Seems the Mystic Forge is similar to a recyling bin for items hehe:
“The Mystic Forge uses a combination of random and non-random recipes. For example, if I throw four rare swords into the forge, it will return a random sword to me with a chance for that sword to be substantially better. There are also specific items that can be created by throwing four very specific items into the forge with no random involved at all. Players should look for clues hidden around the world and in items for what combinations might work. ” ~massively
http://massively.joystiq.com/2012/06/05/massively-exclusive-eric-flannum-talks-guild-wars-2-beta-change/