Fantastic glasses and where to find them

This winter wasn’t about snow or ice. It was about the black material that came in cartloads from an island in the narrow sea. Dragonglass, the common folk called it. It was black, pointy and the weapon of choice for the wildlings in Game of Thrones. Here, Vineeth Venugopal, researcher at the intersection of AI and materials science and Matmatch contributor, gives examples of types of glass that wouldn’t look out of place in a science-fiction novel.   

We may not have dragon glass, but obsidian is a common volcanic glass found in many parts of the world. In Naturalis Historia, written in 77 AD, Pliny the Elder writes that, “among the various forms of glass we may reckon obsidian glass, a substance very similar to the stone found by Obsidius in Ethiopia.” 

It is not known when Obsidius made his discovery, but glasses, in general, had already been known for hundreds of thousands of years by the time of Pliny. In fact, obsidian objects have been discovered in archaeological sites dating to 700,000 BC, making glass one of the earliest engineered materials known to man. 

At the same time, glass did not become a household material until much more recently. During the middle ages, you needed to go to a church to behold the splendour that was stained glass. 

Today, however, we have an explosion of various types of glass around us. We drink from soda lime glass. We wear lenses made of Pyrex or flint glass. We dress our buildings in float glass, crown glass and laminated glass. We heat chemical reagents in borosilicate glass. Automotive glasses are tempered. Chemically strengthened glass, such as Gorilla glass, has been developed especially for smartphones. 

There are glasses that come in all colours and are selectively transparent in the electromagnetic spectrum. There is UV glass and infrared glass. There is even electrochromic glass that can be made to change colours at the press of a button.

Soda lime silica glasses

Technically, glass isn’t a material but a state of matter. Just as all materials can be solid, liquid or gas, many materials can be a glass. 

Most of our glasses start from sand that is melted to a liquid at high temperature. This molten sand is made to cool down rapidly – so rapidly in fact that the molecules in the material don’t have the time to fall into the rigid configuration of a solid. What results is an atomic arrangement that is between that of a solid and a liquid.

Quartz consists of a neatly arranged hexagonal lattice while glassy silica is messy and chaotic. Almost all the electrical, thermal and mechanical properties of glass come from this structure. 

The challenge, however, is that sand melts only at around 1700 degrees Celsius, which makes it expensive and difficult to work with. Adding sodium carbonate (soda) brings down the melting point to 1300 degrees Celsius, creating a soda glass. 

However, this glass is soluble in water, which makes it useless for a large number of applications. 

The addition of lime solves this problem by creating a chemically stable mixture. This type of glass with around 70 per cent sand, 18 per cent soda and 12 per cent lime (calcium oxide) is, therefore, called soda-lime-silica glass. It is today the most prevalent type of glass in the world forming the bulk of our containers, windowpanes, bottles and jars.

High-performance thermal glasses

Typical soda-lime glasses cannot withstand sudden variations in temperature. If you pour boiling water into a glass bottle, it will most likely break. 

Adding boron oxide has been found to improve the thermal properties of glass while retaining its transparency. This is vital, for example, when you need to observe an exothermic reaction in a test tube. Borosilicate glass allows observation without the danger of a chemical spill. 

Today, the introduction of glass-ceramics has resulted in glasses that have outstanding heat conduction and thermal shock-resistant properties. For example, the NEXTREMA glass-ceramics from Schott AG can withstand temperatures of up to 950 degrees Celsius with an overall thermal expansion of less than one per cent. 

Glass-ceramics have tiny ceramic inclusions embedded in an amorphous glass matrix. The ceramic inclusions suppress thermal expansion and provide high heat conductivity which together ensure that these materials can withstand huge changes in temperature. The overall volume of these inclusions is usually less than one millionth the total volume, ensuring that the glass remains optically transparent. 

Like the children of the forest, we have come a long way from dragonglass. To find a glass that’s suitable for your application, visit www.matmatch.com.