MITRA Forum Question Details

​I have always assumed that the darkening resulted mainly from the high oil absorption. The mention of manganese soaps is interesting, and it is true that lead soaps cause white lead to become more transparent. As to particle migration, I find that dubious. I am interested in what other moderators say about this. Also, with so many manufacturers moving from natural earths to hues mixed from manufactured iron oxides and other pigments the issue is fast becoming mute.

In my experience as a paint maker, ​the observation by Kurt Wehlte cannot be applied acc​ross all umbers (Colour Index PBr7), because of the variation of their chemical composition due to the presence of accessory minerals, such as calcite, quartz, clay, etc. In the line of Rublev Colours Artists Oils, we have 8 different umbers all of which behave and appear remarkably different. The essential minerals that classify earth pigments as umber is the presence of iron oxide (FeO3) as the mineral goethite associated with small amounts of manganese dioxide (MnO2), typically as the mineral pyrolusite. Pyrolusite is a black or dark grey mineral that gives raw umber its greenish tint and burnt umber its dark brown tone. While it is true that umbers in general require a large amount of oil to form a paste, and hence have a large oil absorption value and high critical pigment volume concentration (cPVC), this does not mean all umbers exhibit this same tendency—darken in oil as they age.

The opinion that umbers darken has been expressed elsewhere, such as by Jan Esmann, which I believe relies on Kurt Wehlte’s observation:

Th​e umbers and the sienas darken with age (even within a relatively short time, that is 10–20 years) because of their large oil content. It is therefore advisable to grind ones own using good alkyd (most alkyd is thicker than linseed oil), poppy or walnut oil. Some umbers need an addition of a plasticizer in order not to settle or jelly in the tube, but I have observed that one brand of raw umber did not change, while an other brand stiffened in a few weeks and had to receive additional oil. Probably this difference stems not from the umbers, but from the presence of clay or chalk. Traditionally one recommends 2% wax or aluminum stearate melted into the oil. Kurt Wehlte recommends substituting umbers with mixtures based on burnt siena in order to reduce the oil content (and avoid subsequent darkening), but this seems ill advised since burnt siena requires even more oil than the umbers. Replace them with mixtures based on manganese brown or mars brown. Burnt siena can often be replaced with burnt green earth.

Source: Jan Esmann Oils and Pigments . 2012. pp. 18, 19

Kurt Wehlte apparently based his observation of umber on paintings by Max Doerner and his students. I view this observation with much caution as paintings by Doerner and his students were known to exhibit problems quite early as observed by Helmut Ruhemann, a consultant restorer for the National Gallery of London. He commented on Doerner’s methods described in his book, The Materials of the Artist and their Use in Painting. After commenting about Doerner’s technique, he went on to note much of the the work of Doerner’s students using his method was in poor condition.

Thanks George. I guessed as much. Also, Doerner, geez, he certainly caused so many more problems than he ever prevented. Tempra grassa and mische technique, uggg.​

We need to be precise in our terminology and pigment characterization. Van Dyke brown, bitumen, asphaltum, soft coals, and Cassel earth, as well as simple tar all have bituminous components. These are very separate from umbers. No actual umber has bituminous components, so this is not the case here. Umbers are natural iron oxides with clay components that contain a varying amount of manganese dioxide, as previously stated. The “tarry” effect is not in play here.