Collecting native earths

[Salamander]

When collecting native earths, how do you know if you've stumbled upon ochre, umber or sienna? Sometimes there is a fine line between each of these colours.

[Connelley]

When collecting native earths, how do you know if you've stumbled upon ochre, umber or sienna? Sometimes there is a fine line between each of these colours.

According to Kurt Wehlte's Book "The Methods and Techniques of Painting" (Kremer Reprint), it's difficult to differentiate between ochres, umbers, and siennas. The ochres are mostly opaque and soft. The umbers are also mostly opaque until roasted. The siennas are more transparent. They are all iron oxides with varying amounts of clay. The umbers contain manganese, and the siennas contain more silicates. No easy tests to tell one from the others.

Connelley

[Salamander]

Thanks for the confirmation Connelley, that's pretty much what I was thinking.
-Eric