There are two main types of safety glass used in the home:
Toughened (tempered) glass
Four to five times harder to break than ordinary annealed glass, and if it breaks it does so safely by disintegrating into thousands of very small pieces with dulled edges. The normal thickness of 4mm is what is commonly used in most sliding patio doors and front and back doors in the replacement industry today.
The other type of safety glass is laminated, which looks much like ordinary glass but has an almost indiscernible tint. The slight tint is the result of the sandwich structure of laminated glass where two layers of 3mm glass are used with a tough plastic interlayer called polyvinyl butyrain (pvb). The combination of the extra thickness, now 6.4mm overall, and the plastic interlayer is what gives it the slight tint. Although laminated glass uses ordinary non toughened annealed glass, when hit hard enough the outer layer of glass may crack, but the broken pieces will adhere firmly to the interlayer, and so stop splinters of glass flying off. This glass is particularly suitable where it is important to ensure the resistance of the whole sheet after breakage such as: shop-fronts, balconies, stair-railings, roof glazing.
There are two types of laminated glass: PVB and resin laminated glass:
• PVB laminated glass is two or more sheets of glass which are bonded together with one or more layers (PVB) under heat and pressure to form a single piece.
• Resins laminated glass is manufactured by pouring liquid resin into the cavity between two sheets of glass which are held together until the resin cures.
Identifying safety glass:
All installed safety glass should be clearly marked with the British Standard test reference BS6206, with the letter L for laminated, and T for toughened, together with the company registration number of the supplier. If you want to identify (older) safety glass that may not have been marked at the time there are some tests which will give an indication, but I would stress should not be relied upon as being definite, and these are: Toughened – look at the glass carefully side on, a distortion, not too dissimilar in looks to sheet glass, should be noticeable where the glass has been heated and cooled.
Also with toughed glass the gas marks will be noticeable when looked at through polarised sunglasses NOT ordinary sunglasses. With Laminated, the only real test is an accurate measurement, which should show up a thickness of 6.4mm and not 6mm as with ordinary float glass.
Marking of Installed Safety Glass.
All glass installed in critical locations (see earlier diagram) should be marked to comply with regulations. If no mark is evident, it is behind the bead or it is blurred then it does not comply and must be replaced immediately.
The following information must be marked on the glass
|Safety Glass Type
|| Marking In Accordance With
BS 6262 – 4: 2005
| Thermally toughened soda lime silicate safety glass
|| Glass Co.
EN 12150 1
| Laminated Safety Glass
|| Glass Co.
EN 14449 2
| Safety Wired Glass
|| Glass Co.
EN 572 – 9 3
| Adhesive Backed Polymeric filmed Glass
|| Glass Co.
EN 12600 AF 2
Transparent glass used in the manufacture of insulating glass units is identical to that used traditionally for single glass and will, therefore, have a similar level of quality.
Both panes of the sealed unit shall be viewed at right angles to the glass from the room side standing at a distance of not less than 2 metres but for toughened, laminated or coated glasses (not less than a distance of 3 metres) in natural daylight and not in direct sunlight with no visible moisture on the surface of the glass. The area to be viewed is the normal vision area with the exception of a 50mm wide band around the perimeter of the unit.
Flat transparent glass, including laminated or toughened (tempered) glass, shall be deemed acceptable if the following phenomena are neither obtrusive nor bunched:
• Totally enclosed seeds, bubbles or blisters; hairlines or blobs; fine scratches not more than 25mm long, minute embedded particles.
• Obtrusiveness of blemishes shall be judged by looking through the glass, not at it, under lighting conditions as described above.
When thermally toughened glass is viewed by reflection, the effect of the toughening process may be seen under certain lighting conditions. The visibility of surface colouration or patterns does not indicate deterioration in the physical performance of the toughened glass. Because of the nature of the toughening process, distortion will be accentuated when the glass is viewed in reflection or incorporated in insulating glass units.
Visible double reflection can occur under certain lighting aspect conditions, especially when viewed from an angle. This is an optical phenomenon arising from multiple surface reflections in sealed units.
The manufacture of flat laminated glass does not usually affect the visual quality of the glass incorporated in insulating glass units. However, the faults generally accepted above may be increased in number if several glasses and interlayers are used in the production of laminated glass. When viewed under certain light conditions, insulating glass units incorporating clear or tinted flat laminated glass may show a distortion effect caused by reflection on the multiple surfaces of the components of the laminated glass.
The appearance of the optical phenomenon known as Brewster’s Fringes is not a defect of the glass, and can occur with any glass of high optical and surface quality. This phenomenon is a result of the high quality now being achieved worldwide by modern methods of glass manufacture. Brewster’s Fringes occur if wavelengths of light meet up with each other when they are exactly 180º out of phase – an example of the phenomenon known to physicists as the interference of light. The effect is similar to, although usually much smaller than, the interference fringes which can sometimes be seen on toughened glass windscreens. Brewsters’ Fringes occur when the surfaces of the glass are flat and the two panes of glass are parallel to each other, i.e. when the light transmission properties of the installation are of a very high order.
This phenomenon is not a defect of the product, being dependent on the laws of physics and not on the quality of the insulating glass. In fact it arises because modern glass made by the float process is flat, therefore, free of the distortion inherent in sheet glass. The occurrence of Brewster’s Fringes is in its nature rather like (though very much more rare than) the fact that under certain conditions, the observer will see a reflection of himself in any window or door – and no-one could claim that this was a defect of glass.
Note: Patterned Glass
The above criteria do not apply to patterned glass, as due to the method of manufacture, imperfections such as seeds and bubbles are deemed to be acceptable. Visual quality standard for installed insulating glass units constructed from flat transparent glass.
Technical & Compliance Manager