Whenever errands pull Bolinas fiber artist Ashley Eva Brock up to Petaluma, she’ll swing by a few tucked-away spots in the bushland to retrieve her supply of oak galls. Growing along the branches of oak trees, these apple-shaped galls are a natural source of golden dye that become enlivened with a few simple twists of chemistry. (When she adds iron into the equation, the gold turns into varying shades of charcoal.) “It’s a really dramatic, changing dye and I love it,” she said. “Plus it’s free!” Oak gall dye will comprise part of the agenda for an upcoming class on revitalizing clothing through dyes, which Ms. Brock will hold on Dec. 17 and 18 at the Regenerative Design Institute in Bolinas. The other colorant in the lesson plan will be indigo, which Ms. Brock calls an “unusual and special dye” for the way it changes before your eyes from yellow to green to blue, when activated with oxygen. She’ll begin the class by explaining the transformative properties of these organic plants and gallnuts; then, students will practice on fabric. “I don’t necessarily consider myself a dye expert,” said Ms. Brock, whose resumé nevertheless showcases years of fashion experience, including a stint as a dyer and textile manipulator for the San Francisco Opera. “But I really try to provide textures to experiment and learn. It’ll be a facilitated playtime.” Ms. Brock herself majored in fashion design at the California College of Arts, where there was a focus on sustainability. She said she designed her class so that students learn the notion of “upcycling,” or bringing value back into materials that would otherwise be discarded. “That way [the students] bring a second life to their wardrobe,” she said. “When you get somebody to have a relationship with their clothing, it keeps it out of landfills.” Two indigo and oak gall dye workshops with Ashley Eva Brock will be offered on Saturday, Dec. 17 and Sunday, Dec. 18, both from 11 a.m. to 4 p.m. For information and registration,  visit ashleyeveabrock.com or email [email protected].