Advice on Curved Glass

Dimensional Tolerances

Curving GlassA tolerance of +/- half of the thickness of the glass to a maximum of +/- 6mm is required by Novaglaze in accordance with industry standards.

There are no standards for curved glass as there are with flat glass but Novaglaze manufactures to the guidelines produced by Pilkington and the Glass and Glazing Federation which are accepted as an industry standard. To view the guidelines please click here.

Novaglaze continues a long European tradition of leading the world in glass bending. The process of “sag bending” has been evolving for many years now. 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 curved glass, particularly when viewing a moving object through the glass. All curved 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 curved glass will split direct sunlight into striped shadow.
Some variation in edgework may be discernible on exposed edges where different machine and / or hand forming is required for manufacturing. Such variation will be kept to a minimum.



Laminated glass

The process can either be CIP (cast in place) using a clear tape around the edges and around the holes between the two glasses. The junction between the clear tape and the resin is just visible inboard from the edge of the glass and the holes. There can be minor imperfections such as fine bubbles at resin/tape interface. One bubble will be acceptable per m run or per hole area. One bubble of size of size 2-3mm will be acceptable in the body of each plate but not in the visible area as defined by an oval touching the center of all four sides. Or EVA; film placed between the glass which is heated and softened and then bonds itself to the glass EVA is ideal for glass which may be exposed to moisture. Distortion when laminating toughened or heat strengthened glasses together slight visible distortion in transmission due to the small lens effects will be noted with increase in viewing angle.

The phenomenon is not normally a problem in roof glazing, but may be discernible in vertical glazing. Site inspection should be from a distance of 3m. The air in all sealed units expands and contracts in hot and cold weather causing the glass to bow out and in respectively and again reflections will reflect this movement. The clear tape around the edge will, of course be totally within the edge detail of the sealed unit. Site inspection should be from a distance of 3m.



Toughened glass

Besides the points mentioned above, when tempered horizontally toughened glass can have surface distortion produced by a reduction in surface flatness known as roller wave. This is generally more noticeable in reflection. Glass which is thicker than 6mm may show signs of small imprints/pick-up from the rollers. Anisotropy (iridescence) when tempering the process produces areas of different stress in the cross section of the glass.

These areas of stress produce a bi-refringent effect in the glass, which is visible in polarised light. When viewed in polarized light, the areas of stress show up as coloured zones, sometimes known as leopard spots. This occurs in normal daylight, this depends on the weather and the angle of the sun. This is more noticeable either at a glancing angle or through polarized spectacles.

To view a copy of the Industry guidelines Novaglaze manufacture to, click here.



Energy surcharge

The Energy surcharge (ESC) is a variable rate introduced by the European Glass manufacturers to reflect the increase in the energy costs associated with the manufacture and distribution of glass. It is NOT a delivery charge and applies to all sales whether delivered or collected, after November 2008.

The amount is charged per kilogram weight of glass supplied and is set once a quarter based on the IPE Brent Crude Oil Price Index in London.

We have no control over the setting of the rate, which goes down as well as up.