i'm not sure it is a good idea to use a sharpie. i don't think they are archivally safe. as i have said before i am a bookbinder in my real life and i have seen much old paper, which was made of rag, that was damaged or destroyed by the use of inks which contained acid. i can only assume that cotton fabric would be at as great a risk as rag paper.
please does someone have the low down on this issue.