Commentary for Berakhot 103:16

ידים שניות הן ואין שני עושה שלישי בחולין אלא על ידי משקין

The hands are [only unclean in] the second degree, and [anything unclean in] the second degree cannot make a third [unclean grade] with things non-holy except through the medium of a liquid.

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot