What Armor Optimization applies to a Mithral full plate?

Question

The Description of Mithral Armor describes this effect:

Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light.- SRD

The Heavy Armor Optimization feat says:

When you are wearing heavy armor, lessen the armor check penalty of the armor by 1 and increase the armor bonus by 1

There are also feats with identical wording for Medium and Light Armor.

SO

If my intention is to wear Mithral heavy armor would I take the Heavy Armor Optimization or Medium Armor Optimization?

Answer 1

In addition to the other answers, I would say that from a role-playing perspective it makes sense that Heavy Armor Optimization fits. The description of the feat is

You have trained extensively in heavy armor, and you have learned to take advantage of the protection it offers.

To me this implies that your character has learned to use the extra protection of the armor to their advantage when fighting. Some examples of using this to your advantage might be

• Heavy gauntlets on your hands allow you to use your hands to deflect blows
• If a swing or thrust is directed to an especially resilient place (i.e. your chest) you know that it will deflect easily and won't need to brace yourself as much
• You can focus your dodging to moving your vulnerable spots away from the attack rather than moving your entire body.

Any of the above still applies to Mithral Heavy Armor. Even though it weighs less, you can still capitalize on the advantages you get by being more armored.

Answer 2

The Sage says that mithral heavy armor is medium armor even in this case…

The Dragon #355 Sage Advice column “Official Answers to Your Questions” includes this exchange:

Would the Heavy Armor Optimization feats… apply to heavy armors made of mithral?

No.

Mithral heavy armor effectively becomes medium armor for all purposes, which means that benefits accruing to the wearer of the heavy armor wouldn’t apply when wearing mithral heavy armor. (79–80)

(The Sage at the time is Andy Collins.) Thus, according to the Sage, a creature wearing mithral heavy armor can't benefit from the feat Heavy Armor Optimization (Races of Stone 41).

This reader suspects that the Sage is basing this ruling partially on this exchange from the Dragon #335 Sage Advice column “Official Answers to Your Questions”:

Is a character proficient with light armor, such as a rogue, considered to be proficient with mithral breastplate? What about a character proficient with medium armor, such as a barbarian—is he considered proficient with mithral full plate armor?

The description of mithral on page 284 of the Dungeon Master’s Guide is less precise than it could be in defining how it interacts with armor proficiency rules. The simplest answer—and the one that the Sage expects most players and DMs use—is that mithral armor is treated as one category lighter for all purposes, including proficiency. This isn’t exactly what the Dungeon Master’s Guide says, but it’s a reasonable interpretation of the intent of the rule (and it’s supported by a number of precedents, including the descriptions of various specific mithral armors described on page 220 of the Dungeon Master’s Guide and a variety of NPC stat blocks).

Thus, a ranger or rogue could wear a mithral breastplate without suffering a nonproficiency penalty (since it’s treated as light armor), and each could use any ability dependent on wearing light or no armor (such as evasion or the ranger’s combat style). A barbarian could wear mithral full plate armor without suffering a nonproficiency penalty (since it’s treated as medium armor), and he could use any ability dependent on wearing medium or lighter armor (such as fast movement).

The same would be true of any other special material that uses the same or similar language as mithral (such as darkleaf, on page 120 of the Eberron Campaign Setting). (80)

Thus, for example, the Sage would have the creature receive no benefit from the feat Heavy Armor Optimization were the creature wearing mithral full plate. If such a ruling stands in your campaign, and you must still take a feat that applies to your armor, consider instead the feat Armor Specialization (Player's Handbook II 75), Efficient Defender (Cityscape 60), Guerrilla Warrior (Heroes of Battle 97–8), or Improved Armor Dance (Dragon #315 96). (However, if you were aiming for the feat Deflective Armor (RS 137), may I suggest instead the feat Parrying Shield (Lords of Madness 181) or Shield Ward (PH2 82)?)

…But you can take the Sage's advice or leave it

What authority you ascribe to the Sage's rulings is up to you. (See also this question.) Since the feat Heavy Armor Optimization is itself less than optimal, since no feat named Medium Armor Optimization exists, and since no feat offers exactly the same benefits of the feat Heavy Armor Optimization to a creature wearing medium armor, this DM would be comfortable with ignoring the Sage's advice and letting a creature that prefers to wear mithral full plate benefit from the feat Heavy Armor Optimization.

Answer 3

Heavy armor optimization.

It's still heavy armor. It's just treated as medium for movement and other limitations. "Which feats apply" is not a matter of movement or other limitations.

Answer 4

Mithral specifically applies to changing limitations of heavy armor. However, the feat in question does not apply to limitations, but the type of armor. This dm would rule that mithral full plate, while treated as medium armor for movement, penalties, class abilities, etc (since those are all "limiting" classifications), would still fulfill the requirements for heavy armor proficiency since heavy armor does not provide a limitation in this case.