There was a time, within the span of my participation in the color field, when it was common for colorists to practice visual identification of colorants by their curve shapes. With the advent of computer control of color, the capability to identify colorants by shape has become mostly a lost art. I can remember when persons such as Henry Hemmendinger and Hugh Davidson might be holding quite fascinating conversations on the subject at a wine and cheese hour of an ISCC meeting with people like Jackie Welker, who was an absolute devotee of visual identification of colorants. She would go so far as to prepare a colored specimen with an off-beat color combi- nation in advance of a meeting and bring its spectrophotometric curve to the meeting. Then she would go around the room asking others “What is it?”. Your expected answer was to take the form, “Molybdate orange, phthalo blue and rutile.” If you didn’t have at least one modifier for each color name, you were rated an amateur.
It may be of interest to look back at the kind of thing that was considered in identifying colorants. The item best remembered by me is that of the difference between the curve shapes of the rutile and the anatase crystal forms of titanium dioxide. Rutile has a deep absorption trough in the low wave region between 360 (or lower) and 420 nm. Anatase has none, and its curve is thus level with the rest of the high reflectance curve through all of this region. This was of importance in those days, because anatase had an additional property that it chalked upon exterior exposure so effectively that a hard rain would wash it clean. It was thus the preferred white pigment for outside exposure of straight white colors, while rutile was used indoors and in tinted colors outdoors. Thus, an immediate and definitive distinction identifying each crystal positively was useful. Times have changed. Today vehicles have improved for outdoor endurance, and the same material that is used for white is tinted to colors in the store. Anatase is no longer used. In fact, I am told it is no longer made in the United States. It can, however, still be obtained from China and India.
Another telltale curve anomaly positively identifies phthalo blue and distinguishes it from cobalt blue. Cobalt blue can be made to be identical in color with phthalo blue and it is a lot less expensive, but phthalo blue is excellent for exterior exposure. When it is considered that it is an organic pigment, it is nothing short of phenomenal for exterior exposure properties. Cobalt blue is quite impermanent in outdoor exposures, being highly sensitive to acid rain, under whose influence it bleaches readily. Here then is the reason that we might care to identify each of these curves from each other. Phthalo blue has a pronounced secondary reflectance maximum in the 680 to 720 nm region. Cobalt blue completely lacks this hump. Virtually no other portion of the two curves are different but this region, but identification by this difference is definitive.
Kraft paper is a word in the paper industry for that kind of brown paper we know is often used to make what we call ‘brown paper bags’. Kraft paper is widely used also to make the outside surfaces of corrugated cardboard boxes, and many other uses. Kraft paper has a distinctive signature spectrophotometric curve. It is almost a straight line from low to high on a plot of reflectance on a wavelength scale ascending from left to right. Human hair and human skin have much the same property, that of the straight-line curve. This is different from colors made with red and yellow oxide whose curves are mildly spectrally selective while the same colors made with red and yellow organics are very spectrally selective. This leaves us with three distinct sets of colorations from straight line to mildly twisting selectivity to strong selectivity, and one can look at the curve of any one of these tans, or beiges, and know exactly what its pigmentation is.
I close with one additional thought. Two of the three scenarios I have presented may only be well detected by spectrophotometers with a spectral range of 360 to 750 nm, or more. Think about that when you are about to deprecate the regions outside of 400 to 700 nm as having little weight in human vision.
Hugh S. FairmanEditor’s note: When I tried to learn this art in short courses, the instructors tended to emphasize absorption maxima, which could show up as reflectance minima (if you were lucky) or as reflectance points of inflection (for more challenging mixtures of colorants). It was not an easy art to learn! [MHB]