Picture of a hand using a glass touchscreen

Everyone knows what glass is…or do they? We are all surrounded by glass in a myriad of forms and serving a diverse array of functions, from jars and glasses to windows and TV screens, but do we really know much about it? Glass has become so ubiquitous that it is widely accepted as an everyday commodity. Consequently, most of us take it for granted without considering what it is or how it shows such amazing properties and versatility.

Through the ages, glass has provided answers to many technological challenges and enabled unbelievable advances across most areas of our lives.1,2 However, despite being at the center of an ongoing success story, glass receives little attention from the majority.

What is glass?

Glass is commonly categorized as a type of ceramic, but it is not like any other ceramics. Ceramics generally have a crystalline structure and are opaque, whereas glass has a non-crystalline atomic structure and is transparent. Furthermore, glass exhibits a range of remarkable properties that set it apart from other ceramics. In a perfect state, glass is mechanically very strong, even when subjected to extreme changes in temperature, and has a hard surface that is resistant to abrasion and corrosion. Paradoxically, it is also elastic, being able to give under stress (up to a breaking point) and then rebound to its original shape. Glass also has extensive optical properties, is heat-absorbent and an electrical insulator.3

Glass is so unique that it cannot be simply defined. It is neither a crystalline solid nor a liquid; it is a disordered, amorphous solid. It is this amorphous structure that gives glass its unique properties. Neither can the composition of glass be described since there are infinite varieties of glass. A current database lists over 350,000 types of known glass and new workable glass compositions are being developed every day.

The production of glass

Glass is formed when the constituent parts are combined by intense heating and then rapid cooling. The rapid cooling immobilizes the atoms of the glass before they have a chance to assume a regular crystalline structure. This can occur naturally, as in the case of fulgurite that is formed by lightning striking sand, and obsidian that arises from the rapid cooling of volcanic lava.

Man-made glasses are produced from varying mixtures of oxides. Although the precise chemical composition varies widely between different types of glass, it typically includes three components: a former, a flux and a stabilizer. Glass formers, such as silicon dioxide (silica), make up the largest proportion of the mixture and provide the transparency. Fluxes, such as sodium carbonate (soda) lower the temperature at which the formers will melt. Stabilizers, such as calcium carbonate (lime) provide the strength and make the glass water resistant. Without the inclusion of a stabilizer, water and humidity will attack and dissolve the glass.4

Immediately after glasses are batched and melted, they are slowly and evenly cooled. This process is known as annealing. This is an important step that enhances the strength of the glass by reducing internal stresses. It ensures that sections of varying thickness cool at the same rate. This avoids the development of steep temperature gradients that could cause the glass to crack.

Types of glass

The precise chemical composition of the mixture melted to produce glass determines the mechanical, electrical, chemical, optical, and thermal properties of the final product. Glass can thus be manufactured with broad-ranging characteristics. Through careful selection of the basic initial mixture and additives used in production, glass is produced with properties and structures to meet the requirements of specific applications.3

Although there are many thousands of different glass compositions, glass can be categorized as belonging to one of the six basic types, based on the chemical composition that endows it with specific properties.5

Soda-lime glass

Soda-lime glass is the most common, and least expensive, type of glass, accounting for 90% of all glass made. It usually contains 60–75% silica, 12–18% soda, and 5–12% lime. This is the type of glass used to make bottles and windows. It is mechanically strong but does not have good resistance to high temperatures, sudden changes in temperature, and corrosive chemicals.

Lead Glass

Lead glass (more commonly known as crystal) contains at least 20% lead oxide, which makes the glass brilliant, resonant, and heavy. Although lead glass, like soda-lime glass, will not withstand high temperatures or sudden changes in temperature, it exhibits excellent electrical insulating properties. Consequently, it is commonly used for electrical applications. It is also used for thermometer tubing and art glass.

Borosilicate glass

The addition of at least 5% of boric oxide to a silicate glass gives it high resistance to temperature change and chemical corrosion. Borosilicate glass is not as convenient to produce as either lime or lead glass, but is useful for pipelines, light bulbs, photochromic glasses, sealed-beam headlights, and vessels for laboratory or kitchen use.

Silica glass

Removal of almost all the non-silicate elements from borosilicate glass after normal melting and forming produces 96% silica glass. The resulting pores are sealed by reheating the glass to 1200° resulting in glass that is resistant to heat shock up to 900°C. 96% silica glass is used for the outer panes of the forward windshields of space shuttles to enable them to withstand the high temperatures reached during atmospheric re-entry.6

Aluminosilicate glass

Similar to borosilicate glass is aluminosilicate glass that includes aluminum oxide in its composition. Aluminosilicates are more difficult to manufacture than borosilicate glass, but have even greater chemical durability and can withstand higher operating temperatures. Aluminosilicate glass can also be used as a resistor in electronic circuits.3

Fused silica glass

Fused silica glass is the most difficult type of glass to produce, and so it is the most expensive of all glasses. Fused silica glass is pure silicon dioxide in the non-crystalline state and can withstand temperatures up to 1200°C for short periods. Fused silica is used to create astronomical telescopes, optical waveguides, and crucibles for growing crystals.6

Glass additives

Additives can be used to change the characteristics of glass. This may be done be for aesthetic purposes, for example, heavy metals, such as lead or manganese may be added to give the glass color. It may also be altered for functional purposes, for example, the addition of selenium makes the glass a light-sensitive conductor of electricity; a feature that forms the basis of photocopying.

Versatility of glass

Glass has an extensive range of potential forms and shapes. Its desirable properties can be manipulated during manufacturing, such as mechanical strength and chemical stability. This has led to the development of novel glass formats for use across an entirely new scope of applications.

Controlled-pore glass, which is porous glass with a sharply defined and adjustable pore size, can be used as a support for solid-phase oligonucleotide synthesis7 and as a stationary phase for a variety of chromatography techniques.8,9 Hollow glass biospheres have unique optical properties that have enabled the development of new research techniques, which hold huge potential for analytical devices of the future.10

Glass is also increasingly being adopted for a range of applications in medicine and dentistry. Bioactive glass is biocompatible and demonstrates antimicrobial activity. Furthermore, it can bond with both soft tissue and bone to promote healing. Bioactive glass has thus become an invaluable tool in tissue engineering and bone implants11 as well as in dental reconstruction procedures.12 It is also used in toothpaste and dental fillings to strengthen enamel and reduce bacterial colonisation.13

The future of glass

Glass has become the material of choice for solving a range of technological challenges. It is lightweight yet has the potential for strength, durability and optical clarity and its precise properties can be fine-tuned to meet a specific need. It can also be produced in a range of very different formats, including flat sheets, fine tubes, beads, and powder.

The versatility of glass has enabled incredible achievements, but the journey has by no means reached its end. With new production techniques and types of glass being continually developed, potential applications of glass products continue to expand and facilitate further remarkable advances. We are already benefitting from great interactive user experiences through the glass screens of mobile phones and tablets, but prototypes are now in development for touch-activated glass surfaces through which a range of digital devices can be accessed. Similarly, glass screens have been developed that provide a medium for virtual and augmented reality experiences.

Scientists continue to take advantage of the unique characteristics of glass, redefining what is possible. The latest projects include cleaning up nuclear waste by vitrification and using glass to develop safer batteries.

With a long and successful history, glass is still an active field of discovery and innovation with a future of exciting and ever-expanding capabilities.

Mo-Sci is a world leader in high-quality precision glass technology and produces a wide range of specialist glass products, the precise composition of which can be tailored to meet specific requirements.14


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  14. Mo Sci Corporation website. http://www.mo-sci.com/en/products