As there is an increasing demand for transparent building elements, there are prominent uses of glass in contemporary architecture. Glass was traditionally used as a single pane in conjunction with a load-bearing frame. However, as we can see today, glass is also used as a primary structural member in the form of glass fins, walls and beams. Glass is a magical building material, according to Laufs Wilfried & Luible Andreas, because it has various applications in doors, windows and building façades depending on its characteristics and properties. It provides people with a variety of options based on factors such as safety, security, environmental functions such as self-cleaning, sunlight and heat transparency, visibility and qualities such as scratch resistance, among others. This article will concentrate on the properties of glass.
What is glass made of?
Glass is a semi-transparent, hard, brittle, lustrous material that is formed by the igneous fusion of silica (typically sand) with an alkaline sodium or potassium salt and other ingredients. It appears to be used for glazing the windows of grander buildings during the Roman Empire. Based on the important characteristics and properties of glass, it is regarded as the best future building material. There are various types of glass, all have different features, thus performing in various applications.
Properties of glass
Glass is a nearly perfect elastic solid at ordinary temperatures, an excellent thermal and electrical insulator and highly resistant to many corrosive media. However, its optical properties vary greatly depending on the light wavelengths used. Many of the properties that distinguish glass from other solids are ultimately due to the more or less random order of atoms. One distinguishing feature is the iso-tropicity of properties, which means that properties like tensile strength, electrical resistance and thermal expansion are of equal magnitude in any direction through the material.
As a glass-forming melt cools through the transition range, its structure relaxes i.e. changes from liquid to solid in a continuous manner. The properties of solid glass reflect the extent to which this structural relaxation has occurred. Glass retain a memory of the temperature-time schedule during the transition. Firstly, we will discuss physical properties of glass.
Glass is weather resistant, able to withstand the effects of rain, sun and wind. It has the ability to absorb, reflect and refract light, allowing us to control and manipulate natural light to influence our daily activities and regulate our mental and physical health.
Because of its low thermal expansion value, it has excellent dimensional stability. In other words, its volume change with temperature change is very small when compared to other materials.
Is glass a conductor or insulator?
Because of its good insulating response against visible light transmission, it is an excellent insulator against heat, electricity and electromagnetic radiation. Certain types of glass have a high resistance to UV, infrared and x-ray transmission. When used with the appropriate thickness, it has an excellent resistance to sound transmission.
Melting point of glass
Only at extremely high temperatures, glass can be molded. It completely melts or liquefies at temperatures ranging from 1400°C to 1600°C, depending on the composition of glass.
It can withstand the effects of chemical reactions in various environments or acidic effects. It is highly resistant to most chemicals, including inorganic alkali and acid solutions such as ammonia and sulfuric acid.
Hardness and brittleness
Glass is a hard material because it has a high impact resistance when subjected to a load. However, it is a brittle material because it breaks immediately when subjected to load.
Transparency of glass
One of the properties of glass that creates a visual connection with the outside world is transparency. With the advancement of technology, clear glass can also be modified to become opaque.
Color and shape
It can be blown, drawn and pressed into any color, shape or variety, and is available in the market depending on its use, dimensional requirements and safety requirements.
Glass compressive strength and tensile strength
Glass has a compressive strength of 1000 N per Sq.mm at 200 C, which is very high. It means that a 10 ton load is needed to break a 1 cm cube of glass. Glass has a much lower tensile strength than it does compressive strength. Tensile strength resistance for annealed glass is 40 N per Sq.mm at 200 C temperatures and 120 to 200 N per Sq.mm at 200 C temperatures for toughened glass.
Fire resistant glazing
Fire protection is provided by modern glazing products for up to 120 minutes. When exposed to temperatures above 120°C, the transparent glazing becomes opaque. This is accomplished with the assistance of special transparent gels.
Glass Young’s modulus
The stiffness of a material is measured by its young’s modulus. So, greater the value of Young’s modulus, the stiffer the glass. At 200 degrees Celsius, the young’s modulus of glass is 70 GPa.
It is also possible to change some of the properties of glass to make it suitable for different purposes.
Poisson ratio of glass
Poisson’s Ratio, also known as lateral contraction co-efficient, is directly related to elongation and contraction of material when a load is applied in one direction. Glass’s cross-section area decreases as it is stretched. Glass has a Poisson’s ratio of 0.22.
Glass Linear Expansion
Linear expansion is defined as stretch per unit length for a temperature change of 10 degrees Celsius. The linear thermal expansion coefficient of glass is 9 x 10-6 m / 0o C.
Density of glass
The atoms in a glassy solid are packed less densely than in a corresponding crystal, resulting in larger interstitial spaces between atoms. These interstitial spaces collectively constitute free volume and they are responsible for a glass’s lower density when compared to a crystal. The density of silica glass is about 2% lower than that of its closest crystalline counterpart, the silica mineral low-cristobalite. The addition of alkali and lime, on the other hand, would cause the glass’s density to steadily increase as network-modifying sodium and calcium ions filled the interstitial spaces.
As a result, commercial soda-lime-silica glasses are denser than low-cristobalite glasses. Density closely resembles additive behavior. At 200o C, the density of building glass is around 2500 kg per cubic meter, giving flat glass a mass of 2.500 kg per square meter per mm of thickness.
Elasticity and plasticity
Because glass is isotropic, only two independent elastic moduli are normally measured: Young’s modulus, which measures a solid’s ability to recover from lengthwise tension or compression and shear modulus, which measures a solid’s ability to recover from transverse stress. Young’s modulus and shear modulus are not strongly affected by the chemical composition of oxide glasses.
Viscosity of glass
Glass viscosity, measured in cm-g-sec units, which is known as poise, decreases as temperature rises. For example, the temperature at which a gob of molten glass can be delivered to a forming machine, is equivalent to the viscosity of 104 poise. The viscosity of 107.65 poise defines the softening point, at which the glass may slump under its own weight, 1013 poise defines the annealing point and 1014.5 poise defines the strain point. By cooling further, viscosity will rise rapidly beyond 1018 poise, where it cannot be measured.
Thermal expansion of glass
Glass typically expands when heated and contracts when cooled. Glass’s thermal expansion is critical to its thermal shock performance (its performance when subjected suddenly to a temperature change). When a hot glass specimen is suddenly cooled, such as by immersing it in iced water, great tension may develop in the outer layers due to shrinkage relative to the inner layers. This stress may cause cracking. The thermal endurance of a glass is defined as its resistance to such thermal shock; it is inversely related to the thermal-expansion coefficient and the thickness of the piece.
Soda-lime silicates and alkali lead silicates, which have high expansion coefficients, are particularly vulnerable to shocking. Using Pyrex-type sodium boro-silicates or vitreous silica improves thermal shock resistance. Mirror substrates for space-based telescopes frequently require materials with expansion coefficients close to zero in order to avoid dimensional changes caused by temperature fluctuations. A silica glass containing 7.5 percent titanium oxide has a thermal expansion coefficient close to zero and performs well in this application.
Because of atomic vibrations (phonon mechanism), the thermal conductivity of oxide glass does not increase significantly with temperature. Radiation conductivity (thermal conductivity due to photon transport), on the other hand, increases dramatically with temperature. For specific photon wavelengths, radiation conductivity is also inversely proportional to the absorption coefficient of a glass. As a result of the relatively high radiation conductivity of molten clear glass, it is possible to melt to depths of nearly two meters in continuous glass tanks without the risk of frozen glass at the bottom. On the other hand, colored glasses have a high photon absorption coefficient and therefore, they need to be melted either to shallow depths from the bottom of the tank.
Electrical conductivity of glass
Although most glasses contain charged metallic ions capable of carrying an electric current, their movement and electrical activity are hampered by the high viscosity of glass. Thus, glass is a good electrical insulator, though this property varies with viscosity, which is affected by temperature. Glass’s electrical conductivity does, in fact, increase rapidly with temperature. As a result, in glassmaking, it is possible to melt soda-lime-silica glass electrically after it has been heated to approximately 1,000 °C via auxiliary means.