Source : Webster's Revised Unabridged Dictionary (1913)

Sienna \Si*en"na\, n. [It. terra di Siena, fr. Siena in Italy.]
   (Chem.)
   Clay that is colored red or brown by the oxides of iron or
   manganese, and used as a pigment. It is used either in the
   raw state or burnt.

   {Burnt sienna}, sienna made of a much redder color by the
      action of fire.

   {Raw sienna}, sienna in its natural state, of a transparent
      yellowish brown color.

Source : WordNet®

burnt sienna
     n 1: a shade of brown with a tinge of red [syn: {reddish brown},
          {sepia}, {Venetian red}]
     2: a reddish-brown pigment produced by roasting sienna