The Collaborative International Dictionary of English v.0.48:

Siccative \Sic"ca*tive\, a. [L. siccativus.]
   Drying; causing to dry. -- n. That which promotes drying.
   [1913 Webster]

WordNet (r) 3.0 (2006):

siccative
    n 1: a substance that promotes drying (e.g., calcium oxide
         absorbs water and is used to remove moisture) [syn:
         desiccant, drying agent, drier, siccative]