Editing Sienna (section)

==Earth colors==
Like the other earth colors, such as [[yellow ochre]] and [[umber]], sienna is a clay which is partially composed of [[iron oxide]]s. In the case of sienna, the most prevalent iron oxides are [[limonite]] (which in its natural state has a yellowish color), and [[goethite]]. In addition to iron oxides, natural or raw sienna is also composed of [[manganese oxide]], which makes it darker than ochre. [[Aluminium oxides|Aluminum oxides]] have also been found in the soil at very low levels.<ref name="Helwig-2007">{{Cite book |last=Helwig |first=Kate |title=Artists' Pigments |publisher=Archetype Publications |year=2007 |isbn=978-1-904982-234 |volume=4 |pages= |chapter=Iron Oxides}}</ref> When heated, the limonite and goethite is dehydrated and turns partially to [[haematite|hematite]], which gives it a reddish-brown color.<ref>{{cite book
|url=https://books.google.com/books?id=6Z3bAXEM_xwC&q=sienna&pg=PA161
|title=Blue and yellow don't make green
|last=Wilcox
|first=Michael
|publisher= School of Color
|pages=172, 161
|year=2002
|access-date=October 19, 2009
|isbn= 0-9679628-7-0 }}</ref>
Sienna is lighter in shade than raw umber, which is also clay with iron oxide, but which has a significantly higher content of manganese (5 to 20 percent) making it greenish brown or dark brown in color. When heated, raw umber becomes burnt umber, a very dark brown.<ref name="Roelofs-2012"/>

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File:Monteriggioni0001.jpg|The clay soil of Tuscany (here near [[Monteriggioni]])  is rich in limonite, or hydrated iron oxide, the main component of sienna pigment.
File:LimoniteUSGOV.jpg|[[Limonite]], a clay containing iron oxide, which gives sienna pigment its color.
File:Hematite.jpg|[[Haematite]]. When roasted, limonite is converted partially to [[haematite]] and its colors turns more reddish.
File:Pigment sienna burnt iconofile.jpg|After heating, the pigment becomes the color burnt sienna.
</gallery>