This came up with my group when I explained the dangers of burning impure metals. We have a feruchemist in the group and the others were wondering if he could check potentially dangerous metal sources - for example, if the group found a steel bar and wanted to know if it was allomantic steel. The question was, what would happen if a feruchemist tried to store and tap an impure metalmind.
Since feruchemists don't ingest their metalminds, I don't think it they would have to worry about the poisonous side effects. I'd assume it just makes their storing less effective, like only half the charges stored can actually be tapped again. Depending on the type of metalmind, the additional benefits for tapping more charges could also be less.
Any thoughts on that?