Just like the title says.
If you can't why not and is there a work around or do you house rule it?
Could you put one (armour rune) on a shield boss or spike and put THAT on a shield?
No. An armor potency rune could not go on any of those things.
Because they are not armor.
Just like the title says.
If you can't why not and is there a work around or do you house rule it?
No.
Shields All magic shields are specific items with a wide variety of protective effects, as described in their entries. Unlike magic armor, magic shields can’t be etched with runes.
Could you put one (armour rune) on a shield boss or spike and put THAT on a shield?
No.
Shield boss and spike are weapons that will accept weapon runes, not armor runes.
Shields were modified to provide extra tankiness not in pumping AC so that you're unhittable (although it does improve AC with the raise shield action), but by giving you options to significantly reduce the damage you take.
Pumping AC in 1e led to some players being challenged only by super high attack bonus enemies that absolutely shred everyone else, leading to un-fun fights where you either automatically get hit or never get hit.
Shields were modified to provide extra tankiness not in pumping AC so that you're unhittable (although it does improve AC with the raise shield action), but by giving you options to significantly reduce the damage you take.
Pumping AC in 1e led to some players being challenged only by super high attack bonus enemies that absolutely shred everyone else, leading to un-fun fights where you either automatically get hit or never get hit.
Yes, the design philosophy in between editions has more or less changed to "if you have level appropriate armor nonethless expect to get hit more often than not". So armor is extremely less about not getting hit and more about not getting crit. And the additional AC from the shield helps here too.