Glass Type Definitions
Annealed Glass (Float Glass)
Annealed glass is glass which, immediately after it has solidified into the required form, while still at high temperature, is slowly cooled in a carefully controlled temperature regime in order to reduce to a minimum the internal stresses in the glass. The resulting glass can be worked. It is, in fact, ‘ordinary’ glass as taken from the production line and stored in stock plates.
These plates will be subsequently cut to size, and the cut sizes may be then treated as required, e.g. by tempering.
Saudi American Glass Safe T Lam® laminated glass consists of two or more panes of float glass bonded together by heat and pressure with one or more sheets of a tough flexible pvb (polyvinylbutyral) interlayer, sandwiched permanently between the glass sheets.
When laminated glass is broken the glass fragments remain attached to the pvb interlayer and are thus prevented from causing serious injury that may occur with non laminated annealed glass. Laminated glass is regarded as a safety glass and safety glazing material.
The performance of Safe T Lam® laminated safety glass can be varied by changing the number, thickness and type of each of the glass components and the number and thickness of the pvb interlayers.
Tempered (Toughened) Glass
Tempered glass is produced by heating annealed glass to approximately 650oC, at which point it begins to soften. The surfaces of this heated glass are then cooled rapidly.
The technique creates a state of high compression in the outer surfaces of the glass and, as a result, although most other characteristics remain unchanged, the bending strength is increased by a factor of four or five times that of annealed glass.
When broken, the tempered glass fractures into small pieces (called dice). As these particles do not have the sharp edges and dagger points of broken annealed glass, it is regarded as a safety glass and safety glazing material.
Tempered glass must be cut to size and have any other processing, such as edge polishing or hole drilling, completed before tempering, because attempts to ‘work’ the glass after tempering will cause it to shatter.
Heat Soak Tested Glass
Tempered glass carries a small risk of “spontaneous fracture” which can occur a long time after the glass is produced and has been installed. The heat soak process involves heating up the already tempered glass to a high temperature, typically (280oC +/- 10oC) and keeping it at this temperature for a period of time long enough to fracture a large proportion of the panes which may otherwise have fractured in service.
Heat Strengthened Glass
Heat strengthened glass is produced by a similar process to that used for tempered glass. However, the strength developed is about half that of tempered glass and it is therefore sometimes referred to as “Semi tempered glass”. Heat strengthened glass is not generally considered to be prone to spontaneous fracture and does not therefore require heat soak testing.
The product is ideal for use where thermal over-stressing of annealed glass is predicted and where the safety characteristics of tempered glass are not required. It does not meet the criteria for safety glazing because its breakage pattern resembles that of annealed glass. When broken, correctly heat strengthened glass exhibits fracture patterns running to the edge of the glass pane thus leaving no substantial islands of unsupported glass to fall from the building.
Wired glass is not float glass, but is manufactured by a continuous casting and rolling system which effectively laminates a fully welded steel mesh between two layers of molten glass. Wired glass can be supplied with a rough cast (Nominal 7mm thick) obscuring finish that is achieved during manufacture or it can have both surfaces polished flat and parallel to give a clear (Nominal 6mm thick)glass appearance.
Wired glass is designed for use in fire rated glazing systems where the glass is securely clamped at the edges. In the event of fire the glass may crack and even soften but is retained in the frame by the wire mesh.
Standard wired glass is not a safety glazing product, there are however wired glass products that incorporate a heavier wire mesh that is designed to achieve a safety glass rating in accordance with the requirements of various international safety glazing standards.