Blades and Walls of Fire

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I think it actually has a lot to do with the surface area of the organic material.

Now you're just being nit-picky!

Of course, there is a lot more to it then just a percent ratio, such as the molarity of the acid mixture, the type of organic material, the acid itself, whether the acid is poured on the material or the material is dunked into the mixture's container, blah blah blah.

My point is, that two acid fogs or two acid arrows should be stacking in that the actual ratio of acid is increased. A solution of 5 M sulfuric acid squirted from a glass squirtgun will do more damage the more you squirt someone, as Missing Minds stated above. A sulfuric acid mist where you have 1,000 parts per billion will do X damage, therefore 2,000 parts per billion will logically do more then X damage.

Edit: As a note, I think that Wall of Fire should not stack with itself. But I do think that Blade Barrier should stack with itself.