What really surprised me was how good this fibre was - I was expecting it to be like golden taklon or Dalon or any of the other synthetic watercolour fibres that are all essentially mock sable - this is SO fine - it's hard to see in the photo below but the prolene fibres are finer than human hair and springy but they don't have a snap to them - so the sword is flexible and floppy, and, when wet, the fibres lock together (that's the best way I can describe it, they don't physically lock like hog bristle, however) and one can draw beautiful lines and calligraphy with it. I'm going to buy some more prolene brushes now I think as it's a lot of fun!
I've just shot a video that I'll be posting in the coming few days that shows what you can do with a prolene sword.
My Pro-Arte prolene sword blender - I wanted to show it against some other fibres. From left to right - Dalon synthetic round, white nylon (stained by Phthalo Green) round, prolene swordliner and kolinksy one-stroke. They're sitting on a white goat hair skyflow, just for contrast.