I noticed a few of the materials don't list if/when/what bonuses apply for shields when logically you'd think they would.
I.E. fire forged steel, would you apply the armor bonus, the weapon bonus, or neither?
Logically I'd say the benefit, if any, for say a fire forged large shield should be at least as effective as light armor. RAW, it seems shields seem to be forgotten half the time.
RAW has nothing to do with logic, it is mostly balance in the view of the designer.
Also, if you care crafting a shield for offensive purposes, it is a particular type of weapon and so you would use that category for creation purposes. If it is armor, then if the material has no listing for 'shield' it provides no mechanical benefit.