Book excerpt: In Clues in the Calico Barbara Brackman unveils a much-needed system for dating America's heirloom quilts. She tells how, by collecting and observing quilts and finally analyzing her computer file on close to 900 date-inscribed specimens, she arrived at the system. And through this telling she also imparts a colorful, stunningly illustrated history of quiltmaking along with a good bit of entertaining social history and the newest findings in textile research.