boil silk
湅 centers on boiling silk as a specific processing step, likely to soften or refine the raw material.
湅 combines 氵 (water) with 柬, which likely contributes the sound and the idea of selecting or processing. The character likely originally referred to boiling or treating silk in water.
Water 氵 is used to boil and process the silk represented by the right side 柬. Picture a pot of water heating raw silk to soften it.
For レン, imagine boiling silk in a large cauldron while a friend named Ren stirs it: Ren -> レン, and the water bubbles around the silk.