Real Cerulean Blue (PB 35, cobalt stannate) is a compound of cobalt and tin. It’s an expensive pigment with good lightfastness. Mixing it thoroughly with a small amount of linseed or walnut oil will bring it to a smooth, controllable consistency. It’s useful for painting flesh tones of light-complected people.
Beware of a paint labeled: “cerulean blue hue”. It’s a mixture of phthalocyanine blue and zinc white or titanium white, or both. If it isn’t real cerulean blue, it isn’t worth trying to save or use. Fake cerulean made with phthalocyanine blue does horrible things in flesh tone mixtures.
Note: there is also PB36 made from oxides of cobalt and chromium. Some paint manufacturers label it cerulean blue but the proper name should be cobalt chromite blue. PB35 is opaque whereas PB 36 is more transparent, has higher tinting strength, and is darker in value than PB 35.
Cobalt blue is one of the most lightfast of the blue pigments, and from that perspective is not adequately replaced by mixtures of phthalocyanine blue and synthetic ultramarine, neither of which are as lightfast.
Prussian Blue, considered the earliest of our synthetic, inorganic modern pigments, was first synthesized by the paint maker Diesbach in Berlin around 1706. It goes by other names: Berlin Blue, Paris Blue, Antwerp Blue, Milori Blue, Brunswick Blue, Turnbull’s Blue, Chinese Blue, among others. Its qualities include: transparent, very good lightfastness, high oil content, non-toxic and very fast drying.
In response to someone asking if Prussian Blue can be a substitute for Ultramarine Blue: “Prussian blue is quite different from ultramarine, so would not make a good substitute for it. Prussian blue inclines toward green, whereas ultramarine inclines toward purple. If you need blue in your paintings, You’ll probably need ultramarine too.”
The natural Lapis has an ancient history, Ultramarine was first synthesized in 1828;
— the Natural mineral form of synthetic PB29. It has always been and still is a very expensive pigment. Extraction from the rare semi-precious gem stone is labor intensive & time consuming;
* Although Lapis Lazuli (after purifying) and Ultramarine Blue are essentially the same chemically, Lapis Lazuli is the natural mineral source and has different working properties, it also contains iron pyrite crystals that can give it a slight ‘sparkle’. Lapis Lazuli is also a much more expensive pigment so it is misleading to label Synthetic PB29 as Lapis Lazuli.