Wall of Stone indicates that a 5x5 square of stone has Hardness 8 and 15 hp per inch of thickness, while Wall of Iron says that such a square of iron has Hardness 10 and 30 hp per inch of thickness.
Therefore a sheet of Iron 1/30th of an inch thick has 1 hp and 10 Hardness, while 1/15th inch of stone has 1 hp and 8 Hardness.
When an attack breaks through one layer of a wall, the hardness of the subsequent layer is taken off the breakthrough damage before being applied to the next layer.
A wall of 10 slices of stone sandwiched between 10 sheets of iron is an inch thick, has 20 hp, and a semi ablative damage hardness of 180. It is Fully capable of stopping a 200 damage attack before being destroyed, but is somewhat vulnerable to weaker attacks. If a single attack does 11 damage, it will break the first layer, exposing those behind it, dropping the stats to 19hp; hardness 170. If one attack does at least 20 damage, it breaks through the first 2 layers, etc. etc.
Manufacture:
- Cast Fabricate to turn a pile of Iron into 20 gauge sheets
- Slather a very thin layer of mud onto the backs of the sheets
- Press the sheets together, with the mud in between
- If the Armoring is destined for a use with a curved facing, such as a ship hull, flex the sheet metal mud thing into an appropriate shape.
- Cast transmute mud to rock on the composite
a ninth level wizard can Fabricate enough sheet metal for 8 5x5 panels with one casting.
A single transmute mud to rock can cover 960 of these panels per level
if you use wall of Iron for raw materials, one casting supplies 36 panels at lvl 9.
Benifits:
Non magical, can't be dispelled
made with cheap materials
permanent
high damage resistance
can be moved, unlike a wall of force.
appears as a normal iron plate.
successive layers grant protection from rusting.
use gm fiat against him. if he says that shatter affects the whole thing, then make whole should repair the whole thing, including layers already lost. if the opposite, spells won't have line of effect except on the front layer.
costs no xp
Cons:
Some DM's may not let you take knowledge (Materials Science) or Craft (Composite Armoury)
complicates taking damage quite a bit. (I simplify, and assume every full 10 damage on an attack breaks through 1 layer.)
not as quick as a wall of iron, or as durable as a permanent wall of force