Trass is a dark grey mortar similar to Portland cement. It was used in the 17th and 18th centuries (in the 17th century mixed in equal parts with hydraulic lime; in the 18th century mixed with two or three parts of hydraulic lime for each part of trass). The author rediscovered trass while excavating on the north side of the nave of St. Paul's Cathedral in 1969-71, presents a history of the composition and use of trass, and lists its locations in St. Paul's Cathedral.