When machine quilting on dark fabric I tried every kind of marker I could find. None of them worked. I either had to press too hard to get a decent line or they didn't stay on as I repositioned my quilting sandwich. Then I remembered hearing about the sliver of soap. It worked great! Easy to mark, stayed put long enough to finish the quilt, and no problems washing it out. It would probably be best to use a soap without a lot of moisturizing lotions etc. to avoid potential problems. That reminds me, I want to pick up a bar of Ivory just for this purpose.