DIY Solar Furnace Plans

This DIY solar project uses a fresnel lens to create a high-temperature solar furnace for soldering, jewelry work or firing pottery.

| July/August 1974

©1959 by D.S. Halacy, Jr., and originally published by the Macmillan Company as a chapter of the book, Fun With The Sun. Reprinted by permission of the author in 1974. 

Most of us learned at an early age that a magnifying glass would do more than simply enlarge the type in a book. A converging lens magnifies heat as well: that is, it concentrates the sun's rays into a small space. By changing a relatively large amount of warmth into a small amount of heat, we can burn our initials in wood, start a campfire or — as with our reflector stove — cook a meal. It is this same principle of concentration that makes possible the huge solar furnaces in operation today. A lens, or mirror — whichever the case may be — intercepts a certain amount of sunlight, focuses all of its heat energy onto a tiny spot and thus produces temperatures as high as 6,500 degrees Fahrenheit.

Centuries ago, scientists were able to melt metals at temperatures in the neighborhood of 2,000 degrees even with the comparatively crude furnaces they were then able to construct. Modern technology has made it possible for the solar furnace to create the hottest temperatures available to man for any appreciable length of time. Of course, such methods as the "shock tube" and plasma jet techniques do produce higher temperatures ... but only at great cost and for limited periods of time.

In France a 35-foot solar furnace has operated for many years, and has performed satisfactorily in melting metals for industrial use. The largest furnace in the United States as of 1974 is that of the Army Quartermaster Corps, which is about the same size.

Obviously these monster units cost fortunes to erect, but anyone can build himself (or herself) a workable solar furnace in a few hours for about ten dollars. Far from being a toy, the result is a practical tool that will melt metals with fairly low melting points, do soldering jobs and even heat a kiln for firing ceramic jewelry.

Back in 1770 the French experimenter, Lavoisier, had convex lenses made especially for him by the same St. Gobain glassworks that is still in operation today. These curved pieces of glass were joined together at their rims and filled with wine to make them equivalent to solid lenses. Light passing through them was refracted or bent in toward a common focal point to heat whatever samples the scientist placed there.

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