All the sashweights i have seen in NY are cast iron. my house has double hung windows and sash weights, but all the sash ropes have been replaced with chains that don't wear out.

sashweight were also used a weapons, since they were cheap and readily available. thugs (now is a good word!) would bash your head in with a sashweight in the early years of the last century in some of the less savory neighborhoods of large cities.

most modern double hung windows are spring-loaded.