The toughening (tempering) process can also produce an area of surface haze or pitting. This is caused by the weight of the glass on the ceramic rollers in the furnace and it occurs mostly with heavier glass.

Heat Soaked Glass:

During float glass manufacture, impurities in the glass batch can result in inclusions in the finished product. These inclusions are so small that they are normally invisible to the naked eye and yet they can cause spontaneous breakage in Toughened Safety Glass.

The most notorious of these inclusions is nickel sulphide (NiS) crystals (stones), which can be contained in the raw material during the production of the glass.

The glass toughening process requires the glass to be heated to just below its softening point and then rapidly cooled. Because this heating and rapid cooling process induces substantial tension in the glass, nickel sulphide stone inclusions in the tension core can cause spontaneous breakage, as they are known to change phase (expand) sometime after the toughening process.

Heat soaking involves heating the Toughened Safety Glass to 290°C for a given period of time, then slowly cooling it. This process accelerates the expansion of nickel sulfide stones, and at this temperature, glass panels with nickel sulfide stones are likely to shatter.

The purpose of heat soaking is to reduce the incidence of Toughened Safety Glass breaking spontaneously after installation. While the Heat Soak process does not guarantee there will be no spontaneous breakage after glazing, it is a safeguard for specifying glass in areas where safety from glass fallout is a concern and/or access for replacement is difficult.

Why Heat Soak?

In each case the specifier must assess the risk and consequences of failure before deciding whether to specify Heat Soaked Toughened Safety Glass.

Heat soaking will reduce the incidence of failure due to nickel sulphide inclusions, therefore reducing the associated replacement, maintenance and disruption costs and the risk of the building being classified as unsafe; and due to the additional processing – attracts a premium.

When compared to the alternatives or the actual cost of replacing broken Toughened Safety Glass in the field, there is substantial justification for the cost of the additional process. The following applications should be considered for heat soaking:

  • Structural Balustrades (Railings)
  • Infill Balustrades – if fallout is an issue
  • Sloped Overhead Glazing
  • Spandrels – if not Heat Strengthened
  • Structural Glazing with Spider or other fittings
  • Commercial Exterior Frameless Glass Doors

Chemically Toughened Glass:

This is a form of toughening that produces a toughened layer on the surface of annealed glass. It is produced by immersing the glass into a bath of molten potassium nitrate. There is an exchange of ions between the glass surface and the potassium nitrate resulting in an increase of tension in the glass surface thus producing the toughening effect. The advantage of chemically toughening glass is that complex shapes of glass can be crafted before the toughening process and then toughened in a separate operation.

There are some disadvantages with chemically toughened glass:

  • The toughening is only on the surface of the glass
  • The tensions in the glass surface relax back to an annealed state over time
  • On large glass surfaces patches of discoloration are produced and become visible in certain lighting conditions
  • Chemically toughened glass is not readily available in New Zealand and is produced by few companies overseas
  • Chemical toughened glass is NOT a Grade A safety glass unless laminated

Insulating Glass Units:

Insulating Glass Units (IGUs) are designed to provide thermal insulation for building envelopes. They are used to reduce building heat loss and heat gain depending on the climate and IGU combination. Insulating Glass Units are sometimes called Double Glazing, Double Glazed Units (DGU) or Sealed Insulating Glass Units (SIGU).

The glass in an IGU is assembled with a separating spacer around the edges to keep the panels of glass separated by the required amount. The edges of the glass and separating spacer are then sealed with a secondary sealing compound. Commonly the air in the space between the panes of glass is replaced with a dry inert gas such as argon. This prevents condensation developing within the unit and also increases the thermal performance of the unit by up to 10%.

There are a number of optical effects associated with IGUs: Brewster’s Fringes, Newton’s Rings & Multiple Reflections – additional details under Colour Variations & Optical Abberations below.

Laminated Glass:

Laminated glass is a process that involves a thin layer of plastic sheet adhering two or more glass panels together. If the laminated glass panel is broken, the plastic interlayer is designed to hold the broken glass together, thus making it a safety glass. Annealed , heat soaked and toughened safety glass, either flat or curved can be laminated together.

Glass can be either laminated by a cast-in-place resin system, or a sheet film system (PVB & EVA).

A resin system requires clear or white-edge tape around the perimeter, of which you can see a faint line approx. 6mm in from the edge. A film system eliminates the requirement for edge tape, however is more vulnerable to moisture ingress up to 9mm into the edge of the panel that can cause the film to become opaque.

Laminated Glass Edge Information:

All types of laminated glass have an imperfect visual appearance at the edge. This is because there are multiple layers of glass and plastic tapes or resin terminating at an edge. All these elements have different properties of hardness and clarity.

  • Where a high quality edge is required the glass should have a Flat Polish finish with arrised edges and the laminating tape or film would be recessed to the edge of the arris.
  • This results in the polished edges of the glass being the most visible element and provides the highest standard visually.
  • This makeup can be used for both annealed glass and TSG.
  • This type of edge is typically used with frameless balustrades and decorative panels in the residential and corporate sector.


Glass Edge Information:

  • The most basic edge is where the glass is scribed with a manual cutter and snapped along the scribe. The glass edge has a shiny finish but is not flat like a polished edge. The sharp edges are arrised to reduce the danger of being cut on the sharp edge and reduce the risk of the glass edge chipping. This method is most suitable for straight cuts and regular shapes on thinner glass up to 10mm. Over 10mm the cut tends to flair with the cut no longer being 90 degrees to the surface of the glass. This edge type is used when the edge of the glass is not visible and is being glazed into a frame.
  • A water cut edge is the result of the glass being cut with a high pressure water cutting machine. These machines are computer controlled and are ideal for cutting very thick glass or complex shapes and laminated glass. The glass edge has a sanded appearance due to the garnet sand used in the cutting process. Typically the sharp edges would be arrised afterwards to reduce the danger of being cut on the sharp edge and reduce the risk of the glass edge chipping. This edge type is used when the glass is of a more complex shape and the glass edge will not be visible.
  • A polished edge is the result of the glass being processed by a Polishing Machine. There are two main types of polishing finish. A Straight Line Edger machine produces a Flat Polish where the edge of the glass is polished flat with a sharp flat angled bevel. Only straight edges can be polished with this machine. A CNC Polishing Machine is a computer controlled cutting and polishing machine that uses a CAD file to produce the shape required. This machine produces a more rounded edge with a radii edge. These edge types are used where the glass edge is exposed or visible and a high quality finish is required. TSG will typically have a rough arrised edge for flat glass or a polished edge for curved glass to reduce failure during the toughening process.

Notes On Curved Glass:

Please note that curved or bent glass may:

  • have fine distortions or markings (such as stretch marks) which appear on or in the glass
  • show the presence of “roller wave” (a surface distortion produced by a reduction in surface flatness) or “roller pickup” (which can be shown as small imprints in the surface)
  • be some degree of visual distortion that will vary depending the glass type, thickness, and shape
  • have strain patterns and iridescence apparent under certain lighting conditions or viewing angles

The characteristics described above (and any similar characteristics) are not defects, but are a result of manufacturing processes (including bending, heat-strengthening, toughening, etc.) and are acceptable characteristics of such glass. The presence of distortions, marks or other manufacturing effects shall not, directly or indirectly, give rise to a warranty claim or any other claim whatsoever against Glasshape®.

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Color Variations in Glass

Clear Glass:

All glass is essentially made from the “Float Glass” process but the mixture of raw materials can vary slightly. The most noticeable is the iron content and this can make some clear glass more green the others. The iron content can be reduced (low iron glass) to produce a premium, extra clear glass.

Tinted Glass:

The inclusion of metal oxides creates tinted glass in a range of colors (bronze, grey, blue, green), but some manufacturers have slightly different formulations and tint colors. In general the bronze and grey colors are similar but the others can vary and may be challenging to match. In addition the tint varies with the thickness and this adds further complications to color matching. For uniform tinted glass, design for one thickness and source from one supplier.

Laminated Glass:

Clear laminated glass is very similar to clear monolithic float glass of the same thickness, and in most  cases no colour variation is noticeable. However, it can happen in some lighting conditions.  Tinted PVB laminated glass does not match tinted float glass as it’s the PVB interlayer that is tinted and not normally the glass. In addition there are several suppliers of tinted interlayer and their colours and appearance in certain lighting conditions can vary. The laminate can be made with tinted glass to match if required, and some are offered in this format as standard.

Toughened (Tempered) Glass:

The toughening (tempering) process does not alter the clear or tint color but can introduce to tempered glass thermal stress patterns that are visible through polarized light, often known as leopard spots.

Reflective & Coated Glass:

Coated glass colors do change when viewed at different times of the day, depending on the weather, surrounding reflections, building orientation and the angle at which the glass is viewed. In addition the appearance of reflective glass is distinctly different if the coating is glazed outside on surface 1 or inside on surface 2. The appearance of clear Self Cleaning and Low E coated glass is also slightly different to clear float but this is not normally significant. Some Low E glass can exhibit a blue haze, especially noticeable if part of the glass is shaded.

Insulating Glass Units (IGU):

An IGU will not normally look different from monolithic glass if using the same outer glass. However, the introduction of a Low E coating can slightly increase reflectivity and the internal pressure changes in the unit can create flexing in the glass that changes reflected images. In addition some rare visual effects are possible, such as;

  • Brewster’s Fringes, which is a light refraction phenomenon seen as a rainbow effect
  • Newton’s Rings, which is a circular rainbow effect evident when the panes are touching in the center

For more information refer data sheet on IGU Design Limitations.


As a standard IGU has four reflective surfaces, a higher level of reflectivity occurs and multiple images in reflection may be created. This will be more apparent when viewed on an angle to the glass and is an inherent property of the unit. Due to the sealed airspace of an IGU differences in temperature and atmospheric pressure from the time of manufacture will cause the IGU to act as a lens. This can cause significant changes in the images reflected from the windows due to glass deflection. The appearance is of a convex distortion when the glass is bowing outwards and a concave distortion when bowing inwards. The effect will be more noticeable when reflective coatings are incorporated within the IGU, and in larger units.


Haze is the scattering of light rays when visible light passes through a transparent material. The amount of haze in ordinary glass is very low and is not detected by the human eye. High performing IGUs often incorporate Low E coated glass. With any coated glass it is possible to see the presence of the coating under certain lighting conditions. When bright sunlight shines directly onto partly shaded, coated glass and there is deep shade on both sides of the glass, haze may be visible and usually has the appearance of a blue-grey film or dust on the glass. The shaded area will be free of the effect, giving a clean appearance in the shadow. The effect will be more noticeable on some types of coated glass than on others. Vacuum or sputter coated Low E glass products generally have a very low amount of haze, Pyrolytic coatings tend to have higher levels of haze that can be more readily seen by the human eye. Haze is not a manufacturing flaw, rather an industry known and recognised inherent feature of IGUs.

Optical Aberrations

Newton’s Rings:

Newton’s rings are named after Sir Isaac Newton who first studied the phenomena. These are interference patterns caused by the reflection of light between two surfaces, a spherical surface and an adjacent nearly flat surface. They appear as concentric rings of rainbow colors and occur only near the center of a unit.

In a large IGU the two glasses may be displaced to touch or nearly touch in the middle by an increase in atmospheric air pressure due to; insufficient pressure equalization during manufacture, heat treated glass bow, incorrect airspace for the unit size, or inadequate glass thickness. This effect is normally a manufacture or specification fault and replacement is required.

Brewster’s Fringes:

Brewster’s Fringes are reflected light phenomena which occur if wavelengths of light meet up at 180 degrees out of phase. This can occur when high quality float glass is used with surfaces which are optically flat and both panes of glass are parallel in the IGU. Light reflected within one glass can combine with that similarly reflected within the other, with such small path differences as to cause interference.

The effect is of faint colored bands or irregular shapes, which can be located anywhere over the surface. It is rarely noticeable in normal lighting conditions.

Brewster’s Fringes are not a manufacturing fault but can be generally avoided by using IGUs of unequal glass thickness.

Roller Wave Distortion:

IGUs containing heat treated glass may exhibit a similar level of distortion as heat treated monolithic glazing. This is normally visible on an oblique angle perpendicular to the direction of the toughening furnace rollers, and more obvious in reflection than in transmission.

Where IGUs contain two or more panes of toughened glass this effect may be increased. For this reason, units containing multiple panes of toughened glass particularly where reflective or coated glass is involved should be evaluated for possible visual issues associated with roller wave distortion.

Where thermal stress prevents the use of annealed glass in both panes, the use of heat strengthened glass instead of toughened glass (tempered glass) may be considered as often the roller wave is reduced. Roller wave is not a manufacturing fault but an industry recognized and accepted inherent feature of heat treated glass.

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