Another idea along these lines is that combat spells work like “counterfactual weapons”—they get through iff a blade would have worked instead. But in any case Harry should have investigated that before trying to level up Str. As per the current version, he probably did check that metal would stop spells (otherwise the whole plan would be mere stupidity); it’s not such a big leap to try styrofoam while you’re at it.
Another idea along these lines is that combat spells work like “counterfactual weapons”—they get through iff a blade would have worked instead. But in any case Harry should have investigated that before trying to level up Str. As per the current version, he probably did check that metal would stop spells (otherwise the whole plan would be mere stupidity); it’s not such a big leap to try styrofoam while you’re at it.
Muggle artifact prohibition?
He said it was only allowed because people wore it as regular clothing. Unless he could find records of wizards wearing styrofoam, that won’t work.