I decided tonight to clean my only two MM Corn Cob pipes. Based on online instructions I settled down with some pipe cleaners, rubbing alcohol, paper towels and a little time. After cleaning both pipes I gave them a quick rinse under the sink to get out any small loose debris and quickly wiped them off. I noticed that both pipes were leaving a purple-ish black residue on the paper towels. I'm assuming this is some sort of finishing and I'm curious what exactly I've done to my pipes. Is this anything to worry about?