The smooth crystalline substance is extremely slick. Bobbins are the critical tool in tying since they must keep smooth tension on the thread spool and have super smooth inserts to apply the thread to your fly. These bobbins can handle these tasks with ease. They have delrin feet for smooth tension on the thread spool and have dual ceramic funnel inserts for smooth application of thread to fly. These funnel shaped inserts allow for a smoother application of thread as the bobbin is rotated and they cause less breakage than traditional pipe or straight edged inserts.