THE ACTION OF LIGHT ON DYED COLORS.

That light can effect radical changes in many substances was known to the ancients.  Its destructive action on artists’ pigments, e.g., the blackening of vermilion, was recorded 2,000 years ago by Vitruvius.  Since that time it has been well established, by numerous observations and experiments, that light possesses, in a high degree, the power of exerting chemical action, i.e., causing the combination or decomposition of a large number of substances.  The union of chlorine with hydrogen gas, the blackening of silver salts, the reduction of bichromate of potash and of certain ferric salts in contact with organic substances, are all familiar instances of the action of light.  In illustration of this, I show here some calico prints produced by first preparing the calico with a solution of potassium bichromate, then exposing the dried calico under a photographic negative, and, after washing, dyeing with alizarin or some similar coloring matter.  During the exposure under the negative, the light has reduced and fixed the chromium salt upon certain parts of the fiber as insoluble chromate of chromium (Cr_{2}O_{3}CrO_{3}) in the more protected portions, the bichromate remains unchanged, and is subsequently removed by washing.  During the dyeing process, the coloring matter combines with the chromium fixed on the fiber, and thus develops the colored photograph.

The prints in Prussian blue are produced in a similar manner, the sensitive salt with which the calico is prepared being ammonium ferricitrate, and the developer potassium ferricyanide.

Investigation has shown that the most chemically active rays are those situated at the blue end of the solar spectrum; and although all the rays absorbed by a sensitive colored body affect its change, it is doubtless the blue rays which are the chief cause of the fading of colors.  Experiments are on record, indeed, which prove this.

Depierre and Clouet (1878-82) exposed a series of colors, printed and dyed on calico, to light which had passed through glasses stained red, orange, yellow, green, blue, and violet, corresponding to definite parts of the spectrum.  They found that the blue light possessed the greatest fading power, red light the least.

More recently (1886-88) Abney and Russell exposed water colors under red, green, and blue glass, and came to the same conclusion.

But the chemical energy of the sun’s rays is not the sole cause of the fading of colors.  There are certain contributory causes as important as the light itself.