Possible Clues:

• The inert part of gunpowder?
• Fool touching some gunpowder
• I rent it in the gunpowder
• Inert kind of saltpetre
• Two elements turned into saltpetre
• Sodium nitrate
• In autumn it requires a preservative
• Sounds as if it might be darker in gunpowder
• Perhaps I could rent this to make it sound more like a knight, perhaps
• Saltpetre used in unit retreat