Mages have three armor spells, each geared toward protecting us in different way. Of course, since we're mages, we also found ways to cram damage increasing aspects into them. The trick is learning which spell to use and when. The choice is largely determined by which spec you're using and which aspect of the game you happen to be playing.
The first armor spell we master is the one associated with the fire tree at level 34. Molten armor is our pure DPS armor. It increases our critical strike chance by a flat 3%. Deciding when to use it is fairly simple.
This spell is focused on putting out damage without regard for defense, mana, or anything else. If you're playing a fire mage (who generally don't suffer from mana issues) or a frost mage in a PvE environment, you're going to want to use this armor.
In longer fights, or in mana-intensive encounters, you may find yourself needing to switch to Mage Armor, but in most cases, it's better just to stick with Molten Armor when raw damage numbers are the goal. Unless you're consistently running out of mana in an encounter, you simply can't afford to lose that 3% crit.
Defensively, the benefits of Molten Armor are nice but situational. The buff reduces your chance to be critically hit by 5%, essentially giving you a large chunk of resilience. It also does a small amount of fire damage to anyone who attacks the mage. In most cases, you're simply not going to be throwing up mage armor for its defensive qualities. This is an offensively geared buff, and you'll be using it for that purpose.
Our primary defensive armor spell, Frost Armor is learned at level 54. Since this spell is being reworked in patch 4.1, and since patch 4.1 is relatively imminent, we'll be talking about the abilities of this spell post-patch, rather than its current incarnation.
Defensively, Frost Armor is the armor of choice. In just about every PvP situation, this is the armor spell you're going to want to use. Look down. Is someone stabbing you? Yes? Then you should be using Frost Armor.
In patch 4.1, this spell will grant you a flat 15% reduction to damage taken, instead of the significantly crappier 20% armor increase it provides now. I know a lot of tanks who would gladly give up one of his less-vital appendages for a buff like that. It's a no-strings-attached damage reduction, and because it's changing to a flat percentage, we never have to worry about scaling issues -- 15% is always going to be 15%.
It also provides the rather nifty bonus of inflicting a slowing debuff called Chilled on anyone who strikes you. This reduces your enemy's movement speed by 30% and his attack speed by 20% (down from 25% after the patch -- but the good news is that the attack speed reduction will also affect ranged attackers). Immeasurably handy, this slowing effect is of incredibly high value in PvP and while soloing. It provides kiting control and escape opportunities galore.
Of course, the downside to Frost Armor is that it provides almost nothing offensively. This is a survivability spell, nothing more. The Chilled effect currently provides opportunities to proc Fingers of Frost, but that small bonus is going away once the patch hits. Not that you're going to care much when that rogue is knifing you. You're just going to be happy you have a chance to hitch up your skirts and flee.
The final mage spell we learn (at level 68, just in time to take with us to Northrend), Mage Armor is the armor spell for arcane mages. Its major contribution to cause of blowing things up is constant mana return, to the tune of 3% of your maximum mana every 5 seconds, always, forever. For fire and frost mages, this is a reactionary armor, selected only when the circumstances of the encounter dictate that mana is going to be an issue. But for arcane mages, this is a way of life.
Since Mana Adept equates more mana into more damage, any mana return translates directly into a damage buff. Would the 3% extra crit from Molten Armor also help arcane mage damage? Yes, but only until the arcane mage ran out of mana 3 seconds later. Mage Armor is, quite simply, the only choice for arcane mages in PvE.
Defensively, Mage Armor provides some pretty impressive magical protections. It adds an extra 44 points to all of your magical resistances and reduces the duration of all magical effects on you by 35%. Though nice, this is an option only in situations when you know you'll be taking large amounts of spell damage and very little in the way of physical damage. Since that scenario is incredibly rare and almost impossible to predict in PvP, Frost Armor is usually still going to be your best PvP bet. Still, if you manage to see that magical CC spell coming in time, and you're out of CC breaking abilities, and your reaction time is a whole lot better than mine, it might be worth your while to throw up a quick Mage Armor to reduce the time you're affected.
Post patch 4.1, our armor spells will no longer cost any mana whatsoever. In fact, in recent years, these spells have become less like actual spells and more like simple class features. They're instant, cost no mana, and they can't be dispelled or stolen.
Thanks to that removal of any mana cost, it's now a more viable option to switch armors on the fly. This makes such concepts as switching between Mage Armor and Molten Armor in longer fights and switching from Frost Armor to an offensive armor in PvP and back again a bit more realistic.
Right now, only Mage Armor and Molten Armor have specific glyphs, but patch 4.1 amends that. In general, since you'll always have your main armor spell up, it's a good idea to use one of your glyph slots for the respective glyph for that armor spell. These are all highly effective glyphs, and the new Frost Armor glyph is definitely intriguing.
Glyph of Molten Armor This is a flat damage increase, lifting the crit chance bonus from 3% to 5%. More than worthwhile, this one's a no-brainer.
Glyph of Mage Armor Again, if you're an arcane mage who uses Mage Armor (and if you aren't, why not?), this is required glyph. It causes your Mage Armor to regenrate 20% more mana every 5 seconds, lifting that 3% of your max mana to 5%. Mage Armor combined with this gflyph are the only things that allow the Arcane Tree to continue to function in this post-Mana-Adept era.
Glyph of Frost Armor The new glyph for the coming patch, this one is a counter to the tendency of some mages to feel the need to switch between Frost Armor and Mage Armor in PvP for mana purposes. By spending a major glyph slot on this, mages will now be able to enter PvP with their best defensive armor spell up and still be getting a decent amount of mana return (2% of max per 5 seconds). This translates directly into more dead warlocks per match. I haven't done the math on that, but I believe it to be true.
Glyph of Armors A nice little minor glyph, this one is a good option for any mage. It increases the duration of all of our armor spells by 30 minutes, putting them at a full hour. Which is nice. Hey, it's better than most minor glyphs.
Every week, Arcane Brilliance teleports you inside the wonderful world of mages and then hurls a Fireball in your face. Start off with our Cataclysm 101 guide for new mages, then find out which spec is best for raiding, get advice from the poor mage's guide to enchants, and learn how to keep yourself alive.