What JC said .... Cortland Sylk is similar to regular fly lines. Silk are woven silk fiber's.

As far as advantages to silk lines: they cast very nicely with grass rods but only float for maybe 4 hours then you either have to dry them out and re-coat with red muslin or swap ends by removing and switching ends on the reel. I use silk lines for nostalgia only. Their cost is significantly more than synthetic lines but if treated properly have a useable life span much more than synthetics. Kinda evens up the cost issue.

At my age 4 hours is about my limit without a shoreside lunch or sit so silk works good for me in that regard