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,…