A gorgeous, useful middle blue which is a bit more opaque than some of the other pigments. It has a solidity that is hard to describe but it is useful for portraying both objects and atmosphere. Cobalt Blue (Cobalt Aluminate Blue Spinel) or Thenard's Blue is a mainstay of the palette- a beautiful middle blue that varies is color but often tends to lean slightly toward the more blue-purple side. This pigment was a "staple part of the Impressionist palette," along with ultramarine, according to experts on the time period. While some would mix the scale of blues using phthalo and ultramarine, we enjoy a hard-to-describe quality about cobalt related to its reflectivity. Some list cobalt blue as semi-transparent, but we tend to think of it as semi-opaque relative to other blue pigments such as phthalo blue or ultramarine.
Cobalt blue PB28 comes in a variety of colors ranging from teal to middle blue to deep blue. Most Cobalt Blues are of the middle blue variety.
In terms of health and safety, it is listed as toxicity class B, which means it is toxic, so do use great caution. The Artist's Guide to Health and Safety has a section about some of the hazards associated with cobalt pigments. The author's most recently updated writing can be found through her site.
Cobalt blue tends to have a very high amount of oil, and it is interesting that it still dries quickly due to the oil content, at least in linseed oil. PB28 is one of the colors which may be found in walnut or poppy oil in an effort to avoid some of linseed’s natural yellowing tendency. Some give Cobalt Blue a moderate dry time, but it does seem a bit faster than many others. Both factors -- the oil content and the drying-- are good to keep in mind if you are painting in oil and working in layers.
As opposed to other pigment categories where there might be a light, medium, or deep for each pigment, Cobalt does seem to have a range within PB28 but there is also a separate pigment for Cobalt Blue Deep, PB74. Also Cobalt Chromite Blue is different, the pigment code for Cobalt Chromite is PB36.
Cobalt Aluminate Blue has a reflectance in the near-infrared, making it a cool pigment. One sometimes hears about spinel colors and it was interesting to find out that PB28 has a spinel structure.
