Raw Sienna

Synonyms:

Raw Sienna, Terre De Sienne, Terra Di Siena, Rohe Sienna

This earth, which is found chiefly in Tuscany and the Hartz, is a particularly rich variety of yellow ochre, and contains a large proportion of a ferric hydrate. The late

G. H. Hurst ('Chem. News,' vol. lix., p. 172) gave three analyses of raw sienna. In these the range in the percentages of the more important constituents is as follows:

Raw sienna is prepared for use by crushing, sifting to remove sand, grinding and washing in the same way as in the case of yellow ochre. It should be noted that when lumps of fresh raw sienna are first broken and exposed to the air, their surfaces acquire a slight olive-green nuance. In order to avoid any disadvantage which might arise from such an alteration taking place subsequently, it is a good plan to expose the crushed earth to the air and light previous to its final preparation as a pigment. Before grinding it in oil, it should be cautiously dried at 60° C. to remove the greater part of the accidental or hygroscopic moisture.

Raw sienna in thick washes is somewhat deeper in tint and of a warmer and browner hue than yellow ochre. Owing to a trace of organic or peaty matter which it contains, raw sienna is liable to become rather less brown and more yellow by long-continued exposure to strong light. It is generally without action on other pigments, and is available for water-colour, oil, and tempera painting; it sometimes fails in fresco. A given weight of raw sienna requires a larger proportion of oil than any other pigment; the finished oil-paint contains only 30 percent of pigment.

Raw sienna is not subject to adulteration, but it is well to be certain that the pigment has been well washed.

1	Includes traces of organic matter.