In the Hobbit films, the 13 dwarves, along with Bilbo Baggins have a very narrow escape from being cooked alive by mountain trolls. At the last moment, Gandalf the Grey wizard comes along and saves them by hewing a stone and exposing the trolls to sunlight, which turns them to stone. Once the danger of the encounter has passed, they realize that if there are trolls around, then there must be a troll hoard hidden in the nearby cave.
When they go inside to explore, they find Sting, the sword that saves Bilbo and Frodo’s life on many occasions, as well as Glamdring and Orcrist. He explains to his companions that they are made of no ordinary steel and that ‘These were forged in Gondolin, by the high elves of the first age. You could not wish for a finer blade.’ He then gifts sting to Bilbo, saying that it will help keep him safe by glowing blue when orcs or goblins are near. So what is it about these words that makes them glow?
First, it is important to note that not all elven swords glow, it is only a few select and very precious blades, the ones forged in Gondolin. There are a few theories about why these particular blades might glow in such a way. Many Lord of the Rings fans believe that it might have something to do with the runes that are inscribed upon the metalwork. In the same way that the invisible markings on the One Ring of power can only be seen when the letters glow, the same could be said for the illumination of the swords. Many of the swords are described as having faint light around their edges, but the original texts of Tolkien never specify whether it is the metal itself that is glowing, or a specific part of the sword such as the fine and intricate lettering often found on the hilt or the fuller of the weapon.
This is an interesting concept when referring to Narsil, the blade that was broken when it cut the ring from the finger of Sauron during the battle of the Last Alliance. There is no mention of the sword having any phosphorescence when it is wielded by Isuldur, the original owner of the blade, and one of the kings upon whom the Argonath are based. But when the blade is renewed and reforged in Rivendell, and its name and lineage were set upon its length in elvish runes, the blade is described by Aragorn as glowing with a soft light like a flame around its edges. This supports the idea that it is the runes themselves that glow, as the sword only glows after these additions are made, but Anduril shines with a sort of reddish hue in the sunset and a pale white in the moonlight, rather than the traditional blue glow to warn that orcs or goblins are near.
However, some other fans have much more sinister ideas about why the weapons glow when the enemy is near. There is a theory that the light comes from infusing drops of orc or goblin blood into the blade whilst it is being forged. It is said that the drops of blood react with the different metals in the blade, which changes the color of the pigment inside them. In this way, a little bit of the essence of the evil creatures remains in the blades, in the same way that a part of the essence of Sauron remains in the One Ring, which allows him to endure, and to flee to Angmar where he is able to regain his strength until he can resume physical form.
However, the essence included in the elven blades is mingled with their own power and will, which is thought to turn the orc blood into a deadly weapon against the enemies, which is why they are so good at cutting down evil creatures, and earned names like ‘Goblin-cleaver.’ Tolkien never actually specifies within the writing of his works why the blades glow blue to warn of approaching enemies, but there is also a resemblance to the doors that can only be seen when moonlight is upon them.
Celebrimbor was an elven smith, creating beautiful crafts from metal, wood, and stone. Alongside Narvi, a Dwarven smith from Moria (before it was taken over by the Balrogs) Celebrimbor created the intricate stone doors that are the entrance to the Mines of Moria, and the entrance to Erebor, the lost kingdom that they try to reclaim from Smaug the dragon in The Hobbit. As we can see from when the fellowship sits outside the door, trying to answer the riddle of the elvish inscription, the words are an important part, but perhaps, considering that the doors which glow are made of stone, and the swords that glow are made of steel, and that the swords were made by elves, whereas the doors were made by Dwarves, it is something to do with the ancient craft of smithing, the actual process that goes into making them rather than the objects themselves that give them their luminosity.