The Collaborative International Dictionary of English v.0.48:

Alchemistic \Al`che*mis"tic\, Alchemistical \Al`che*mis"tic*al\,
   a.
   Relating to or practicing alchemy.
   [1913 Webster]

         Metaphysical and alchemistical legislators. --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alchemistical
    adj 1: of or relating to alchemists [syn: alchemistic,
           alchemistical]