Copper
Both stationary (as in wash tubs), portable and for early washing machines was chosen because it conducted heat well, and didn't rust. Other options included galvanized metal which does rust sooner or later, and or wood. The latter was BOL and avoided when possible for many reasons, but it was cheap.
Copper tubs in any case came either lined with tin, or just left bare. Tinning gave greater protection against staining laundry from verdigris caused by oxidation of copper.
Manufactures of various wash tubs/machines of all sorts usually offered a range. Copper was TOL, then came galvinized metal and finally wood.
Once science of applying porcelain onto metal, copper at least for washing machine tubs began to fall out of favor. The metal was costly, required frequent polishing (if anyone bothered), and had other issues. Indeed porcelain on metal would soon replace galvanized metal and wood as well. Though late as the 1940's some commercial (and perhaps domestic) washing machine makers were still offering machines with wood inner tubs and other bits.
Of course stainless steel and now plastic has replaced porcelain on metal, so there you are....
