Many Stone Age societies around the world used natural glass, such as volcanic obsidian glass, to make sharp cutting tools. However, according to archaeological evidence, the first true fused glass was created in coastal northern Syria, Mesopotamia or Egypt.
The history of the earliest known glassware can be traced back to at least 2000 BC. For example, beads were created during the production of earthenware, a vitreous material.
Since then, the process of making glass has changed dramatically. Advances in materials science and manufacturing techniques have made it possible to produce glass with special reflective, refractive, and transmissive properties that can be used in prisms, optical lenses, and optoelectronic materials.
Today, glass is produced using two basic techniques: the Float Glass process (also known as float glass), which involves floating molten glass on a layer of molten metal, and glass blowing, in which molten glass is blown into a bubble.
Molten glass is produced by heating ordinary sand (mostly containing silicon dioxide) at very high temperatures until it melts and becomes a liquid. When the sand cools, it does not return to its original state. Instead, it turns into an amorphous solid, a noncrystalline solid in which the atoms and molecules are not organized into a specific lattice structure.
Let’s dig deeper and find out what materials and processes are involved in both methods, and what their future holds.
Method 1: The float glass process
In this method, a sheet of glass is made by floating molten glass on a layer of molten metal, such as tin or lead. It is used to produce sheets of glass with uniform thicknesses and flat surfaces. Let’s look at the step-by-step process:
1. Smelting 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 change the chemical and physical characteristics of glass.
These ingredients are mixed in a batch process in the desired proportion. The entire batch is then fed into a furnace where it is heated to nearly 1,500 °C. Most furnaces hold more than 1,000 tons of material.
As the glass melts, its temperature is stabilized to 1,200 °C to check its relative density or specific gravity.
2. Tin bath
The molten glass from the furnace flows into the float bath, a bath of molten tin, through a ceramic edge called the spout edge. The amount of glass poured over the tin is controlled by a slide, usually known as a brush.
Tin is the preferred choice for this process because it is cohesive and does not mix with molten glass. It also has a high specific gravity. However, it oxidizes in air to form tin dioxide, which sticks to the glass. To prevent this oxidation, tin is treated with hydrogen and nitrogen.
The glass flows over the tin bath and forms a floating strip of equal thickness and smooth surfaces on both sides. The temperature is gradually reduced (to 600 °C), and the glass ribbon is removed from the bath by rollers.
The thickness of the outgoing product can be adjusted by changing the speed of rotation of the roller and the flow rate of the glass. The rollers are usually placed over the molten tin to adjust the thickness as well as the width of the glass strip.
Some glass is made reflective. In such cases, either a soft or hard coating is applied to the surface of the cooled tape.
3. Annealing
As soon as the glass comes out of the bath, it passes through a lyer furnace, a long furnace with a continuous temperature gradient. This allows the glass to anneal without deforming. It also prevents the glass from cracking due to temperature changes.
In particular, this process changes the chemical and physical properties of the glass, reducing its hardness and making it more malleable.
4. Inspection
Using advanced inspection techniques, millions of inspections can be performed throughout the glass production process. Most of these involve detecting stresses, sand and air bubbles that reduce glass quality.
Today, there are thousands of systems that can accurately monitor optical quality, distortion, tension, thickness and flatness of glass at the earliest stage of the production process.
5. Cutting
At the exit of the “cold end” of the ler furnace, the glass is cut and shaped according to customer requirements using specialized equipment. Large sheets of glass are cut on a semi-automatic, computer-controlled glass cutting table. These sheets are then manually cut into individual sheets of glass.
While most classroom cutters use a small, sharp wheel of tungsten carbide or hardened steel, some use diamond to create the split.
Using float glass
Float glass has become the most popular form of glass in consumer products. It can come in a variety of colors and degrees of opacity. It has 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 automotive glass. It also has many applications in modern architecture in both residential and commercial buildings.
Recent advances in float glass, such as ultra-thin float glass, are opening up new applications in electronics and technology. Aluminosilicate compositions such as Gorilla Glass (which contains silicon dioxide, aluminum, sodium and magnesium) are used in various smartphones and other electronic devices.
Method 2: Glass Blowing
In this glass-blowing method, molten glass is blown into a bubble using a blowing tube. It is used to produce bottles and other containers.
How does it work?
Inflation refers to the process of expanding a molten piece of glass by injecting a small volume of air into it. Because the atoms in liquid glass are bound together by strong chemical bonds in a disorderly and disordered grid, the molten glass is viscous enough to be blown out. As it cools, it slowly hardens.
To facilitate the blowing process, 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 harder to blow. (Natron is a natural substance containing sodium carbonate decahydrate and sodium bicarbonate.)
When blown, thicker layers of glass cool more slowly than thinner ones and become less viscous than thinner ones. This makes it possible to produce blown glass of equal thickness.
Over the past couple of decades, more efficient and effective glass-blowing methods have been developed. Most of them involve the same steps:
Step 1: Place the glass in an oven and heat it to 1300 °C to make it malleable.
Step 2: Place one end of the blowing tube in the oven and roll it over the molten glass until a “blob” of glass sticks to it.
Step 3: Roll the molten glass over a marver, a flat metal plate that is made of polished steel, graphite or brass and attached to a wooden or metal table. The marver is used to control the shape as well as the temperature of the glass.
Step 4: Blow air into the tube to create a bubble. Gather more glass over this bubble to make a larger piece. Once the glass is the desired size, the bottom is ready.
Step 5: Attach the molten glass to a rod of iron or stainless steel (usually known as a point) to form and transfer the hollow piece from the blowpipe.
Step 6: Add color and design by dipping it in broken colored glass. These crushed pieces quickly adhere to the base glass because of the high temperature. Complex and detailed patterns can be constructed using a cane (colored glass rods) and murrin (rods cut crosswise to reveal patterns).
Step 7: Take the resulting piece back and roll it out again to give it the desired shape.
Step 8: Remove the glass from the glass tube using steel tweezers. Usually the bottom part of the blown glass is separated from the rotating blow pipe. It can be removed from the blowpipe with a single touch.
Step 9: Place the blown glass in the annealing oven and let it cool for several hours. To avoid accidental cracking, do not expose it to sudden changes in temperature.
This method requires a great deal of patience, persistence and skill. It requires a team of experienced glass craftsmen to create complex and large pieces.
Environmental Impacts
The main environmental impact of glass manufacturing is due to melting processes that release various gases into the atmosphere. For example, the burning of fuel or natural gas and the decomposition of raw materials results in the emission of carbon dioxide.
Similarly, the decomposition of sulfates in batch materials produces sulfur dioxide, which contributes to acidification. The decomposition of nitrogen compounds releases nitrogen oxides, which contributes to acidification and smog. In addition, tons of particles are released into the atmosphere during evaporation from raw materials and melted components.
Other factors, such as emissions of volatile organic compounds and the generation of solid waste during production, also cause environmental problems.
However, recycled glass can solve many of these problems. It can be recycled several times without significant loss of quality. Every 1,000 tons of recycled glass can result in 300 tons less carbon dioxide emissions and energy savings of 345,000 kWh.
On a smaller scale, recycling one glass bottle can save enough energy to power a 20-watt LED lamp for an hour.
Although both production technologies have greatly improved in terms of efficiency, further reduction of dust particles, carbon dioxide and sulfur dioxide emissions is still a major environmental challenge in sheet glass production.