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 and pointy. It was the weapon of choice for the wildlings.
Despite its colour, it gleamed in the sun like metal. It had edges and folds like rock but it shattered on the ground like ice.
That friend of the King of the North – the fat one, Sam they called him – had apparently killed a white walker with dragonglass. By now every child north of the twins knew that it was the only thing besides Valyrian Steel that could kill a white walker.
With death at the door, dragonglass was the only hope for the long night.
The men spoke with wonder of the deep mines from where it came. Glass small and large glinted in the darkness, nestled like jewels in the crevices of ancient rock.
In the legends of the common folk, dragonglass is what you get when rocks are melted by dragon fire. The Old Valyrian for dragonglass, after all, is ‘zīrtys perzys’, which literally translates to ‘frozen fire’. Is it any wonder then that it can kill the dead once more?
The maesters however knew better. It wasn’t the heat of dragon fire that melted the rock, but the liquid fury of volcanoes, bleeding from ancient wounds. They called it by its real name: obsidian.
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 this ‘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, as seen in the image below.
Quartz (seen on the right), consists of a neatly arranged hexagonal lattice while glassy silica (on the left) 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 °C, which makes it expensive and difficult to work with. Adding soda (sodium carbonate) brings down the melting point to 1300 °C creating a ‘soda glass’.
However, this glass is soluble in water, which makes it useless for a large number of applications. Think of windows that dissolve in the rain!
The addition of lime solves this problem by creating a chemically stable mixture. This type of glass with around 70% sand, 18% soda and 12% 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, window panes, bottles and jars.
Molten glass is like bubble gum, in that it can be blown into any shape with heat, air and the right equipment.
The hand-blown ‘crown glass’ was highly popular before machines took over the job of glass blowing. Some of the legendary stained glasses in medieval cathedrals were made from crown glass.
Blowing air into a blob of molten glass expands it into a hollow globe or ‘crown’. The crown is heated and spun rapidly so that it flattens into a sheet that is then cut into rectangles. The spinning inevitably results in the edges being thinner than the center. This is why old stained glass windows have been found to be thicker at the bottom than the middle.
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 °C with an overall thermal expansion of less than 1%.
The video below demonstrates the high thermal shock resistance of three NEXTREMA line glass-ceramics: transparent, opaque and translucent. A sheet of each of these NEXTREMA glasses is heated in an oven to 350 °C. They are then taken from the oven and submerged into cold water. A traditional glass shatters when exposed to such an extreme variation in heat. The NEXTREMA glass-ceramics, however, come out unscathed with no visible marks of their violent past.
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.
To learn more about glass-ceramics, read our dedicated article, Glass-Ceramics: Properties, Processing and Applications.
Bran the builder to Bran the broken
Like the children of the forest, we have thus come a long way from dragonglass. Like the people of Westeros and Essos, we build cities with our materials and paint them with our joys, sorrows and longings.
With the millions of metals, ceramics, glasses, polymers and composites at our feet today, we are not the backwaters of the seven kingdoms. We are the inheritors of Valyria: a glorious land teeming with magic and marvels.
As the story that began with Bran the builder ended with Bran the broken, we write for ourselves an even greater epic. The centuries-old, ever-growing story of our materials.
I work in the cutting edge area of applying machine learning and artificial
intelligence to one of the earliest endeavours of human civilization – in understanding, exploiting and developing new materials.