This article appeared in the
in The Textbook Letter, March-April 1997.
During a typical sales pitch, the huckster puts a lump of frozen food (or a lump of ice) onto his "defrosting tray," puts another lump onto a tabletop, and demonstrates that the lump on the tray thaws much more rapidly -- even though the tray doesn't use electricity or any other evident source of energy. The tray, the huckster says, is a marvel of modern technology.
In fact, the tray is merely a hunk of cast aluminum, and the huckster's demonstration is bogus. Frozen food lying on an aluminum tray thaws more rapidly than food lying on a tabletop because the tabletop is made of wood or plastic; these materials are poor conductors of heat, and they are very slow in transferring heat to the food from the surrounding air. Metals are far more effective in this role, because they are far better conductors, so the huckster's aluminum tray wins every time.
There is nothing remarkable here, and one can produce the same result by laying a piece of frozen food onto any thick piece of aluminum or copper. A heavy aluminum griddle or a heavy copper pan, for example, will work just as well as any "defrosting tray." The trays don't have any unusual properties, and the claims used in promoting the trays are bunkum.
This doesn't mean that the trays are wholly worthless, though. If you teach physics or physical science, you can use a "defrosting tray" to build a memorable lesson that will combine thermodynamic theory and hands-on lab work with some consumer education. All that you'll need will be twenty dollars, for buying the tray, and a copy of Robert L. Wolke's article "A Real Cool Con," which appears in the March-April issue of Skeptical Inquirer. In only two pages, Wolke -- a professor of chemistry, emeritus, at the University of Pittsburgh -- explains the pertinent physics, describes his own testing of a "defrosting tray," and suggests some other experiments that you and your students can perform easily. His article is masterly, witty, and a pleasure to read.
William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes often about the propagation of quackery, false "science" and false "history" in schoolbooks.
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