Can a strong wide beam flamethrower topple/collapse a stone wall?

Answer

We don’t often see a fire-breathing dragon walking down the street…but what if we did? The makers of Game of Thrones can already imagine it vividly. The walls of the Red Keep and the houses in Kings’ Landing are shattered by dragon fire during the final season. [Spoiler Alert]

When the bells of Kings’ Landing herald the city’s surrender, something breaks in Daenerys Targaryen (Dany to the friends). The city that cost her Rhaegal and Missandei hoped to escape the pain of destruction and arson. The Red Keep is completely untouched up to this point and is staring at Daenerys as a personal challenge. This is when the gods flip a coin, and Daenerys decides to launch a devastating flight over the city with Drogon. Dracarys…fire. Drogon makes his way through the city like a living flamethrower, with one building after another spectacularly shattered by the dragon fire. The Red Keep does not escape this fate either. Touched by the fire of Drogon, the masonry explodes in a fantastic and cinematographically sound way. An impressive image that alludes to an enormous power that would be present in the flame beam produced by Drogon.

However, during Drogon’s long devastating flight, there’s a little voice somewhere in the back of your mind that’s getting bored and wondering aloud, “All well and good, but that can’t be in real life, can it?” And before you know it, you’re on the trail: “Dragons don’t really exist, so…”. But let’s assume that dragons do exist [1] and spit fire [2]. Is it then to be expected that the houses of Kings’ Landing will come to an explosive end? This depends on a number of factors: (1) how hot is the dragon fire and (2) what type of stone the buildings are made of.

Let’s start with the dragon fire. The temperature of this can, as for any other object, be determined on the basis of the heat radiation. The higher the temperature, the shorter the wavelength of the emitted light. With an infrared camera we can visualize this radiation from our own body, while a metal rod heated in the furnace of a forge glows red hot. Drogon’s flames are orange-red in color. Do we use Planck’s formula for a black radiator [3] then we discover that at a temperature around 5000°C the radiation with an orange-red color has the highest intensity, and at a temperature of more than 6500°C this maximum lies with radiation with a blue color. Strange but true, the fire of the zombie ice dragon Viserion is therefore hotter than that of Drogon. At this 5000°C, however, two important remarks have to be made. (1) Our eyes see only a very small part of the electromagnetic spectrum, this part we call the visible light. We cannot see everything that falls outside of it (think of the infrared from just now). This means that for temperatures where the maximum of the curve in the equation is outside of visible light, we will see the color (if intense enough) that is within visible light. So an object with a temperature of 20000°C (radiates most in the ultra-violet) we will see as blue, while an object with a temperature of “only” 1000°C we will see as red. (2) The color of a flame depends not only on the temperature but also on what is burning. In a combustion process, the molecules of what burns are ionized. Because of this they will also emit light [4]. This is why a wood fire or a match shows a yellow-red flame without reaching a temperature of 5000°C, or why a gas stove with blue flames is no hotter than the surface of the sun.

With these two caveats in mind, the temperature of the Drogon-built flame column can be estimated at a meager 1000-1200°C while Viserion’s blue flames reach a slightly higher temperature of 1500-1600°C. Now that we’ve made this estimate, it’s interesting to note that wrought iron, which is used in sword making, has a melting temperature of at least 1482°C.[5] So Drogon will have to step up a gear if he wants to melt the Iron Throne (oops).

Now that we have a better idea of ​​the temperatures we’re dealing with, we can look at the impact on Kings’ Landing’s architecture. The brick as we know it has a very long history.[6] It is also one of the simplest and most common fire retardants. Because bricks themselves are manufactured at a high temperature, since they are fired at a temperature of 800-1100°C, they can also withstand the same temperature. (Ironically, slow seasonal temperature change is considered one of the main enemies of the brick…but that’s another story [6]). This means that brick buildings (the basement of the Red Keep, under which Cersei and Jaime are buried, was clearly brick) must be able to withstand Drogon’s fire to some degree…at least the bricks. In contrast, the mortar that holds the bricks together will expand and crack, causing collapse.

What if the houses are built of natural stone? We all know the granite steps of older buildings or the marble fireplaces. But sandstone and limestone are also often used in building. Think of medieval castles such as the Gravensteen in Ghent, which is built from limestone. In the case of Kings’ Landing, we can take a look at the building materials of the real film location (Dubrovnik, Croatia), and then the old city turns out to be mainly built of limestone.[7] Although these rocks themselves can withstand high temperatures, porous rocks can explode when heated. They do this not so much because of their own composition, but because of the presence of water in the pores. In softer rocks such as limestone and sandstone, this porosity allows water to penetrate into the rock. When this rock is heated very quickly, the trapped water will heat up and evaporate very quickly. The pressure built up as a result can become so great that the stone in question will break or, in extreme cases, explode.[8]

Thus, the jet of fire produced by Drogon may well be capable of explosively destroying the limestone buildings of Kings’ Landing, if they contain sufficient water and are heated sufficiently quickly.

dracarys…

[1] Where do the dragons come from. https://www.smithsonianmag.com/science-nature/where-did-dragons-come-from-23969126/

[2] Although there are no fire-breathing animals on earth, there are specimens that come pretty close, such as the bombardier beetle. https://www.youtube.com/watch?v=TgqF-ND2XcY

[3] https://en.wikipedia.org/wiki/Black-body_radiation

[4] https://en.wikipedia.org/wiki/Flame

[5] https://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html

[6] “Long-term Performance and Durability of Masonry Structures” FM Fernandes, https://doi.org/10.1016/B978-0-08-102110-1.00001-7

https://sci-hub.se/10.1016/B978-0-08-102110-1.00001-7

[7] https://en.wikipedia.org/wiki/Walls_of_Dubrovnik

[8] This exploding usually consists of a fairly unimpressive pop, after which the stone is cracked into two or more pieces. There are several examples of this on youtube.

https://outdoors.stackexchange.com/questions/801/how-to-avoid-exploding-rocks