Artifact Armor is the set of armor which provides each job with its classic "Final Fantasy" look. In addition, Artifact Armor increases the stats and abilities that are typically seen as definitive to that job. Each set is unique to each job, and are obtainable only through a series of job-specific quests.
The first piece of Artifact Equipment is the Artifact Weapon, which is obtainable at level 40. The remaining pieces of Artifact Equipment are armor pieces for the head, body, hands, legs and feet, and are obtainable at level 50, and equippable throughout levels 50-60.
For original Final Fantasy XI and Rise of the Zilart jobs, four of the pieces are obtained through quests, and two of the pieces are obtained though Treasure Coffers. For Treasures of Aht Urhgan and Wings of the Goddess jobs, two of the pieces are obtained through quests, and the remaining three are obtained by trading materials to specific NPCs: four of the five trade crafting items as in a normal synthesis, while Scholar "trades" key items obtained in specific zones.
Artifact Armor Quests
The Artifact Weapon is obtained via a quest. You must be at least level 40 in the relevant job to activate this quest. In order to obtain the quest, you must talk to the starting NPC as the relevant job. This AF piece is always a weapon (except for Scholar which has a spell Klimaform, and Puppetmaster which has the Animator). Once you have activated the quest, you may complete it as any job.
First Armor Piece
The first piece of Artifact Armor is usually level 52 and is obtained via a quest. You must be at least level 50 and have completed the AF weapon quest to activate this quest. In order to obtain the quest, you must talk to the starting NPC as the relevant job. You may complete this quest as any job. (Note that for both the Ranger and Thief jobs, the piece is actually level 54, with the hands being level 52; this has no effect on quest order, as the first armor piece quest must still be started before the hands quest.)
The Artifact Hands are obtained via a quest. You must have the first armor piece quest at least started in order to obtain this quest. You can only have one Artifact Hands quest active at one time, regardless of job, and this quest must be completed before you may activate an Artifact Hands quest for another job. In order to obtain the quest, you must talk to Guslam at the Durable Shields shop in Upper Jeuno. Unlike the other Treasure Coffer 'quests', you can complete all parts of this quest as any job, including opening the coffer for the Old Gauntlets.
There are two other pieces of Artifact Armor obtainable from Treasure Coffers. These AF pieces are linked only to the activation of the first Artifact armor quest for that job. Unlike the other parts of the quest, you must be the correct job while opening the coffer in order to obtain your Artifact Armor.
The last piece of Artifact Armor is obtained via a quest. It is usually the level 60 piece, but in some cases it is the level 58 piece when the hands are the level 60 piece of Artifact Armor. You must be at least level 50 and have the first armor piece quest completed to activate this quest. In order to obtain the quest, you must talk to the starting NPC as the relevant job. You may complete this quest as any job; however, certain portions of some of the quests require abilities that only those jobs have, so in some cases it may be necessary to use the appropriate job. The Dragoon last armor piece quest requires the use of Call Wyvern, and the Black Mage one requires the use of Manafont.
ToAU/WotG/SoA Artifact Armor
Artifact Armor acquisition differs slightly for the three jobs added in the Treasures of Aht Urhgan expansion (Blue Mage, Corsair, and Puppetmaster), the two added in the Wings of the Goddess expansion (Dancer and Scholar) and the two from Seekers of Adoulin expansion (Geomancer and Rune Fencer). As with other jobs, some Artifact pieces are acquired via quests. However, instead of the rest being found in coffers, you trade materials to a specific NPC to have the piece crafted (except Scholar, which requires obtaining certain key items at Indescript Markings in the field). Also, unlike every other job, the artifact armors from the two jobs of SoA are all level 99.
If you need to re-acquire a piece you threw away or upgraded to Artifact Armor +1, you can repeat the artifact quests by talking to Vingijard in Lower Jeuno. You do not need to discard your other AF first. You may reset the quests and start them again, even if you still have all of your original AF. But you will need to either discard your af set or store it at an Armor Depository. You must have completed all the quests and opened all the coffers for a particular job before you can reset the quests.
To upgrade your Artifact Armor to Artifact Armor +1, talk to Sagheera in Port Jeuno (J-8) near the Windurst Dock. She will offer to repair your worn Artifact Armor and make it all shiny and new. To do so, you will need to bring her your artifact armor, some ingredients dropped from Limbus and a crafted item. In addition, she will charge you a fee for her services in Ancient Beastcoins.