Blackout Glow

As war darkens or dims U.S. cities, their onetime incandescence will be replaced more & more by the ghostly luminescence of pigments which give off light in the dark. Today luminescent pigments are being produced in the U.S. at 500 times their rate before the war. Their makers, well nigh luminescent with delight, hope to produce before year's end 100 times as much as they did in 1941.

Luminescent pigments outline the instruments in the lightless cabins of all night-flying U.S. bombers. In blacked-out fortresses and cities, they are more than a novel convenience; they are a necessity. In darkened Britain, during the early months of blackout, the death rate from falls, collisions and other accidents was almost twice that of well-lit peacetime years. Still high, it would be far higher without the wan yellowish gleam of curbs, guide rails, doorways, signs and even pedestrians' lapels and trouser cuffs touched with luminescent pigments.

The many sorts of luminescent materials are classified by the stimulus which makes them glow. Some materials are affected by sound, radio waves, slow oxidation, cathode rays, or shaking. But all commercial luminescent pigments are photoluminescent: they glow only after stimulation by light. If the pigment glows for several minutes or hours after exposure to light, it is phosphorescent. If the afterglow is very brief--perhaps only 1/10,000th of a second -- the pigment is fluorescent. Hence fluorescent substances glow only when continuously exposed to invisible ultraviolet rays ("black light"), which they reflect as visible light.

But both types of photoluminescence are similar physical phenomena. Many luminescent materials are crystalline. Impact of light waves moves some electrons away from their normal position within the latticed clumps of molecules called phosphors. Then, as each electron moves back to its original orbit, it emits light waves. Length of afterglow depends upon the time taken by the electrons in returning.

Grinding sometimes destroys the crystalline structure of luminescent pigments, but one way that they can be dispersed, crudely ground, through oil or varnish to form paints is with the help of wetting agents (TIME, Jan. 5). Many of the phosphorescent pigments now manufactured in the U.S. are sulfides, chiefly of strontium, zinc, barium, calcium, cadmium, etc.; and a large group have formulas that are military secrets. The fluorescent pigments are chiefly beryllium and zinc silicates, cadmium borates, calcium and magnesium tungstates, and organic dyes.

In their pure form some luminescent pigments glow quite feebly, but they brighten up startlingly when only .0006% of certain metals such as copper are added. Yet if a similar infinitesimal .0006% of iron is also present, the afterglow is dimmed by one-third. Since iron could easily be floating about the laboratories as dust, great care must be taken to guard the purity of the ingredients while they are being compounded in electric furnaces. Result: luminescent pigments are costly--price last week was from $12 to $25 per lb., or $10 to $60 per gallon for paints.

At that, luminescent pigments are now much cheaper than when they were dramatized in the U.S. by Showman Florenz Ziegfeld in 1922. Phosphorescent lace gowns worn by damsels in the Follies were treated with pigments imported at $185 per lb. from France. With cheaper French pigments the Maginot Line was marked phosphorescently within. A cheap luminescent paint, an organic material synthesized from coal tar, was announced last year by Germany's I. G. Farbenindustrie. U.S. manufacturers know the formula, but believe that increasing production of their own better pigments will greatly reduce prices.

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