Clay poultice is used to draw copper staining out of the stone in the St. Patrick’s Cathedral restoration project. The clay is mixed with chemicals, applied to the stone, and allowed to dry. As the clay dries, the chemicals pull copper stains out of the stone (as seen on the left of this photo). Once the clay is dry, it is washed away, leaving a cleaner, whiter marble surface. -Sarah Rosenblatt, Preservationist at MBB