Image: Amanda LaPergola's series of drawings show her reaction to "The Rains of Castamere." Jen from Cocktail Party Physics confesses to "two full days lying on her virtual chaise in a stupor," but does she give up on GoT? No way, she seeks comfort in science: What makes dragon glass so special? How hot does dragonfire have to be to burn Harrenhal?
Like everyone else, physics buzz blog is also on a GoT-roll:
Though it is said that Balerion's fire was so intense it burned black, really the hottest of flames is white. Assuming his flame was in fact white hot, as all available art seems to indicate, it burns with a minimum temperature of around 2800 degrees Fahrenheit. Most castles in the UK were built with granite so assume the walls of Harrenhal in Westeros are also made of granite. The melting temperature is around 2300 degrees Fahrenheit so the walls of the castle, if exposed to continuous white flame, could actually melt. Not just crack and crumble; become liquid.
How soon can the dragons melt melt the walls of the Red Keep of King's Landing? Not yet. Their flame is still orange, and if the depiction is accurate, orange flames burn at about 2000F, not hot enough yet. It seems dragon fire gets hotter as they mature.
The same temps needed to melt granite walls can create dragonglass. Obsidian is a mineral created "when lava rich in feldspar and quarts is cooled very quickly and there isn't enough time for the molecules to align neatly and form a crystal. Instead, the molecules harden in a disordered fashion and become glass." Aside from magic, dragonglass kills a White Walker because White Walkers apparently have a very low core temperature. Steel becomes brittle at -120F and shatters. Obsidian doesn't shatter in cold temps, as long as the hit is straight on without shearing.