Through history, the finest Raw Sienna has been transparent with a jewel-like clarity in the undertone.  This was due to the fact that the pigment taken from Sienna, Italy was comprised of a natural iron oxide in combination with silica, or mica.  The result was a warm, golden earth tone of great clarity.  In the late nineteenth century, that vein of pigment in Sienna was exhausted, forcing manufacturers to use Sienna pigments of greater opacity and far less clarity.  Winsor & Newton has made use of a variety of oxide pigments, both natural and synthetically derived, to achieve the clarity and brilliance that should be exhibited by Siennas. 
            Raw Sienna is light and warm yellow, with low to moderate tinting strength.  It is ideally suited to glazing and clean mixes.

Natural Iron Oxide, PY43; Synthetic Iron Oxide, PY42
AA – Extremely Permanent; ASTM I
Series 1
Transparent