Does anybody know what this tough film is in my tumbler?? I usually get some degree of it after tumbling fired brass. Cleans up with simple green after a 10 minute soak and some elbow grease. Black part is waxy somewhat. Tumbled 250 cases in .caliber .38 for 24 hours. Brass primed and previously fired Used Dillon Rapid polish (same result with Frankford Arsenal). Corn Cob was the media NEW dryer sheet split in thirds. cool basement @ 52 degrees and 63% humidity. Any details I left out? Just curious as brass looks good even after 5-10 years but I always had this unanswered question. Many Thanks!