PathFinder, Mithril Shield

[Griffin]

I was reading around in the d20pfsrd while I was thinking on the build of my paladin, which I intend to create as a master of shields (with feats suchs as shield master, shield slam, saving shield etc, etc.) however at some point I realized the cost to fight with a weapon and heavy steel shield, which means that I will have -4 on each attack roll until I get the Shield master feat, but then again my main weapon (or weapon in case I used the shield as main hand) will still has the -4 (considering the main weapon to be a short spear or a heavy mace, maybe even a long sword, but not likely)

However, then I remembered the special materials part and I checked Mithril, it does say that armors made of mithril are one category lower, but, does this applies on shields?

The rule also says that it doesn't affect weapons, but a shield is an armor and weapon at the same time, so I'm not sure how this will work, because, if it does reduce the shield category, it will be reduced to light, as the following shield downwards is the light steel shield

[Blisstake]

Rules as written, applying mithral to a shield would only reduce the arcane spell failure, armor check penalty, and the maximum dexterity bonus. It would also adjust the weight, but not change any categories that the shield falls into (it doesn't fit inside the traditional light/medium/heavy armor classifications, and it explicitly doesn't affect the classification of any weapons.)

That being said... try talking with your DM about it.