GLASS EXPERTS

Knowing the Glass

The basic physical properties of glass are:


Density: 2,500 kg / m3

Linear expansion coefficient: 88 x 10-7m / oC

Thermal permeability: (u-value) 5.2 - 5.8 watts / m2 * oC (depending on thickness)

Thermal conductivity: (K-value) 1.05 watts / m * oC

Resistance to temperature differences: at 6mm a glass that 

has been artificially heated will break from thermal self-breaking if immersed in water 

with a temperature difference above 55οC.

 

Compressive strength: 1000 MPa.

It is extremely high and means that in order to crush a glass cube with a volume of 1cm3, 

one must apply a weight of 10 tons.

Tensile strength: 40 MPa (this number rises to 200 MPa for heat-resistant glass) clearly 

reduced in relation to compressive strength.

Melting point: about 730oC.

Measure of elasticity: (Young coefficient) 70 GPa. This coefficient expresses the theoretical 

force (per unit area) that should be applied to a glass specimen to double its original length.

Transverse shrinkage coefficient: (Poisson ratio μ) 0,22

Hardness: 6.5 on the Moh scale (diamond has 10, sapphire 9 and gypsum 2).

Dielectric constant: for 6mm glass at 21oC

1 GHz 6.0
10 MHz 6.5
1 KHZ 7.4
10 Hz 30.0

Refractive index: 1.52 (this coefficient varies depending on the wavelength of the light in question)

Luminous reflection: (Loss of light due to reflection) about 8% to 10%.

When light passes through a separating surface of 2 optical media with different refractive indices (eg air-glass), 

it is always reflected. The percentage of light reflected depends on the refractive index of the media and the angle of incidence of the light.

Luminous permeability: about 85% for 6mm white glass.

Ultraviolet permeability: at a wavelength of 340nm 41%, at 315nm 1%.

Chemical resistance: Glass is extremely resistant to most acids except hydrofluoric and high temperatures and phosphate.

However, it shows sensitivity to alkalis. Also soluble sulfur impurities from corrosion of metals tend to adhere permanently to the free surface 

of the glass which must be removed as soon as possible.

 

Self-breaking Glass Shock:

Causes

Thermal shock is a phenomenon that occurs when the central area of ​​a glass is heated (either artificially or naturally) while its perimeter area 

is kept cool and not subject to expansion. In this way a tendency is gradually created on the surface of the glass.

Self-breaking is the result of the accumulation of excessive voltage due to heat in a glass pane. The amount of thermal voltage depends on the 

temperature difference between the coldest and warmest area of ​​the glass, as well as on the distribution of the temperature gradient on its surface.

A glass that has suffered a heat shock is easy to recognize by the shape of the fracture which is very characteristic. This type of fracture always 

starts vertically from one of the sides of the glass and extends in a straight line for 2 to 5cm. It then branches in one or more directions. 

The number of these branches indicates the magnitude of the thermal stress developed on the glass.

FACTORS THAT INCREASE THE POSSIBILITY OF HEAT SHOCK

Any reason that contributes to the development of temperature differences on the surface of a glass should automatically be characterized as 

an "aggravating" factor. Such are:

Climatic conditions that favor the absorption of heat from the glass with long hours of constant exposure to the sun.

The orientation of the building, with the worst being the east where usually in the first months of spring the phenomenon is observed, the cold glass 

from the drop of the temperature at night, to be heated unevenly by the unobstructed rising sun.
The position of the building, when shadows from surrounding objects fall on it, such as e.g. other buildings, trees, parked large vehicles etc.

Certain types of colored and / or coated glass have an inherently higher risk of heat shock due to the increased energy absorption they exhibit. 

For this reason, it is advisable to control the energy absorption coefficient during the selection of the most suitable for the respective application 

of glass.
Dimension and thickness (the bigger, the worse)
Frame, type (aluminum, PVC, wood), Color, framing (4-sided, 2-sided, structural)

Perimeter condition. Small cuts and sores can be fatal. Perimeter grinding, even with non-diamond tools, always improves behavior.
Exterior shading, when they do not fall evenly over the entire surface of the glass. These can be either involuntary (balconies, cantilevers, canopies, etc.) 

or voluntary (blinds, awnings, stickers, etc.)

Interior shading, such as curtains, blinds, etc. which reflect amounts of heat to the outside and increase the thermal tendency. To reduce this phenomenon, 

it is recommended to provide ventilation of the space between the glass and the source of internal shading.
Internal cooling / heating sources, such as air ducts, air conditioners, ventilation of electronic devices, etc., when they are directed directly at the windows 

can cause serious problems.

Treatment

When the general conditions indicate an aggravating condition of heat shock, it is good to take preventive treatment / protection measures. Such are:

Perimeter sanding. Improves conditions by about 10% to 15%

Perimeter sanding with diamond tools. 20% to 25% improvement

Heat reinforcement. 70% -80% improvement

Thermosetting. 90% improvement

 

Heat permeability coefficient U-Value (or coefficient K)

Heat transfer through a surface in all three ways (contact, mixing and radiation) is attributed to the U-value coefficient (also known as the K coefficient). 

This is the degree of heat loss (in Watt), through a surface of 1m2, for a heat difference of 1 degree Kelvin, between indoor and outdoor.

The total thermal insulation of a window depends on the thermal insulation of the frame, the thermal insulation of the glazing and the thermal breakers 

and is given by the coefficient Uw, while

the thermal insulation of the glazing alone is given by the coefficient Ug.

 

Evolution of the thermal permeability coefficient of glazing


A glazing unit with single glazing 6mm thick, has U = 5,7W / (m2.K)

In the early 1950s, the first double glazing appeared, which was assembled and installed on site. Later in the 1960s, factories produced hermetically 

sealed 

double glazing. The idea of ​​double glazing was to create a gap filled with dry air between two sheets of glass, since the glass has a thermal 

conductivity 

of 1W / (mK) while the air is only 0.025W / (mK), thus improving the insulating properties and to reduce the coefficient of thermal permeability (Ug) 

of a glazing

A double glazed window, consisting of two common 5mm glazing with 12mm air gap, between them, has U = 2.8W / (m2.K)

Later there was a new improvement with the replacement of air with noble gases (Argon, krypton) which have lower thermal conductivity than air to 

reduce contact 

heat transfer, but also higher density to reduce heat transfer by mixing (make it more difficult to movement).

However, a crucial step in the quality of thermal insulation of glass windows was the development of new coating techniques. By applying a metal 

coating to the 

surface of a glass, we turn it into High-performance or as it is otherwise called Low-emissivity Low-emissivity (Low-E).

A double glazed window, consisting of a common 5mm glazing and a high performance 5mm glazing, with 14mm gap, filled with gas, has U = 1.1W / (m2.K)

To better understand the gravity of the above Ug values ​​of glazing, we can compare them with the coefficient of thermal conductivity of a wall without 

insulation 

inside, which has about Ug = 1,5W / (m2.K), while a wall with insulation , has Ug less than 0.6W / (m2.K).

 

PERCEPTION OF WHITE LIGHT AND COLORS

DEPTH OF NATURAL LIGHTING

As tenants of different spaces, we need natural light to function in the context of our daily lives, while at the same time we want to save energy 

by using as 

little artificial lighting as possible.

The more natural light we provide in a space, however, the more solar energy enters our space, unwantedly raising its temperature and creating energy 

expenditure 

for its cooling.

So the question that arises is to determine the necessary, for satisfactory natural light, light permeability of our glass, or otherwise to know for 

a specific 

permeability glass what "depth" of lighting we can expect.

In order to quantify this need, a relationship was recorded between the Visible Light Transmittance Lt and the expected illumination depth.

As a reference point, it was determined that the common white glass 6mm with Lt = 87%, provides satisfactory natural light at a depth of about 6 meters 

in the 

space in which it is placed.

If we now consider that we are glazing the same space with a tinted glass of the same thickness with Lt = 43%, then in the relative curve, we see that 

the expected 

depth of satisfactory natural light will be limited to 4.5 meters. As the relationship is not linear, it should be emphasized that when light transmittance 

drops to 

"half" it does not mean that natural light "reaches" to half depth.

 

 

PERCEPTION OF COLORS

The glass can "color" the natural light, either through the colored mass (tinted bronze, etc.), or through the colored membranes (in the case of triplex), 

or through various coatings (in the case of reflective glass of various types).

Studies have shown that the psychological vision of the brain and the eyes affect the way we perceive colors.

When the outside view is made exclusively through colored glass (without the possibility of reference to natural light), the human eye adjusts 

to neutralize 

the effect of colored glass and renders the color differences in the same way that would do it in natural light too!

So the paradox is observed, the white snow remains the same white even if one looks at it through a gray or blue or green glass !!

The curious thing is that if an adjacent window opens, at the same time we would see the same snow purple / blue, as our eye will try to balance 

the white of 

the snow with the color of the glass.