Cast iron pieces made in the mid-1800s or earlier have distinct surface features on their undersides, such as a circular mark called a sprue on the oldest pieces, or a thin, long line called a gate mark on later ones. These marks disappeared as casting techniques improved, but cast iron pieces from the late 1800s and early 1900s can usually be dated by their trademarks and maker's marks.
The rough markings on the bottoms of old cast iron objects indicate where the molten iron was poured into the mold. Most pieces made after 1875 have smooth undersides, because crafting methods changed so that the iron entered the mold on the sides, usually in two places. More recent cast iron may show heavy grinding marks on the sides to smooth out these rough entry points, but this grinding technique wouldn't be available to foundrymen before the mid-1800s.
Manufacturing techniques were perfected in the early 20th century, and pieces from between 1920 and 1940 are considered to be the highest quality. These examples often feature smooth, glassy surfaces due to polishing methods that were introduced to production. Early 20th century cast iron pieces made by recognizable manufacturers, such as Wagner and Griswold, are highly collectible today.