handprint : a color top

the color top

A simple but effective tool for studying additive color mixing is the color top. This simple device was first used by Ignaz Schiffermüller and Munsch in the mid 18th century, and was systematically applied a century later in rigorous color mixing experiments by James Clerk Maxwell and Ogden Rood.

Constructing the Color Top. As the name implies, a color top is simply a rapidly spinning disk whose visible surface is covered with colored material. The spinning of the disk visually mixes the light reflected by the colors on the disk to produce the illusion of a single pure hue. By comparing this mixed hue to a color sample, any color can be analyzed in terms of other colors. Grays can be analyzed as a specific proportion of white and black, and hues can be analyzed as a specific mixture of the additive "primary" colors — red, green and blue.

The top can be constructed in a variety of ways. The main design differences have to do with the method used to spin the disk. The simplest method is shown below.

a simple color top

The top itself consists of a rigid wooden disk, about 8 inches (20cm) in diameter. Glued at its center is a wood dowel with a threaded metal rod at the end, which extends through the disk and out the other side. A wing nut and washer are screwed onto the metal rod to hold the paper samples in place on the surface of the disk. The disk is spun by rubbing the wood dowel between the palms of both hands.

It is easier and more effective to spin the disk mechanically, for example with a variable speed hand drill or fan motor. The dowel can be gripped like a drill bit, or the disk mounted on the fan rotor shaft.

The color samples can be any colored material that will not flutter as the disk is spun. The easiest to prepare are single 8" disks cut from 300 GSM watercolor paper, and painted with the red, green and blue colors used to analyze color samples. The disk must have a hole cut in the exact center, large enough so that it can be inserted over the metal rod in the center of the color top.

More flexible analyses are possible by using three disks, one for each of the additive "primary" hues. Each disk must be cut radially (from the center to the edge), so that disks can be partially overlapped to produce different color mixtures. The proportion of each color visible can be adjusted by turning one disk while holding the other two fixed.

If three or more disks are used, the top can be spun in only one direction — opposite the direction in which the disks are overlapped. However, if you have woodworking tools available, you can make an 8 inch disk of plexiglas with a hole drilled in the center. This can be placed over the colored paper disks to hold them in place while the top is spun, and the top can then be spun in either direction.

Almost any color of red, green or blue will produce acceptable color mixing demonstrations. For results approaching the actual mixture of R, G and B inputs to the perceived color, specific shades of each additive "primary" color are required.

When Maxwell conducted his color mixing experiments, he used these historical pigments:

• Vermilion PR106 (scarlet). This is a moderately saturated scarlet red, no longer available commercially. Pyrrole orange (PO73) is a good modern substitute, both more intense and slightly more yellow than vermilion. Disazo scarlet (PR242 or any cadmium scarlet (PR108) are also excellent. (Maxwell conducted his experiments before orange and red cadmium pigments were available.)

• Emerald Green PG21 (green). This is a bright, bluish green pigment, no longer available commercially. A mixture of phthalo green blue shade (PG7) lightened with a small amount of hansa yellow light (PY3) and titanium white (PW6) is a good substitute.

• Ultramarine Blue PB29 (violet). Maxwell may have used a slightly greener shade of blue than the ultramarine (French ultramarine) used in today's artist's pigments. Winsor & Newton ultramarine blue GS is a close approximation, but any ultramarine blue or deep (bluish) ultramarine violet (PV15+PB29) will do nicely.

To make the analysis disks, scribe an 8 inch circle on the white watercolor paper with a compass, and paint each color at a concentration yielding the maximum color saturation. (See the page on dilution, value & saturation for instructions on how to do this.) When dried, flatten the paper if necessary by moistening from behind and then compressing overnight between weighted towels.

Then cut the disks from the paper, punch or bore the center hole, make the radial cut, and use a protractor to divide the circumference of the disks into ten equal intervals — 36 degrees each — starting from the radial cut as zero.

The ideal analysis colors will produce a pure gray of medium lightness if spun in exactly equal proportions (e.g., one third of the disk is covered by each color). In practice, disks that approximate this color balance are perfectly useful to explore basic color relationships.

locating a color top mixture in a Maxwell triangle

The diagram shows how a specific color top mixture is located within a Maxwell triangle.

A sample color is painted on a smaller disk that is placed over the analysis color disks. The top is spun repeatedly, and the three analysis disks are "dialed" to change the proportions of red, green and blue visible in a color mixture.

When the sample and analysis colors match as closely as possible when the disk is spun, the proportions of red, green and blue are read from the markings along the edge of the colored disks. These proportions define a specific point within the Maxwell triangle (illustrated for cerulean blue).

For a precise match, disks of pure white and pure black can be added, making 5 analysis colors in all. These are necessary to create unsaturated hues such as browns and pastel colors.