The Collaborative International Dictionary of English v.0.48:

Mixtion \Mix"tion\, n. [L. mixtio, mistio: cf. F. mixtion. See
   Mistion, Mix.]
   1. Mixture. [Obs.]
      [1913 Webster]

   2. A kind of cement made of mastic, amber, etc., used as a
      mordant for gold leaf.
      [1913 Webster]