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He places a thin layer of virgin wool fleece at the very top of each mattress, to smooth out the horsehair and make sure none of it pokes out of the fabric covering. When fully stuffed, a mattress can be 20 inches high: "There's so much material inside, you think the fabric is about to explode," says Heer. It's then he begins the process of giving it shape, starting with hand-sewing its rolled edges.