Re: Chemical composition
I thought I'd answered this post before, but it must have gone to computer heaven..So here's my second go-round.
The pigment Identifications indicated on the backs of tubed paint labels is for a general reference, only. It is not at all an indication of color, but only the pigment of which the paint is made.
For example, Grumbacher offers a Cadmium Red Deep, a Cadmium Red Medium, and a Cadmium Red Light. M. Graham offers a "Cadmium Red". Now, no two of these tubed colors is alike--they all differ from each other, yet the Pigment Identification of each of them is "PR108". The color has to do with how the pigment is processed--heated, burned, laked, chemically treated, etc., etc., and, so depending upon the processing, many colors can be created from the same pigment.
Another excellent example is that of Burnt Umber, and Burnt Sienna--two colors that are decidedly different, yet the Pigment Identification of each of them is PBr7.
The Pigment ID is merely a guide, and it roughly indicates the pigment of which the color is made, but not the precise color.
As a "color theorist", my primary concern is that of color. I generally care a lot less about what caused it to be that way. I am usually quite concerned with lightfastness. I also don't care much about opacity, or transparency. A transparent paint can always be made to be more opaque by the artist, but it is more difficult to get an opaque paint to behave more as a transparent one.