smelt metals, forge, refine
鍊 centers on applying heat and skill to metal: smelting raw ore, forging it into shape, and refining it to improve quality.
鍊 combines 金 (metal) with 柬, which likely contributes the sound and may suggest selecting or sorting, fitting the idea of refining metal.
The left side is metal 金, and the right side 柬 looks like a bundle being sorted. Picture a smith sorting raw metal pieces and heating them in a forge to refine and shape them.
For レン, imagine a metalworker named Len who refines ore in his forge: Len -> レン, and his work with 金 makes the metal pure.