I have a Martin OM28V which I've owned for about seven years or so. This morning after playing it for a few minutes I noticed the palm of my right hand was covered in a black dust, like graphite. At first I thought it couldn't be from the guitar but sure enough I rubbed my hand on the strings and this black stuff rubbed off onto my hands. The strings are Curt Mangan strings and they've been on the guitar for about six months or so. I play the guitar a few times a week but I have never had this problem with Mangan strings or any other brand of strings for that matter, on electric or acoustic guitars. FWIW I actually really like these strings soundwise--they really have great punch and seem to last longer than other strings I've tried. What is going on here?