Geological Survey, Volume 1, 1897
The manufacture of fire-brick has been one of the characteristic industries of Maryland for 50 years, and the brick made from the fire-clays of the Pottsville formation in Allegany county are regarded as the best in this country. The oldest fire-brick concern in Allegany County, that at Mount Savage, was organized in 1841 and was the first of its kind in the United States. The main fire-clay deposit lies near the bottom of the coal measures and the clay bed ranges from 5 to 20 feet in thickness. It is divided into two varieties, designated as the hard and soft clay. The hard clay is of a gray color, shading almost to black; it is non-plastic, unless ground into an impalpable powder, and disintegrates but little upon exposure to the weather. The soft clay is very plastic, much lighter in color and crumbles rapidly under atmospheric influences. The impurities in this clay are fewer and smaller in amount than in most other fire clays. The two most valuable characteristics of this clay are its freedom from potash and the large proportion of silica to alumina. The clay industry has grown rapidly in importance during recent years, and the value of the output during 1896 was $1, 753, 003.