Since moving to Los Angeles five years ago, the artist Rachel Duvall has been refining an almost scientific approach to handweaving, based as much in foundational considerations like hue and line as in methodical chemical experimentation. She uses only natural dyes and modifiers such as copper and iron to “investigate the subtlety of colors,” she says, though the range she achieves — including a bright neon yellow and purple from fermented lichen she collects herself and then brews in her backyard — is striking.