The glass is a non-equilibrium, non-crystalline condensed state of matter that exhibits a glass transition. The structure of glasses is similar to that of their parent supercooled liquids (SCL), and they spontaneously relax toward the SCL state. Their ultimate fate, in the limit of infinite time, is to crystallize. Glasses are also known as amorphous solids.

The boundary between the glassy state and the liquid state is not as immediate as can be imagined. A liquid has viscosity, a measure of its resistance to flow. As a liquid is cooled its viscosity normally increases, but viscosity also has a tendency to prevent crystallization.

Usually, when a liquid is cooled to below its melting point, crystals form and it solidifies; but sometimes it can become supercooled and remain liquid below its melting point because there are no nucleation sites to initiate the crystallization. If the viscosity rises enough as it is cooled further, it may never crystallize. The viscosity rises rapidly and continuously, forming a thick syrup and eventually an amorphous solid. The molecules then have a disordered arrangement, but sufficient cohesion to maintain some rigidity. In this state, it is often called an amorphous solid or glass.

Glass is a liquid with a viscosity so high that it maintains its shape for very long times, thus preserving the distances and bond angles of an atom with its immediate neighbors.

Glass is called supercooled liquid because there is no first-order phase transition as it cools. In fact, there is a second-order transition between the supercooled liquid state and the glass state, so a distinction can still be drawn. The transition is not as dramatic as the phase change that takes you from liquid to crystalline solids. There is no discontinuous change of density and no latent heat of fusion. The transition can be detected as a marked change in the thermal expansivity and heat capacity of the material.

The temperature at which the glass transition takes place can vary according to how slowly the material cools. If it cools slowly it has longer to relax, the transition occurs at a lower temperature, and the glass formed is denser. If it cools very slowly it will crystallize, so there is a minimum limit to the glass transition temperature.

A liquid to crystal transition is a thermodynamic one; i.e. the crystal is energetically more favorable than the liquid when below the melting point. The glass transition is purely kinetic: i.e. the disordered glassy state does not have enough kinetic energy to overcome the potential energy barriers required for the movement of the molecules past one another. The molecules of the glass take on a fixed but disordered arrangement.

Glasses and supercooled liquids are both metastable phases rather than true thermodynamic phases like crystalline solids. In principle, a glass could undergo a spontaneous transition to a crystalline solid at any time. Sometimes old glass devitrifies in this way if it has impurities.

Thermodynamically, a glass is in a metastable state with respect to its crystalline counterpart. The conversion rate, however, is practically zero.

The common glass “looks” solid, however, was found for example that the ancient window glass was deformed. We can describe the common glass as a very viscous liquid; at room temperature we can consider it a frozen liquid rather than a solid. A useful criterion for distinguishing a crystalline solid from a glassy substance is a crystalline solid that has a well-defined melting point. Unless a pure compound decomposes by heating, it can be characterized based on its melting point, defined in a precise and unambiguous way.

On the other hand, glass does not go through a well-defined transition between its solid and liquid state; on the contrary, its viscosity decreases with increasing temperature until it visibly melts and eventually turns into a fluid liquid.

The decrease in viscosity with increasing temperature is a typical behavior of the liquids, and then the glass behaves as such through a wide range of temperatures.

Material properties of glasses

Glasses can be made of quite different classes of materials: inorganic networks (such as window glass, made of silicate plus additives), metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers.

Glass can be made from pure silica, but fused silica has a high glass transition point at around 1200° C which makes it difficult to mold into panes or bottles.

At least 2000 years ago it was learned how to lower the softening temperature by adding lime and soda before heating, which resulted in a glass containing sodium and calcium oxides. The soda-lime glass used for windows and bottles today contains other oxides as well.

Measuring the glass transition temperature for different glasses is not easy because it changes according to how slowly the glass is cooled. In the case of modern soda-lime glass, a quick cooling will produce a glass transition at about 550° C. There is thought to be a minimum glass transition temperature at about 270° C, and if it is cooled very slowly it can still be a supercooled liquid down to just above that temperature.

Glass such as Pyrex (used for test-tubes and ovenware) is usually based on boro-silicates or alumino-silicates, which withstand heating better and typically have a higher glass transition temperature. Some glasses, such as the leaded variety, have lower transition temperatures.

Physical and chemical properties

Optical properties

Glass is transparent to visible light, it can refract, reflect, and transmit light following geometrical optics, without scattering it (due to the absence of grain boundaries).

Transparency is the main property of glass which allows the vision of the outside world through it. The transparency of glass can be from both sides or from one side only. In one side transparency, glass behaves like a mirror from the other side.

The visible fraction of light that passing through glass is the property of visible transmittance.


Glass is characterized by a high degree of corrosion-resistance. Because of its high water-resistance, it is often used as primary packaging material in the pharma industry since most medicines are preserved in a watery solution. Besides its water-resistance, glass is also robust when exposed to certain chemically aggressive liquids or gases.

Glass disease is the corrosion of silicate glasses in aqueous solutions. It is governed by two mechanisms: diffusion-controlled leaching (ion exchange) and hydrolytic dissolution of the glass network. Both mechanisms strongly depend on the pH of contacting solution: the rate of ion exchange decreases with pH as 10−0.5pH whereas the rate of hydrolytic dissolution increases with pH as 100.5pH.

Glass disease is a degradation process of glass that can result in weeping, crizzling, spalling, cracking, and fragmentation, caused by an inherent instability in the chemical composition of the original glass formula. Properties of a particular glass will vary with the type and proportions of silica, alkali, and alkaline earth in its composition. Once the damage has occurred it is irreversible, but decay processes can be slowed by climate control to regulate surrounding temperature, humidity, and airflow.


The strength of glass depends on the modulus of the rupture value of glass. In the general glass is a brittle material but by adding admixtures and laminates we can make it as more strong.

Glass typically has a tensile strength of 7 megapascals (1,000 psi), however theoretically it can have a strength of 17 gigapascals (2,500,000 psi) which is due to glass’s strong chemical bonds.

Imperfections on glass such as scratches and bubbles decrease the strength of glass. The imperfections (surface flaws) on a piece of glass have a great effect on the strength of glass (even more than other brittle materials).

The chemical composition of the glass also impacts the tensile strength of glass. The processes of thermal and chemical toughening can increase the tensile strength of glass.

U value of Glass

U value represents the amount of heat transferred through glass. If a glass is said to be insulated unit then it should have lower u value.

The U value is dependent upon climatic conditions, which means that the transmittance of heat through a glazing system changes. Therefore glass transmits heat at varying rates depending upon the prevailing climatic condition. In order for a comparison of glass properties based on a U Value, it is important that the climatic condition used to model all the systems are the same.

Centre pane U-values. This is the measurement of energy conductivity through the middle of a pane of glass whether it is single glazed, double glazed or triple glazed, etc. It does not take into account anything at the edge of the glass such as the spacer bar or window frame.

Window U-values. This is the measurement of energy conductivity through the window that is made up of glazing and frame. Here spacer bar plays an important role, as a may sealant. This is why window U-values are improved using warm edge spacer bar but center pane U-values are not.

The higher the U-value of glazing or windows, then the higher the energy conductivity through them. This means that lower U-values mean better performance in terms of insulation.

History of glass

Like many great discoveries, the glass was invented by accident. In the prehistoric era, man discovered it following a volcanic eruption followed by rapid cooling or a lack of prolonged light.

Around 4,000 BC – Pliny the Elder says that the glass would have been born casually on the banks of the Belo river in Syria. In the “Naturalis Historia,” it tells of some Phoenician merchants who lit a fire and accidentally used it as a support for cooking blocks of natural soda. These melted by the heat and, mixing with the sand of the beach, gave rise to the first glassy material.

3,000 BC – Thanks to the long navigation of the Phoenician merchants, the new art was used along the Mediterranean coasts, in Syria, Cyprus, and above all in Egypt, where a great variety of objects were produced using a compound similar to modern glass made with calcium carbonate. Although modern technology and today’s discoveries have partially changed the “Egyptian forms” leading to the codification of new rules, the raw materials known in the III millennium BC are still used today.

1,500 BC – The Egyptians are credited with creating the first cosmetic glass bottle for perfumes and precious essences.

300-200 BC – The technique of “blowing through a cylindrical barrel” (Augustan age) profoundly revolutionizes the techniques of glass production, allowing to obtain the shape of the object by free blowing or in metal, wood and ceramic molds, which reproduce the shape to be modeling. This new technology, still used today, and the evolution of the molds, perfect the glass art the evolution of the molds, allowing the production of objects aimed at a wider social range.

100 AD – The polychrome, translucent, shiny, and opaque glass is disappearing to make way for natural coloring. By inserting manganese dioxide into the glass mixture, the Romans obtain transparency, which will be one of the main characteristics of the material. The production of the first geometric-shaped bottles, called Roman, the true forerunners of modern bottles, dates back to this period.

680 AD – Archaeologists uncover remains of kilns used to manufacture glass dating from this period in York, Britain.

XII-XIII century AD – The first glass window appears in England in 1180. In the following century the colored windows, as an art form, reached their maximum development, with admirable examples throughout Europe.

XV century AD – Already in the Middle Ages Venice became one of the most important glass centers thanks to its geographical position between Western Europe and the East, where it learned the most refined techniques of glass art existing there. The glassmakers were forbidden to leave the island of Murano, to ensure that their secrets remain within the city.

Mid XVI century AD – Molds are spread and scientific experiments are conducted in glass containers. George Ravenscroft invents the barometer and leaded glass and in the same period table glassware, ornamental glass and optical lenses are born.

XVII Century AD – The industrial production of glass becomes. Some factories can produce up to one million bottles a year, although the bottles are still hand-blown.

Early XX century AD – Michael J. Owens invents the first automatic bottle making machine which, which was put into production in Manchester, is capable of producing 2,500 bottles every hour.

Mid XX century AD – Saint-Gobain develops tempered glass and introduces solar glass to the car market. This green anti-glare glass soon becomes the industry standard.

1953 AD – Pilkington discovers the double glazing.

Types of glass

The types of glass used in construction are:

  1. Float glass (clear float, extra clear float (low iron), tinted float, high performance tinted float)
  2. Reflective (coated) glass
  3. Decorative glass (wired and mirror)
  4. Laminated glass
  5. Toughened and heat-treated glass

Float glass

The float glass process is the most common method of flat glass production in the world. This process basically involves melting silicate (sand), lime and soda in a furnace and floating it onto a large bed of molten tin, hence the name float glass.

The process, which originally allowed to produce only 6 mm thick glass, now reaches thicknesses ranging from 0.4 mm to 25 mm. The transition from the melting basin to the tin bath takes place in a controlled atmosphere. The glass floats on the tin spreads and forms a uniform plate.

From this point, the glass emerges in one continuous ribbon and is then cut and further processed to customers’ needs. The different thicknesses are obtained by varying the extraction speed of the glass from the tin bath.

After annealing (controlled cooling), a perfectly transparent finished glass with parallel surfaces is obtained. Float glass is also known as soda-lime silicate glass as these are the major components used in manufacture.

Reflective (coated) glass

Special coatings can be applied to a float glass surface to make it reflective to short wave radiation from the sun and/or long wave radiation from heat inside or outside the building. These coatings are known by a variety of terms, but there are two main types:

  1. Pyrolytic coatings: in this process, semi-conducted metal oxides are directly applied to the glass during float glass production, while the glass is still hot, in the annealing lehr. These coatings are called on-line or hard coatings, and are relatively less harmful to the environment. The best feature of this product is its durability; it can be easily handled like a standard square of glass. It can also be easily cut, heat strengthened or toughened.
  2. Sputtered coatings: in this process, one or more coats of metal oxide are applied under a vacuum to finished glass. The coatings applied by this technique are soft and require protection from the external environment; they are therefore applied on the inner side of glass panes. Their low resistance makes them better off when used in a double glazing system. Cost-wise, this glass is relatively expensive.

Traditional reflective glass has a mirror-like appearance and reflects and absorbs a major proportion of the sun’s direct short wave solar radiation. The degree of reflectivity is dependent on the type of coating and the orientation of the glass. The use of reflective glass is more popular in commercial glazing as it provides superior solar control performance to clear or tinted glass products, and thus improves the energy efficiency of the building.

Low Emissivity (Low E) coatings are traditionally clear and are designed to reflect long wave radiation. They are available in both pyrolytic and sputtered coatings and the performance varies. Some modern reflective glass products have Low E coatings to reflect long wave radiation as well as the sun’s short wave radiation.

Laminated glass

Laminated glass consists of two or more sheets of glass permanently bonded together by a plastic or resin interlayer. Laminated glass offers superior safety. Although it will break on impact, the fragments are held by the interlayer.

The layered nature of laminated glass means that it blocks more noise and UV light than a single glass. It can also be made with Low Emissivity glass and used in Insulating Glass Units for increased environmental benefits.

Toughened (heat treated) glass

Toughened glass is a strong glass that has low visibility. It is available in all thicknesses and when it is broken it forms small granular chunks that are dangerous. There are two different types of heat-treated glasses, heat-strengthened and tempered. The differences between the two glasses are as follows:

  • With tempered glass, the cooling process is accelerated to create higher surface compression (the dimension of force or energy per unit area) and/or edge compression in the glass. It is the air-quench temperature, volume and other variables that create a surface compression of at least 10,000 pounds per square inch (psi). This is the process that makes the glass four to five times stronger and safer than annealed or untreated glass. As a result, tempered glass is less likely to experience a thermal break.
  • With heat-strengthened glass, the cooling process is slower, which means the compression strength is lower. In the end, heat-strengthened glass is approximately twice as strong as annealed, or untreated, glass.
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