What is Glass?

Glass is a brittle, supercooled, non-crystalline liquid. It is a clear amorphous solid, meaning, its atomic arrangements lack a definite pattern and long orderly arrangement. Glass can be thought of as a frozen liquid whose molecular flowability is retarded to much sower rate. Studies and science related to glass is an emerging field, with this, a new branch of materials science and engineering has developed known as, "glass engineering". Glass engineering focuses on the design and development of ceramics, glass, and composites. Engineers in this field test and work on new materials, that can be used in place of the traditional glass in terms of strength and durability. A glass engineer also engages himself in developing new equipment and manufacturing techniques, which can lead to the production of glass with enhanced properties.

The content of this article focuses on the science behind glass production and the different types of glass used in modern day-to-day applications.

Different molecular arrangement in solids.
CC BY-SA 3.0 | Image credits: https://en.wikipedia.org | Sbyrnes321

Glass as a super cooled liquid

Supercooling is also known as undercooling. It is a procedure to cool liquids and gases below their freezing points without turning them into solids.

During glass manufacturing, the glass is cooled from its liquid state to temperatures below their freezing points, but they do not turn into solid. To convert into solid, the glass is cooled further below the glass transition temperature. At this level, the molecular activity ceases and shows amorphous behavior.

Process of glass making

1. Gathering raw materials

The type of raw materials depends on the type of glass to be produced. Some of the raw materials that are used in the glass production are, chalk, soda ash, clean sand, potassium carbonate, lead monoxide, lead sesquioxide, pure sand, salt cake, coke, and ordinary sand. Besides these materials, some amounts of cullets (broken pieces of old glass) and decolorizers are also added to remove the colors of the previously used pieces of glass.

2. Batch preparation

The mixture to be used in glass manufacturing includes raw materials, cullets, and decolorizers. These are first converted into fine powder in a grinding machine and then mixed thoroughly in a mixer machine. This process of mixing is known as batch preparation.

3. Melting the mixture in the furnace

The next step is melting the mixture. This is carried out in furnaces. Pot furnaces and tank furnaces are usually used for melting. The melting is continued till the evolution of carbon dioxide, oxygen, and sulfur dioxide gases cease to liberate.

In the pot furnace, there is no physical contact between the glassmaking materials and the furnace walls. The materials are contained in pots and placed inside the furnace. The walls of the furnace and pots are made of refractory materials. The mixture in the furnace is placed for about 18 to 24 hours inside the furnace. Multiple pots can be used at a time with this type of furnace that cannot be used using ordinary furnaces.

Tank furnaces are generally preferred when using an automatic machine for glassblowing. These machines need a continuous flow of molten glass. Tank furnaces are primarily used for large-scale productions and can accommodate a weight of 2000 tonnes at a time. Tank furnaces are large furnaces and use a variety of fuels to power the system, natural gas is one of the first choices. To achieve high temperatures with added fuel economy, recuperative and regeneration systems are used.

4. Fabrication

In this stage of glass production, the glass is given its suitable shape using a variety of processes. These processes can either be done by hand or machine. Hand molding of glass is carried out for small-scale productions, while the machine is used for large-scale productions. Blowing, casting, drawing, spinning, rolling, and pressing are some of the fabrication processes.

5. Annealing

Slow and steady cooling of glass is known as annealing. Annealing is the most important step after the fabrication process. If the glass is allowed to cool rapidly, it induces internal stress which can lead to breakage at even slight shock and vibrations. During rapid cooling, the outer layer of the glass cools down initially while the internal part remains hot and in the molten state. The interiors try to expand and develop thermal strains which ultimately give rise to the development of thermal stress.

Different types of glasses

Clear glass

Clear glasses are glasses that do not contain any tint, due to which it achieves a clear and transparent finish. The quality and grade of clear glass are measured by the amount of iron content with the raw materials used for making glass. The amount of iron content is measured in parts per million (PPM). The lower the iron content, the pure and clearer the glass is. Clear glass finds its application in constructions, decorative items, furniture, mirrors, and so on. Some of the test kit items used in the detection of coronavirus disease 2019 (COVID-19) are majorly made up of clear glass.

Corning glass

The popular Corning gorilla glass is majorly used in smartphones for shielding the screen of the phone from damages caused by impacts and shocks. Corning glass is produced by Corning Inc., an American multinational company having its headquarters in New York. The Corning Museum in New York is famous for its variety of glass works collection including artworks.

The gorilla glass manufactured by Corning is a thin, lightweight glass, which is damage-resistant and shockproof. It is a chemically treated strengthened glass.


Fiberglass is a popular fiber-reinforced glass whose glass fibers are mixed with the matrix of the thermosetting plastic. The fibers of glass can be woven into glass cloth, randomly oriented, or flattened into sheets. The glass is cheaper and primarily stronger than many metals. These kinds of glasses find their wide applications in making windshields, enclosures, glassware, decorative items, and so on.

Context and Applications

This topic is extensively taught in courses such as

  • Bachelors of Technology in Civil Engineering
  • Masters of Technology in Civil Engineering

Practice Problems

Q1) Which of the following glass is used in smartphones?

a. Lalique glass

b. Corning gorilla glass

c. Fiberglass

d. Clear glass

Answer: Option b

Explanation: Corning gorilla glass is majorly used in smartphone applications.

Q2) Which of the following glass is fiber-reinforced glass?

a. Vitreous

b. Fiberglass

c. Clear glass

d. None of these

Answer: Option b

Explanation: Fiberglass is a fiber reinforced glass.

Q3) Which of the following is true for a glass?

a. It is an amorphous solid

b. It is a supercooled liquid

c. It requires annealing during its production

d. All of these

Answer: Option d

Explanation: Glass is a supercooled amorphous solid, which requires annealing during its manufacturing.

Q4) What are cullets?

a. Broken pieces of old glass

b. Molten glass

c. Silica and sand

d. Decolorizers

Answer: Option a

Explanation: Cullets are the broken pieces of old glass.

Q5) What is the matrix material used in the production of fiberglass?

a. Thermoplastics

b. Plastics

c. Bakelite

d. None of these

Answer: Option a

Explanation: Thermoplastics are the matrix materials used in the production of fiberglass.

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