Likewise the color of the mortar used to lay the bricks can also often help to distinguish between multiple generations of brickwork.  Traditional and hydraulic mortars used throughout the 18th and early-19th centuries were almost always tan colored. In contrast, Portland cement, which is currently used in most mortars, has a distinctive light gray color.  Because Portland cement was not widely used in the United States until the end of the 19th century, finding a brick wall built with gray-colored mortar often indicates that it dates to after the late 19th century.  At Montpelier the duPonts' masons used Portland cement-based mortar in many locations and so it is often easy to identify their work by its distinctive gray-colored mortar.