Many characters were embroiled in heated scenarios in Sunday night’s Game of Thrones finale, but Zombie Viserion took the flaming cake. Dany’s dead dragon-child, whose eyes turned ice blue after the Night King felled him with an ice spear in Episode 7, stunned White Walkers and Jon Snow’s squad alike when he torched the Wall with a stream of blue fire.
Judging by how rapidly they tore the Wall down, those flames were incredibly potent — much more so than flames produced by Dany’s other dragons. It makes scientific sense: Blue flames are generally hotter than orange, yellow, or red ones, and the difference in temperature has a lot to do with the type of fuel that feeds the fire. Viserion’s new hue, it seems, is a result of Night King-induced changes to his inner fuel source.
Flames are an indication of how much heat a chemical reaction is producing, which itself is dependent on the type of fuel being burned and the environment in which it’s burning. Two elements are necessary for combustion to take place: fuel and oxygen.
The warm-colored flames we usually see spew forth out of Dany’s dragons are a side effect of incomplete combustion, which is what happens when a fire doesn’t burn as hot as possible. Incomplete combustion usually happens when the fuel being burned is wood, oil, or coal, and there isn’t enough oxygen available to create a super hot flame.
In these situations, a process called pyrolysis occurs before full combustion takes place. Pyrolysis is the process by which the warmed-up fuel begins releasing some of its component chemicals as gases. Ultimately, it’s the burning of these chemicals that leads to different colored flames.
Yellow and orange flames are usually the result of tiny bits of carbon (basically soot) burning in the heat. Drogon, Rhaegal, and pre-icing Viserion, it seems, had relatively low-intensity combustion going on in their guts, which led to their burping up of flammable bits of carbon.
But not Ice Viserion, whose blue flames belie complete combustion, which can only occur when there’s plenty of oxygen available to allow a flame to get super hot, and the fuel being burned doesn’t release too many additional chemicals during pyrolysis that might lead to a different colored flame. Hot flames release electromagnetic radiation (known as black body radiation emission) in the visible part of the electromagnetic spectrum, and as temperatures increase, the colors they emit shift from red to orange, yellow, and, lastly, white-blue.
Physics suggests that Viserion’s inner chemistry changed when he was struck by the Night King’s ice spear: We can assume that whatever fuel once fed into the combustion process inside him has given way to a type of fuel that’s purer and more flammable. (We can also assume that the availability of oxygen inside his torso has remained the same.)
In 2013, an article in Discover speculated that the fuel yellow-flamed dragons use in their combustion reactions is diethyl ether, a flammable compound that could feasibly be synthesized in the body.
A good bet for what might be brewing inside Zombie Viserion is propane, which — as anyone who’s grilled on a gas barbecue or cooked over a camp stove can attest — tends to burn blue, too.