Milled from a member of the Phthalocyanine “family,” arguably the most important discovery in the blue part of the spectrum since cobalt in 1802. Phthalocyanine was chemically synthesized in the late 1920’s, with pigment becoming commercially available in 1935. Chemically, the molecule is related or similar to porphyrin (the structural basis for natural compounds such as chlorophyll). Phthalocyanines form complexes with a variety of metal atoms, most notably with copper (for Phthalo Blue). When “noble gases” like bromine or chlorine are added to the structure, a green pigment molecule results (i.e. Winsor Green [yellow shade] PG36, or Winsor Green [blue shade] PG6).
When first developed, the pigment was identified as have twice the tinting strength of Prussian Blue and from twenty to forty times the strength of Ultramarine. Because of the variety of options for synthesis, the pigment can be “tweaked” in any number of ways to produce different shades.
For generations, the philosophy behind the “Winsor” named colours has been to offer colours that occupy the perfect spectral point of a major and definable hue. For example, Winsor Yellow, Winsor Blue, and Winsor Green have always been formulated to occupy each of those precise spectral positions without bias too far to the warm or cool. As the range of available pigments has increased, so, too, has the range of Winsor-named colours, each intended to fit perfectly within the named colour position (e.g. Winsor Blue [Red Shade], Winsor Red, Winsor Orange, and so on). Because the intent behind these colours has been to hit a specific spectral target, we have not named the colours by pigment (as we do with Cadmium Red or Cobalt Blue), and have made it our policy to make use of the pigments the best hit the spectral target. For decades, Winsor Blue was formulated with the pigment used in making Prussian Blue (PB27). When it became obvious that Phthalocyanine (PB15) was better suited to that spectral position, the change was made to the new pigment.
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