and of course don't forget that the Roman had the technology to produce hot water, that they also used for bathing. I would imagine a suitably fired pot made of clay can be heated in a fire sufficiently for its content to boil. and as Wayland states, the dying process has been around for a very long time, and the heat is needed to activate the dye. I would not be to surprised that originally volcanic hot springs were used either as hot water for non consumption like dying or cleaning..etc, and later for heating water in sealed containers or such like. In japan at one site the hot spring is used to boil eggs which are then sold to the visitors