Cast iron wash pots have been made for centuries. Minor changes in the casting process over the years produced subtle differences in the pots that make it possible to estimate when they were made. One can most easily tell how old the pots are by looking at the markings on their undersides and their handles. As casting techniques were refined, changes were made in the way the iron was poured and the manner in which the pots were ground and finished, making newer pots smoother and more uniform-looking.
Flip the pot upside down so you can see the underside.
Check to see what symbol is stamped there. The oldest pieces, made prior to the mid-1700s, will have a circular "sprue" mark on them. The sprue is the point where the foundry worker poured the molten iron into the mold to make the pot. Even if the pot has been ground and polished, you can still usually detect the sprue.
Observe whether a long, thin line on the bottom of the pot called a "gate" mark is present. Pots with gate marks were made from the mid-1700s until the late 1800s.
Examine whether the bottom of the pot has any marks at all. Pieces made from 1875 to the present time were fashioned so that the iron entered the mold from the sides, creating a smooth bottom with no marks.