refine metals, kneading over fire
煉 centers on applying heat to transform or purify a substance: refining metals by fire, or kneading and working something over heat to change its consistency.
煉 combines 火 (fire) with 柬, which likely contributes the sound and the idea of selecting or sorting, suggesting the process of refining by fire to separate impurities.
Fire 火 on the left and 柬 on the right: picture a smith using fire to knead and refine metal, working it until it's pure.
For レン, imagine a metalworker named Len who refines metals over fire: Len -> レン, and his fire purifies the metal.
brick
purgatory
chamotte brick
ornamental tile; facing bricks
wooden block
elixir of life (prepared from cinnabar in ancient China)
art of making elixirs (e.g. in Chinese trad. medicine); alchemy
firebrick
paving brick
boiled pine pitch and oil (traditionally used as a glue in archery, etc.)
red brick