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What is the price and armor check penalty of a Mithral Breastplate?

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Well, you have the breastplate and the effects of mithral, which you combine to produce a final mithral breastplate:

\begin{array}{l c} \text{Armor} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline \text{Breastplate} & \text{Medium} & 200\text{ gp} & +6 & +3 & -4 & 25\% & 20\text{ ft.}, 15\text{ ft.} & 30\text{ lbs.} \\ & & & & plus \\ \text{Material} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline \text{Mithral} & -1 & 4,000\text{ gp} & 0 & +2 & +3 & -10\% & \text{(varies)} & \text{(half)} \\ & & & & equals \\ \text{Final Item} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline \text{Mithral} \\ \text{Breastplate} & \text{Light}\dagger & 4,200\text{ gp}\ddagger & +6 & +5 & -1\ddagger & 15\% & 30\text{ ft.}, 20\text{ ft.} & 15\text{ lbs.} \\ \end{array}

Unfortunately, they don’t tend to put special materials in neat tables like the one I just made for Mithral, so you have to read carefully to determine its effects, but generally speaking the process is the same as what I’ve done here: you figure out what the material modifies, and just apply each one.

\$\dagger\$ In Pathfinder, mithral does not change the armor’s proficiency requirements, so even though the mithral breastplate is a light armor, you still need proficiency in medium armors to use it properly. This is a new rule in Pathfinder that did not exist in 3.5, and I recommend ignoring it.

\$\ddagger\$ Note that mithral items are always masterwork and the price of that is included in the 4,000 gp, so you don’t have to pay for it separately (i.e. another 150 gp).

About Armor Check Penalty, the rules are less clear: they never explicitly say that the Armor Check Penalty reduction of the mithral includes the Armor Check Penalty reduction of being masterwork armor. The rules for adamantine armor are a little more clear on this point, and the formatting of each material should be consistent, so that’s a clue, but not explicit. Consulting with various example mithral armors,1 it appears that it is included (i.e. the total reduction in Armor Check Penalty is −3, not −3 for mithral and −1 for masterwork) is another clue, though that’s not necessarily useful since the mithral shirt also weighs less than it should (10 lbs instead of 12½ lbs).

So by strict RAW, this isn’t exactly definitive; the primary source on this is the rules text for mithral itself, which is ambiguous. You could make an argument that the masterwork effect is not included, and then that the example armors are mistaken or intentionally different from what the usual rules would provide, and also that nothing says that adamantine and mithral have to handle this the same way. But that’s a lot to claim, and since the rules for mithral are merely ambiguous, rather explicitly in your favor, I’d be shocked if any DM bought it, but the argument could be made and I might buy it for a TO exercise or something. But then I’d also consider houseruling that you get both (i.e. ACP 0) just because I might be convinced things are better that way.

  1. Thanks to mxyzplk to pointing those example armors out.
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You might want to link the pfsrd rules for mithral ( - also, the ACP should be -1; looking at some standard armors like the mithral shirt and mithral full plate of speed confirms that calculation. Adamantine etc. work the same way (and its entry is a little more explicit on this note). – mxyzplk Jan 11 '14 at 19:21
@mxyzplk Derp, didn’t even think to check example armors, good call. Link’s a good idea too. – KRyan Jan 11 '14 at 20:04
Note that the example Mithral Shirt… also weighs two and a half pounds less than would be found by applying the "mithral" rules to the "chain shirt" base armor. – Matthew Najmon Feb 16 '14 at 0:05
It should be noted that a mithral breastplate still requires medium armor proficiency: "This decrease does not apply to proficiency in wearing the armor." So it's more of a weight class light* – MrLemon Jun 28 at 22:06
@KRyan I agree, it is a very awkward rule, and entirely pointless to boot. – MrLemon Jun 28 at 22:11

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