Many Stone Age societies across the world used naturally occurring glass, such as the volcanic glass obsidian, to make sharp cutting tools. However, according to the archaeological evidence, the first true glass was created in coastal north Syria, Mesopotamia or Egypt.
The history of the earliest known glass items can be traced back to at least 2,000 BC. Beads, for example, were created during the production of faience, a vitreous material.
Since then, the process of making glass has changed a lot. Advances in material science and manufacturing techniques have made it possible to produce glass with specific reflective, refractive, and transmission properties that can be used in prisms, optical lenses, and optoelectronics materials.
Today, glass is made using two main techniques: the Float Glass Process, which involves floating molten glass on a bed of molten metal, and Glassblowing, which involves inflating molten glass into a bubble.
Molten glass is made by heating ordinary sand (mostly contains silicon dioxide) at very high temperatures until it melts and turns into liquid. When the sand is allowed to cool, it doesn’t return to its initial state. Instead, it turns into an amorphous solid, a noncrystalline solid where atoms and molecules are not organized in a definite lattice pattern.
Let’s dig deeper and find out the material and process involved in both methods, and what does their future look like.
Method 1: The Float Glass Process
In this method, a sheet of glass is produced by floating molten glass on a bed of molten metal, such as tin or lead. It is used to produce glass sheets with uniform thickness and flat surfaces. Let’s go through the step-by-step process:
1. Melting and Refining
Common raw materials used to make float glass include sand, dolomite, salt cake (sodium sulfate), soda ash (sodium carbonate), and limestone. Other materials are often used as refining agents to alter the chemical and physical characteristics of the glass.
These ingredients are mixed in a batch process in the right proportion. The entire batch is then fed into a furnace where it is heated to nearly 1,500 °C. Most furnaces can hold over 1,000 tons of materials.
When the glass is melted, its temperature is stabilized to 1,200 °C to check the relative density or specific gravity.
2. Tin Bath
The melted glass from the furnace flows into the float bath, a bath of molten tin, through a ceramic lip called spout lip. The amount of glass poured onto the tin is controlled by a gate, commonly known as a tweel.
Tin is a preferred choice for this process because it is cohesive and immiscible with molten glass. It also has a high specific gravity. However, it oxidizes in air to form tin dioxide, which adheres to the glass. To prevent this oxidation, tin is treated with hydrogen and nitrogen.
The glass flows along the tin bath and forms a floating ribbon with even thickness and smooth surfaces on both sides. As this happens, the temperature is slowly decreased (to 600 °C) and the glass ribbon is pulled off the bath by rollers.
The thickness of the outcoming product can be controlled by changing the roller speed and the glass flow speed. Typically, rollers are placed above the molten tin to adjust the thickness as well as the width of the glass ribbon.
Some glasses are made reflective. In such cases, either a soft coat or hard coat is applied on the surface of the cooled ribbon.
As soon as the glass gets out of the bath, it passes through a lehr oven — a long kiln with an end-to-end temperature gradient. This allows the glass to anneal without strain. It also prevents the glass from cracking due to the temperature change.
More specifically, this process changes the chemical and physical properties of the glass, reducing its hardness and making it more ductile.
Using acute and advanced inspection technology, millions of inspections can be conducted throughout the glass manufacturing process. Most of them involve identifying stresses, grains of sand, and air bubbles that reduce the glass quality.
Today, there are thousands of systems capable of precisely inspecting optical quality, distortion, tension, thickness, and flatness of glass at the earliest possible point throughout the production process.
On exiting the ‘cold end’ of the lehr oven, the glass is cut and shaped as per the customer’s requirement, using specialized equipment. Large glass sheets are cut with a computer-assisted semi-automatic glass cutting table. These sheets are then broken out manually into individual sheets of glass.
While most class cutters use a small, sharp wheel made of tungsten carbide or hardened steel, some use a diamond to create the split.
Uses of Float Glasses
Float glass has become the most popular form of glass in consumer products. It can be produced in various colors and opacities. It exhibits a high degree of light transmission and good chemical inertness.
These properties make float glass ideal for a wide range of applications, such as mirrors, windows, doors, furniture, and automobile glass. It also has numerous applications in modern architecture, both in domestic and commercial builds.
With recent advancements in float glass production, such as ultra-thin float glass, new applications are being discovered in electronics and technology. Aluminosilicate compositions like Gorilla Glass (which contains silicon dioxide, aluminum, sodium, and magnesium) are used by a variety of smartphones and other electronic devices.
Method 2: Glassblowing
In this glass-forming technique, molten glass is inflated into a bubble with the help of a blow tube. It is used to produce bottles and other containers.
How Does It Work?
Inflation refers to the process of expanding a molten blob of glass by injecting a small volume of air in it. Since atoms in the liquid glass are attached via strong chemical bonds in a random and disordered network, molten glass is viscous enough to be blown. It slowly hardens as it cools down.
In order to facilitate the process of blowing, the stiffness of the molten glass is increased by slightly changing its composition. It turns out that adding a small amount of natron makes the glass stiffer for blowing. (Natron is a naturally occurring substance containing sodium carbonate decahydrate and sodium bicarbonate.)
During blowing, thicker layers of glass cool slower than thinner ones and get less viscous than the thinner layer. This makes it feasible to produce blown glass with even thickness.
In the past couple of decades, more efficient and effective glassblowing methods have been developed. Most of them involve the same steps:
Step 1: Place the glass in a furnace and heat it to 1,300 °C to make it malleable.
Step 2: Put one end of the blowpipe into the furnace and roll it over the molten glass until a ‘gob’ of glass sticks to it.
Step 3: Roll the molten glass on a marver, a flat metal slab, which is made of polished steel, graphite, or brass surface attached to a wooden or metal table. The marver is used to control the shape as well as the temperature of the glass.
A marver being used to shape glass
Step 4: Blow the air into the pipe to create a bubble. Gather more glass over that bubble to make a larger piece. Once the glass has been blown to its desired size, the bottom is finalized.
Step 5: Attached molten glass to iron or stainless steel rod (commonly known as punty) to shape and transfer the hollow piece from the blowpipe.
Step 4: Add color and design by dipping it in crushed colored glass. These crushed pieces quickly fuse to the main glass due to the high temperature. Complex and detailed patterns can be built using a cane (rods of colored glass) and murrine (rods cut in cross-sections to reveal patterns).
Step 5: Take the resulting product back to the marvel and roll it again to give it a required shape.
Step 6: Remove the glass from the glasspipe using steel tweezers. Typically, the bottom part of the blown glass is separated from the rotating blowpipe. It can be removed from the blowpipe with just one solid tap.
Step 7: Put the blown glass into an annealing oven and let it cool for a few hours. To prevent cracks from forming randomly, don’t expose it to rapid temperature changes.
Roman blown-glass from 4th century AD
This method requires extreme patience, tenacity, and dexterity. Creating complex and large pieces requires a group of experienced glassworkers.
The major environmental impact of glass manufacturing is caused by melting processes, which emit various gases into the atmosphere. For example, the combustion of fuel or natural gas and decomposition of raw materials results in the emission of carbon dioxide.
Similarly, the decomposition of sulfate in the batch materials produces sulfur dioxide, which contributes to acidification. Decomposition of nitrogen compounds releases nitrogen oxides, which contributes to acidification and smog formation. Furthermore, evaporation from raw materials and molten components ejects tons of particles into the atmosphere.
Other factors, such as emission of volatile organic compounds and production of solid waste during the manufacturing process, also raise environmental issues.
However, recycled glass can overcome many of these issues. It can be recycled multiple times without significant quality loss. Every 1,000 tons of glass that get recycled can lead to 300 tons less carbon dioxide emission and 345,000 kWh of energy savings.
On a smaller scale, recycling a single glass bottle could alone save enough energy to power a 20W LED light for an hour.
Although both manufacturing techniques have evolved a lot in terms of efficiency, further reductions in the emission of dust particles, carbon dioxide, and sulfur dioxide are still the main environmental objectives for flat glass productions.
In 2019, the global glass manufacturing market was valued at $127 billion, and it is projected to grow at a CGAR of 4.1% between 2020 and 2027.
The major factors likely to drive this market growth include the ever-increasing demand for consumer electronics and the penetration of artificial intelligence in consumer and business applications.
Flat glass is expected to play a key role in architectural applications over the coming year.
The recent trend suggests a quick transition in building architecture that maximizes natural daylight through the integration of flat glass in roofs and facades. Since triple silver insulated Low-E flat glass contributes to significant energy savings, it could be extensively used in green buildings across the globe. Solar flat glass is also likely to note significant growth over the next few years.
China is currently the world’s leading exporter of glass and glassware, with its exports amounting to more than 23% share of worldwide glass and glassware exports, at a value of nearly $18 billion. Germany and the United States account for approximately 9% and 7% share of global glass exports.