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Colouring


The normal float glass has a slightly greenish tint. This colouring can mainly be seen along the edge of the glass, and is caused by the naturally existing ferric oxide in the raw materials. By selecting extremely ferric oxide-poor raw materials, or by undergoing a chemical bleaching process, the melt can be turned into an absolutely colour-neutral, extra white glass. Interiors and specialty solar products are the widest areas of application.
Colouring

German English
klares Floatglas Clear float glass

In addition to these three versions of float glass, tinted glass can be produced using coloured mass. Chemical additives in the mixture allow green-, grey-, blue-, reddish- and bronze-coloured glass to be produced during certain production floating line periods. Changing glass colour in the vat naturally means a considerable amount of work and increased cost due to scrap and loss in productivity. Thus, it is only produced for special campaigns.

1.2.2 Properties

Most of today’s glass production is float glass, with thicknesses usually ranging from 2 – 25 mm and a standard size of 3.21 x 6 m that is used for further processing. The glass has the following physical properties:

1.2.2.1 Density

The thickness of the material is determined by the proportion of mass to volume and is stated using the notation “p”. Float glass has a factor of p = 2,500 kg/m³. That means that the mass for a square metre of float glass with a thickness of 1 mm is 2.5 kg.

1.2.2.2 Elasticity module

The elastic module is a material characteristic that describes the correlation between the tension and expansion when deforming a solid compound that possesses linearly elastic properties and the formula symbol “E”. The more a material resists deformation, the higher the value of the E-module. Float glass has a value of E = 7 x 1010 Pa and is defined in EN 572-1.

1.2.2.3 Emissivity

Emissivity (E) measures the ability of a surface to reflect absorbed heat as radiation. A precisely defined “black compound” is used as the basis for this ratio. The normal emissivity found for float glass is E = 0.89, which means 89 % of the absorbed heat is re-radiated (see also > chapter 3.3).

1.2.2.4 Compressive strength

As the term implies, this indicator demonstrates the resistance of a material to compressive stress. Glass is extremely resilient to pressure, as demonstrated by its 700 – 900 MPa. Flat glass withstands a 10 times higher compressive power in comparison with the maximum compressive load.

1.2.2.5 Tensile bending strength

The tensile bending strength of glass is not a specific material parameter, but rather an indicated value which, like all brittle materials, is affected by the composition of the surface being subjected to tensile stress. Surface infractions reduce this indicated value, which is why the value of the flexural strength can only be defined using a statistically reliable value for the probability of fracture. This definition states that the fracture probability of a bending stress of 45 MPa for float glass (EN 572-1) as per the German building regulations list may be maximum 5 % on average, based on a likelihood of 95 % as determined by statistical calculation methods.
o = 45 MPa as per rating with the double ring method of EN 1288-2.

1.2.2.6 Resistance to alternating temperature

Resistance of float glass to temperature differences along glass panes is 40 K (Kelvin). This means that a temperature difference of up to 40 K over the glass pane has no effect. Higher differences can cause dangerous stress in the glass cross section, which may result in glass breakage. Heating devices should therefore be kept at least 30 cm away from glazing. If this distance cannot be maintained, installing one pane safety glass is recommended (see also > chapter 7.1). The same applies if the glazing is massive, permanent and partially shaded, due, for example, to static building elements or to nearby plantings.

1.2.2.7 Transformation area

The mechanical properties for float glass vary within a defined temperature range. This range is between 520 – 550 °C and must not be compared with the pretempering and form shaping temperature, which is about 100 °C warmer.

1.2.2.8 Softening temperature

The softening point for float glass is approx. 600 °C.

1.2.2.9 Length expansion coefficient

This value indicates the minimum change in float glass when temperature is increased, which is extremely important for joining to other materials:
90 x 10-6 K-1 according to ISO 7991 at 20 °C – 300 °C
This value gives the expansion of a glass edge of 1 m when temperature increases by 1 K.

1.2.2.10 Specific heat capacity

This value determines the heat increase needed to heat 1 kg of float glass by 1 K:
C = 800 J . kg-1. K-1

1.2.2.11 Acid resistance

Chart: Class 1 acc. to DIN 12116

1.2.2.12 Alkali resistance

Chart: Class 1-2 acc. to ISO 695

1.2.2.13 Water resistance

Chart: Hydrolytic class 3-5 acc. to ISO 719

1.2.2.14 Fresh, aggressive alkaline substances

For example, this includes substances washed out of cement that completely hardened, and when they come into contact with the glass, attack the silica acid structure that is part of the glass structure. This causes a change of the surface, contact points get rougher. This effect occurs when the liquid alkaline substances dry and is completed after the cement has fully solidified. For this reason, alkaline leaching substances must not come into contact with glass at all, or all points of contact have to be removed immediately by rinsing them off with clean water.