Density: 88 χ 10-7m/°C
Linear expansion coefficient: 88 χ 10-7m/°C
Thermal permeability: (u-value) 5,2 – 5,8 watt/m2*°C (depending on thickness)
Thermal conductivity: (Κ-value) 1,05 watt/m*°C
Resistance to compression: 1000 MPa. It is extremely high and means that to crush a 1cm3 glass volume cube a weight of 10 tons must be applied.
Tensile strength: 40 MPa (this number rises to thermally amplified glass at 200 MPa) clearly reduced in relation to resistance to compression.
Melting point: At about 730°C.
Elasticity: (coefficient Young) 70 GPa . This factor expresses the theoretical power (per unit area) that should be applied in a glass sample, in order to double its original length.
Transverse contraction coefficient: (Poisson ratio μ) 0,22
Hardness: 6.5 Moh scale (diamond has 10,sapphire 9 and plaster 2)
Dielectric constant: for 6mm glass at 21°C
- 1 GHz 6,0
- 10 MHz 6,5
- 1 KHZ 7,4
- 10 Hz 30,0
Bright reflection: (Loss of light due to reflection) about 8% to 10%. The light, when passing through a dividing surface of 2 optical media with different refractive indices, (eg air-glass) is always reflected. The percentage of light reflected, depends on the refractive indices of the materials and the angle of incidence of light.
Bright permeability: About 85% for 6mm glass.
Translucent ultraviolet radiation: in wavelength 340nm 41% , to 315nm 1%.
Chemical strength: Glass is extremely resistant to most acids in addition to hydrofluoric, at high temperatures and in phosphate. However, it is sensitive to alkalis. Also soluble sulfide impurities from metal corrosion tend to adhere permanently to the free surface of the glass which must be removed as soon as possible.
Strength and safety
Strength and safety
Glass in a building will be subject to mechanical loads in the form of wind load and impact. It may also be subject to stresses due to environmental conditions such as temperature changes. The strength properties of glass can be varied by increasing the thickness, heat treatment and combining the glass with other materials to form composites. The strength of glass must be sufficient to resist the loads it is likely to be exposed to. Safety of glass is related to its strength but also takes into account the risk of injury from the failed glass.
Annealed glass is the basic form of glass produced in float glass plants. It has no special properties of strength or safety and on breaking it forms large shards with sharp edges.
Heat strengthened glass
Annealed glass may be strengthened by controlled heating and cooling. Heat strengthened glass is not a safety glass but is roughly twice as strong as annealed glass. When broken it behaves like annealed glass and breaks into large shards with sharp edges.
Annealed glass is toughened by heating it to 650”C and rapidly cooling the surfaces. This compresses the surfaces and increases the strength of the glass. Toughened glass is roughly five times as strong as annealed glass. An important property of toughened glass is the way in which it breaks. Any cracking of the glass leads to a rapid release of the surface compression and toughened glass always breaks into small pieces of glass. Toughened glass complying with EN 12600 is a safety glass. Toughened safety glass should be CE-marked and installed with the CE mark visible. Toughened glass cannot be cut or drilled after toughening and must therefore be cut to size before toughening.
Heat soaked toughened glass
Toughened glass may fail due to the presence of nickel sulfide crystals in the glass. To reduce the risk of nickel sulfide failure, the glass may be subjected to a process known as heat soaking. To be effective the heat soaking process must be strictly controlled.
Annealed, heat strengthened or toughened glass can be laminated in any combination to make a safety or security glass. Two or more pieces of glass are laminated together to give the required properties. The glass may be laminated as a sandwich with a layer(s) of polyvinyl butyral (PVB) between the sheets of glass. Glass can also be laminated by pouring a resin between two sheets of glass. PVB laminates are best suited to flat glass while poured resins are best suited for curved glass. Laminated glass is not as strong as a single pane of glass of the same type and thickness but after failure the broken pieces of glass will be held together by the interlayer. The performance of a laminated glass depends on the type of interlayer used. Some are designed to resist penetration and others solely as safety glasses.
Tempering is the American term for strengthening and toughening. Tempered glass is roughly equivalent to heat strengthened glass and is not a safety glass. 0nIy fully tempered glass has similar properties to toughened glass. Fully tempered glass used as a safety glass should conform to EN 12600.
A glazing with a single glass with thickness of 6mm, has a U-value of 5,7W/(m2.K).
A glazing with double glasses, consisting of two non-thermal insulation glasses with 5mm thickness and 12mm air space, between them, has a U-value of 2,8W/(m2.K).
A glazing with double glasses, consisting of a non-thermal insulation glass with thickness of 5mm, and a low-E 4mm glass, with 14mm space filled with argon gas, has a U-value of 1,1W/(m2.K).
To better understand the gravity of the above Ug glass degrees, we can compare them with the coefficient of thermal conductivity of a wall without insulation inside, which has about a U-value of 1,5W/(m2.K), while a wall with insulation, has a U-value less than 0.6W / (m2.K).
The performance of glass is highly dependent on its condition. The use of damaged glass or insulated glazing units will impair the performance of the facade.
- Feathering where the edge of the glass is not exactly square to the face and may not be plane
- Venting where the edge of the glass is clearly chipped to leave sharp edges around a depression
Visual inspection with a gauge card held against the surface will identify the glass thickness using the reflection from the back face of the glass. A reflected flame will appear differently on coated surfaces.
Suitable methods of identification:
Clearfloat Visual or meter
Coated Visual or meter
Toughened Mark, DSR or polarised light
Laminated Mark, ultrasonic
Glazing unit Mark on spacer
0ff-line coated Visual, meter, reflections
- Neoprene with Shore Hardness 80 to 90
- Plasticized PVC with softness of 35 to 45
- Extruded non-plasticized PVC
- The glass and frame can move without stressing the glass
- Water entering the frame can drain freely
- 3mmfor glass sizes up to 2m
- 5mm for glass sizes over 2m
- 6mm for all drained systems
- 3mm for plastic sizes up to 1m
- 5mm for plastic sizes between 1 and 2m
- 7mm for plastic sizes between 2 and 3m
- In a dry covered area
- Out of direct sunlight
- Stood on edge
- Protected from impact
- Protected from dirt