Black slate gains its color from carbonaceous materials, present during its metamorphic formation. Like green slate it can be divided into two groups, Weathering and Unfading. Weathering slate changes color when exposed to the elements, while Unfading slate does not. Weathering slate is not inferior to Unfading slate– often the reverse is true. The oxidation of the surface that produces the color change is a limited process, affecting barely one hundredth of an inch of the slate.
Vermont Black, also known as Vermont Gray-Black is a Weathering slate. It is a dark slate with lighter striations. It weathers to very attractive tan, buff and brown shades and is widely used on the East Coast of the United States. This is a relatively new slate and has become popular in the last fifteen years.
Penn Black, known further south as Bangor slate, comes from the Lehigh County region of Pennsylvania and is a soft, blue-black slate.
During the building boom of the forties and fifties in Baltimore, developers were looking for an alternative to Vermont green, an excellent slate but one that was costly to transport. Pennsylvania obliged, and over the next forty years the large residential areas of the Mid-Atlantic were flooded with Bangor slate. It does not have the greatest longevity and, as it begins to deteriorate, the whitening edges of leaching gypsum and brown delaminating layers become a familiar sight.
Cathedral Gray slate, so called since the quarries were owned by the Church of England, is an excellent slate, also from Lehigh County, Pennsylvania. It has the smooth, blue-black surface of Bangor slate but with a silvery tinge. It is much harder material, however, and subsequently longer-lasting.
Chapman slate is another Pennsylvanian product, from Northampton County. It is a hard-veined, long-lasting slate, easily identified by the herring-bone striations of quartz. Many homes in Baltimore have this slate, some over a hundred years old.
Peach Bottom Slate comes from the Delta region of Pennsylvania. It was quarried from a slate ridge that runs from Lancaster County, under the Susquehanna River, into York County and across the State line to Whiteford, Maryland. Settled by Welsh slate quarriers in the early nineteenth century, the area was soon known for producing a world-class slate known as Peach Bottom, Delta or Susquehanna Slate. In 1850 it was awarded first prize at the World’s Fair at the Crystal Palace of London. This is a charcoal gray-black slate, favored in Baltimore for use on churches. St. Govan’s Presbyterian Church on the eastern boundary of Homeland is a pristine example of its use. This slate is frequently reclaimed from deteriorating buildings to be re-used on new roofs.
Monson Slate comes from a tiny rural community in the center of the state of Maine. Up until the mid-20th century it was famous for its slate quarries, which produced an excellent slate, sometimes indistinguishable from Peach Bottom slate. There is variety in slate colors, even those from the same quarry, and Monson is sometimes a very dark black.
North Country Black is an unfading slate from Canada, about five miles north of the Maine border. Flat, smooth and consistent in color, this slate has gained great favor and proved its quality on such large jobs as the Railway Museum and the MTA buildings in Baltimore.
Grayson slate, from Arvonia, Virginia, is the most distinguished slate in the United States. It adorns many affluent homes on the Eastern Seaboard. It was also the choice of the State and National government, who mandated that it be used on all of their buildings. It is a deeper black than Peach Bottom and truly is a phyllite, rather than a slate, the next step in the metamorphic process. It is harder than slate, more difficult to quarry and split. It has unknown longevity.
In the North-West Spanish province of Gallicia, the quantity of slate produced is one hundred times that of all American quarries combined. Labana slate has a dark, leaden color, Unfading and durable, the heavy-lifter of the future.