The "Crown" process up to 1850
Surfaces not flat and parallel.

The "Cylinder" process up to 1910.
Surfaces not flat and parallel

The "Drawn Sheet" process 1910 – 1970.
Surfaces not flat and parallel.

The "Polished Plate" process 1850 – 1965.
Both surfaces flat and parallel. No distortion. Intensive machining and labour inputs.

The "Float" Process remains virtually unchanged in principle since  1962. Surfaces flat and parallel. Natural Physical process. Minimal Labour. No machining.

WHAT IS GLASS?
Impact behaviour of annealed glass
– Impact behaviour of annealed glass

How can this situation be resolved to allow safe glazing design?
– Impact behaviour of annealed glass

Primary float glass, as manufactured, is a glass which is totally free from stress which is known as "ANNEALED" condition.

This allows it to be easily cut, drilled and edge-worked. However, annealed glass cannot be used as a structural material and has extremely limited resistance to high wind-load, dead-load or to severe solar exposure. Furthermore, when broken, annealed glass is a lethal material which can cause severe or fatal injury. Question: how can these limitations be overcome? Answer:  by heat-treatment in a modern horizontal roller furnace to induce additional properties into the annealed glass which will make it suitable for use in contemporary design including structural silicone curtain wall  systems and all forms of bolted frameless glazing. How Is This Done?

IMPACT BEHAVIOUR OF ANNEALED GLASS
When loaded, in any circumstance, annealed glass will deflect causing the face # 1 to develop a level of compressive stress while face # 2 is now in tension. As the load increases, the tensile stress in face # 2 also increases. Since glass is very strong in compression, but weak in tension, the face # 2 surface will soon reach it‘s tensile stress limit and the glass will break. All stress forces will be relieved and the result is a potentially dangerous fragmentation. Annealed glass cannot therefore be used for frameless glazing and is restricted for use only in areas which have no legal requirement for safety glass.

HOW CAN THIS SITUATION BE RESOLVED TO ALLOW SAFE GLAZING DESIGN?
HEAT-TREATED GLASS (HS) AND (FT).
Heat-Treated Glass products, whether heat strengthened (HS) or fully tempered (FT) are produced in a very similar fashion using the same kind of horizontal roller furnace employed by CLADTECH INTERNATIONAL.

Briefly, the glass is heated to approximately 700?C and is then force-cooled to create surface and edge compression in the glass. It is by controlling the rate of cooling that glass becomes heat strengthened or fully tempered.

To produce FT glass, the cooling is done very rapidly to induce high surface compression in the glass. To produce HS glass, the cooling process is slower and the resultant compression in the surfaces is much less then FT glass.

Because of the compressive stresses in the surfaces, HS glass is approximately x2 stronger than annealed glass, and FT glass is x 4-5 stronger than annealed glass of the same thickness.

Except for this increase in mechanical strength, all other properties of the glass remain unchanged.

The most dramatic and important difference between HS and FT glass is in the post-breakage characteristics of the two products, as defined by the break-pattern.

If HS glass should break, the pieces will be relatively large and tend to remain in the glazing system until removed for replacement. On the other hand, FT glass will shatter into innumerable small, roughly cubical fragments which do not have sharp edges and are therefore "NON-INJURIOUS".

HS glass is not a safety glazing material, when safety glass is required to meet safety codes, A certified glazing material such as fully tempered or laminated glass must be used.

CLADTECH INTERNATIONAL FULLY TEMPERED (FT) GLASS
FT glass is produced in a horizontal roller furnace in which the glass is heated to around 700 ?C at which temperature it is red-hot and in a plastic condition.

It is then rapidly cooled (quenched) by a force of cold air which causes all the outer surfaces (including the edges) to contract, thus creating a total "envelope" of compressive stress in face 1 and 2.

However this rapid quenching of the surfaces is not immediately conducted to the centre of the glass which remains in a temporary state of expansion but then cools, after a short delay, to a greater degree of contraction than the surface. As a result, the centre zone of the glass is now placed in tension entirely within the compression envelope thus creating a perfect balance of forces.

Clearly, if the FT glass is now subjected to a load, the compressive stress in face # 2 will allow the glass to absorb a much greater force without breaking and, on removal of the force, the glass will return to its original flat condition.

IMPACT BEHAVIOUR OF FT GLASS
Breaking of FT glass will occur when the deflection exceeds the capacity of the compressive envelope to resist the tensile force, or if the glass is impacted by a hard material which penetrates through the outer compressive zone to reach the tensile zone. The sudden release of energy stored in the tensile zone of the FT glass will cause total disintegration of the pane into small, fragments which are non-injurious. This important feature of FT glass means that it is considered by all major International standards to be a "TRUE SAFETY GLASS" for use in all glazing situations where impact resistance and thermal safety are required.


PROPERTIES
–    Is 4 – 5 times stronger than annealed glass of the same thickness
–    Has greater resistance to thermally-induced stress than heat strengthened or annealed glass
–    Typically breaks into small particles which can be handled safely.
–    Suitable for use as a safety glass as defined by
American Standard     ANSI Z-97.1 1984
British Standard    BS 6262 Part A 2005
European Standard    EN 12600 2002
–    Manufacturing conforms to American Standard ASTM C 1048 – 4

APPLICATIONS
–    All types of clear, tinted, pyrolitic-coated and post temperable  sputter- coated glass are available in FT condition
–    FT glass can be used in any window or curtainwall framing system
–    FT glass is a structural glass which can be used for frameless glass facades, frameless glass doors, structural glass balustrades and many types of furniture
–    FT glass can be laminated with a suitable number of PVB interlayers
–    FT glass can be produced with silk-screen ceramic frit designs
–    FT glass cannot be cut or drilled after tempering and any post-tempering operations such as edge-grinding, cutting, sand-blasting may cause sudden, or premature failure.

AVAILABILITY
SAFE GLAZING SIZE
The sizes shown below refer to manufacturing capacity limitations. The actual "Safe glazing size" will depend on design wind-load, dead load, whether single or double glazed, lamination and whether combined with annealed or heat strengthened glass in double glazing.

For confirmation of "safe glazing sizes" please contact the Technical Sales Department at Cladtech International.

Throughout the Middle East Region, Architects and Engineers have turned substantially towards the use of the HS glass for use in facades and windows where full impact-safety is not a requirement. The absence of risk from spontaneous breakage, the better retention in the glazing system (if broken) and the improved surface quality, make HS glass the first product-of-choice for the majority of non-structural glazing situations.

Cladtech International offers HS glass for a wide variety of applications requiring sufficient strength to resist stresses caused by absorption of solar energy and also to resist the forces of deflection under wind-load, dead-load etc.

Due to its lower surface compression stress level, HS glass is unlikely to break spontaneously even if nickel sulphide stones are present in the tensile zone of the glass.

Cladtech International strongly recommends the use of HS glass except for areas which are covered by mandatory safety codes.
Typical Break-pattern

AVAILABILITY
SAFE GLAZING SIZE
The sizes shown below refer to manufacturing capacity limitations. The actual "safe glazing size" will depend on design wind-load, dead-load, whether single or double glazed, lamination and whether combined with annealed or fully tempered glass in double glazing.

For confirmation of "Safe Glazing Sizes", please contact the Technical Sales Department at Cladtech International.