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. Fairman
Editor’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]
