SRD:Masterwork Armor

Masterwork Armor
Just as with weapons, you can purchase or craft masterwork versions of armor or shields. Such a well-made item functions like the normal version, except that its armor check penalty is lessened by 1.

A masterwork suit of armor or shield costs an extra 150 gp over and above the normal cost for that type of armor or shield.

The masterwork quality of a suit of armor or shield never provides a bonus on attack or damage rolls, even if the armor or shield is used as a weapon.

All magic armors and shields are automatically considered to be of masterwork quality.

You can’t add the masterwork quality to armor or a shield after it is created; it must be crafted as a masterwork item.