micron pens are acid free- permenent- do not run or bleed (cause fuzzy edges) artists have been using them for years-
it is always a good idea to test what ever you are going to use to mark fabrics- and i have had quilts written on with micron pens fade a bit with time---generally ones that were written fairly light in the first place- i have gone over them with a little heavier hand (and thicker tip- they come in a fine/medium& heavier tip sizes) and they have been fine. my daughter is enjoying her 10th anniversary this year & at her reception people wrote on muslin squares with micron pens for a special wedding quilt= they wrote things, they drew pictures- they had fun with it= it has held up amazingly well for those 10 years and counting.