NATURAL IRON OXIDE PIGMENTS 4 # SIENNA and UMBER
Post 409 ⇒ by Gautam Shah →
An artist palette and building designers’ vocabulary invariably includes few rich earth colours such as red oxide, ocher, raw sienna, burnt sienna, raw umber, burnt umber and Indian red. These colours in various intensities, transparencies and reductions with white create a language that cannot be matched by synthetic pigments. The iron oxide colours are sun fast, and remain stable on alkaline (masonry or lime) and acidic (metal) surfaces. Browns colours cover a wide visible spectrum, ranging from yellow, orange, to red. The shades are also named using composite adjectives, like red-brown, yellow-brown, dark-brown, etc. Modern day browns derive by mixing green (blue with yellow) and red, or orange and black. Red colouration is linked to the presence of hematite, and yellow colouration to the presence of goethite.
Like the other earth colours, such as yellow ochre and umber and sienna are clays containing iron oxide, called limonite, which offers yellowish colour. These colours are a range of different colours. All standards fail to agree on precise shade for them.
Red oxide usually contains about 70% Fe2O3 . Sienna is a brownish yellow containing about 60% Fe2O3 with some quantity of manganese oxide. Umber is a greenish brown containing some 45% Fe2O3 and 15% MnO2. Umber is named after a department in Central Italy where it was first tested. Umber of good quality, called Turkish Umber, is found on the island of Cyprus. Numerous deposits of colouring earths occur in various parts of the world.
Earth colours Sienna and Umber, are mix of limonite with some amounts of manganese oxide, and so both are darker then ochers. Senna is darker, more towards orange and so thought of as reddish, compared to Umber that is closer to yellow and thought of as brown. Sienna has a higher content of manganese which makes it greenish brown or darker brown. Raw Sienna is transparent and warmer whereas Raw Umber is opaque. Raw Sienna when calcined becomes richer and darker as the burnt sienna. Raw Umber becomes the rich deep or dark brown as burnt umber.
Sienna was mined originally mined near Arcidosso, formerly under Sienese control, in southern Tuscany. It was also called terra rossa (red earth), terra gialla, or terra di Siena. Umber is linked to Umbria in mountainous region in central Italy, and so called terra di Ombra or Umbria. Ombra literally means ‘shadows’, for the purpose for which it was used, shadows or for shade toning other colours.
The creative exploitation umber for creating dark shades happened during the baroque period, in the chiaroscuro (light-dark) style of painting. It was the palette of Caravaggio (1571-1610) and Rembrandt (1606-1669). This nearly ended during 19th C, when Impressionists dazzled with modern brilliant synthetic pigments, rebelled against the dark and dirty umber and other earth colours.