Between 18 they operated a subsidiary firm known as Seth Thomas' Sons & Company that manufactured a higher-grade 15-day mantel clock movement and during that period were major supporters of a New York sales outlet known as the American Clock Company.
After Thomas’ death in 1859, his son Aaron became President and began to add new products to a conservative line.
These clocks have become known to collectors as "Black Mantel Clocks", and were popular from 1880 to 1931.
Seth Thomas made clocks in marble cases for a short time, from 1887 to ca. They also made clocks in iron cases finished in black enamel, from 1892 to ca. Seth Thomas is well known for their "Adamantine" black mantel clocks, which were made starting in 1882.
In the 1860s, French clocks in slate, onyx or marble cases became popular in the United States.
These cases were expensive, so the American clock manufacturers produced similar looking cases made of iron or wood.
These were cased in pillar and scroll cases until 1830, when the bronze looking glass and other styles became popular. They phased out movement manufacturing and began importing many movements from Germany.