History of coal mining
Anthracite (or "hard" coal), clean and smokeless, became the preferred fuel in cities, replacing wood by about 1850. Bituminous (or "soft coal") mining came later. In the mid century Pittsburgh was the principal market. After 1850 soft coal, which is cheaper but dirtier, came into demand for railway locomotives and stationary steam engines, and was used to make coke for steel after 1870.