Illustration from James Oliver's April 4, 1876, patent for an improvement in casting moldboards, patent no. 175,622.
The answer is in a manufacturing process. When we refer to a chilled plow, we’re talking about the way in which a plow’s soil-contacting parts, such as the share and moldboard, were cast.
During the mid-1800s, when steel was scarce and expensive, many plows were made of cast iron. Cast iron is relatively soft and wore rapidly under the abrasive action of the soil. If hardened in the ways then known, the iron became brittle and broke easily when striking a rock.
The answer was to use a “chill” in the mold when casting each share and moldboard. Simply put, the chill consisted of an iron plate that formed the part of the sand mold that fashioned the cutting edges of the share, or the concave wearing surface of the moldboard. When molten iron was poured into the mold, the plate drew off the heat in the part of the casting adjacent to it more rapidly than the rest of the casting.
Because of the rapid cooling, the “chilled” surfaces of the resulting cast iron parts were hard enough to take a good shine and wear well, while the rest of the casting, which had been more slowly cooled, was soft enough to resist the sudden shock of striking an obstruction.
Although the process had been known for several years, James Oliver is generally credited with perfecting and patenting the process in 1857. FC