Mithral + Masterwork interaction

[Alchemist 23]

All Mithral items are considered masterwork so does that apply the masterwork -1 to armor check? Basically do Mithral items actually have a -4 reduction in armor check or is the masterwork rolled into the normal -3?

Given how screwy Pathfinder can be I'm not sure this an unreasonable question.

[Syries]

Mithral does exactly what it says it does. Mithral is already masterwork. It reduces armor check penalties by 3, increases max Dex bonus by 2 and weighs half as much.

[shaventalz]

Existing published items have stats that say mithral's bonuses already include the masterwork adjustment.

Mithral Full Plate (of Speed) - full plate normally has ACP of -6. This mithral full plate has ACP -3.

Elven Chain - chainmail normally has ACP -5. This has ACP -2.

Unless you hold that "of speed" and "elven" actually increase the armor check penalty, the material portion must reduce the penalty by exactly 3.

[Fuzzy-Wuzzy]

Syries wrote:

Mithral does exactly what it says it does. Mithral is already masterwork. It reduces armor check penalties by 3, increases max Dex bonus by 2 and weighs half as much.

^This.

[Artificial 20]

As you say, the masterwork -1 is rolled into the overall -3. That's why the other numeric benefits of mithral have a value of 2, specifically max Dex is up by 2, and spell failure is down by 10% (where spell failure is measured in 5% increments). The mithral part gives -2 reduction to armour check penalties, stacking with the -1 masterwork benefits for -3 total.