The same phenomenon happened with "yakan,"(kettle) or 薬缶 in Chinese characters; it literally means 'medicine can' and was originally used for decocting medicines; however it was used for boiling water as well and people continued to use the original name "yakan" for it.
薬缶を薬煎じの目的以外で、単に湯を沸かす容器の名称に使い続けても、名称は“薬缶”のままである現象と同じことが起こったのである。