I would dry the brown, dirty ones to low case (just barely flexible), then brush off most of the dirt with a soft, dry brush of some sort. The more they are rinsed after browning, the more nicotine gets washed away. The dirt itself won't hurt you or your pipe, but it won't burn, and might taste a bit "loamy". If they've been sprayed with agent orange, or other mystery compounds, I would be disinclined to smoke such low quality leaf.
Bob