Glass is a material of many faces: It is both ancient and modern, strong yet delicate, and able to adopt almost any shape or color. These properties of glass are why people use it to make everything from smartphone screens and fiber-optic cables to vials that hold vaccines.
Humankind has been using glass in some fashion for millennia, and researchers are still finding new uses for it today. It’s not uncommon to hear the oft-repeated factoid that glass is actually a liquid, not a solid. But the reality is much more interesting – glass does not fit neatly into either of those categories and is in many ways a state of matter all its own. As two materialsscientists who study glass, we are constantly trying to improve our understanding of this unique material and discover new ways to use glass in the future.
What is glass?
The best way to understand glass is to understand how it is made.
The first step to make glass requires heating up a mixture of minerals – often soda ash, limestone and quartz sand – until they melt into a liquid at around 2,700 degrees Fahrenheit (1,480 Celsius). In this state, the minerals are freely flowing in the liquid and move in a disordered way. If this liquid cools down fast enough, instead of solidifying into an organized, crystalline structure like most solids, the mixture solidifies while maintaining the disordered structure. It is the atomically disordered structure that defines glass.
On short timescales, glass behaves much like a solid. But the liquidlike structure of glass means that over a long enough period of time, glass undergoes a process called relaxation. Relaxation is a continuous but extremely slow process where the atoms in a piece of glass will slowly rearrange themselves into a more stable structure. Over 1 billion years, a typical piece of glass will change shape by less than 1 nanometer – about 1/70,000 the diameter of human hair. Due to the slow rate of change, the myth that old windows are thicker at the bottom due to centuries of gravity pulling on the slowly flowing glass is not true.
Colloquially, the word glass often refers to a hard, brittle, transparent substance made of fused sand, soda and lime. Yet there are many types of glass that are not transparent, and glass can be made from any combination of elements as long as the liquid mixture can be cooled fast enough to avoid crystallization.
From the Stone Age to today
Humans have been using glass for more than 4,000 years, with some of the earliest uses being for decorative glass beads and arrowheads. Archaeologists have also discovered evidence of 2,000-year-old glass workshops. One such ancient workshop was uncovered near Haifa in modern Israel and dates back to around 350 C.E. There, archaeologists discovered pieces of raw glass, glass-melting furnaces, utilitarian glass vessels and debris from glass-blowing.
Today, scientists are far beyond simply using glass as the material for a cup or a mirror. At the cutting edge of research into glass is the ability to manipulate its complex atomic structure and relaxation process to achieve certain properties.
Because glass is atomically disordered and always changing, any two points on a piece of glass are likely to have slightly different properties – whether it is strength, color, conductivity or something else. Because of these differences, two similar pieces of glass that were made in the same way using the same materials can behave very differently.
To better predict how a piece of glass behaves, our team has been researching how to quantify and manipulate the chaotic and ever-changing atomic structure of glass. Recent advances in this field have had direct benefits to existing technologies.
For example, phone screens do not crack as easily as they did in 2014 in part because new processing techniques decrease the differences in atomic bond strengths to make it harder for cracks to propagate. Similarly, internet speeds have vastly improved over the last 20 years because researchers have figured out ways to make the density of glass used for optical fibers more uniform and, therefore, more efficient at transmitting data.
A deeper understanding of how to manipulate the changing, chaotic structure of glass could lead to big advancements in technology in the coming years. Researchers are currently working on a range of projects, including glass batteries that could enable faster charging speeds and improved reliability, fiberglass wind turbines that require less maintenance than existing turbines, and improved memory storage devices.
It is the atomically disordered structure that defines glass. On short timescales, glass behaves much like a solid. But the liquidlike structure of glass means that over a long enough period of time, glass undergoes a process called relaxation.
Glass, however, is actually neither a liquid—supercooled or otherwise—nor a solid. It is an amorphous solid—a state somewhere between those two states of matter. And yet glass's liquidlike properties are not enough to explain the thicker-bottomed windows, because glass atoms move too slowly for changes to be visible.
It is a solid, albeit an odd one. It is called an amorphous solid because it lacks the ordered molecular structure of true solids, and yet its irregular structure is too rigid for it to qualify as a liquid. In fact, it would take a billion years for just a few of the atoms in a pane of glass to shift at all.
Amorphous solids are hard like other solids, but their molecular structure is disordered, somewhat similar to the molecular structure of liquids. Glass is considered an amorphous solid and is sometimes thought to very slowly “flow” and move like a liquid.
It does not form a crystalline structure. So, the constituent particles of the glass can move. In regular solids, there is no movement of constituent particles under normal conditions. Due to this fluidity property, glass is called as supercooled liquid.
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.
Some people claim that glass is actually a supercooled liquid because as it cools, no first-order phase transition occurred. In fact, a second-order transition occurs between the supercooled liquid state and the glass state, and so a distinction between a "glass" and a "supercooled liquid" must still be drawn.
Contrary to the urban legend that glass is a slow-moving liquid, it's actually a highly resilient elastic solid, which means that it is completely stable. So those ripples, warps, and bull's eye indentations you see in really old pieces of glass “were created when the glass was created,” Cima says.
Read this carefully: "In other words, while some antique windowpanes are thicker at the bottom, there are no statistical studies to show that all or most antique windowpanes are thicker at the bottom than at the top.
Glass is an amorphous form of matter. It is a solid. You may have heard different explanations about whether glass should be classified as a solid or as a liquid.
Why Does Hot Water Cause Glass to Break? It is not the hot water that breaks the glass, but the sudden change in temperature, causing internal stress to be exerted on the material. If these changes occur suddenly, they create internal tension that leads to the breakage of the container.
Glass is submersed in a bath containing a potassium salt (typically potassium nitrate) at 300°C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.
Toughening. Toughened or tempered glass is a type of safety glass. Toughened glass is made by heating annealed (float) glass uniformly through a furnace to temperatures exceeding 600 C. The annealed (float) glass is then transferred to the quenching chamber of the furnace where it is rapidly cooled.
"The material is mechanically a solid but with a liquid-like disordered structure." To reach equilibrium, the glass would need to "relax" back toward a liquid state. But at room temperature, the particles move so slowly that this just isn't practical. "This is what's called a metastable equilibrium," Mauro said.
Glass is called supercooled liquid because glass is an amorphous solid. Amorphous solids have the tendency to flow slowly. It does not form a crystalline solid structure as particles in solids do not move but here it moves. Hence it is called a supercooled liquid.
It is the atomically disordered structure that defines glass. On short timescales, glass behaves much like a solid. But the liquidlike structure of glass means that over a long enough period of time, glass undergoes a process called relaxation.
Contrary to the urban legend that glass is a slow-moving liquid, it's actually a highly resilient elastic solid, which means that it is completely stable. So those ripples, warps, and bull's eye indentations you see in really old pieces of glass “were created when the glass was created,” Cima says.
So what is glass, really — a solid or a liquid, or something in between? "It's neither a true liquid nor a true solid — it has properties of both, but it's its own distinct state of matter," John Mauro, a materials scientist at Penn State, told Live Science.
Specifically, it is an amorphous solid because the silicon dioxide molecules are not packed in a crystal lattice. The reason people thought glass might be a liquid was because old glass windows were thicker at the bottom than at the top. The glass was thicker some places than others because of the way it was made.
Introduction: My name is Duane Harber, I am a modern, clever, handsome, fair, agreeable, inexpensive, beautiful person who loves writing and wants to share my knowledge and understanding with you.
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