Illustration: DANIELS & DANIELS BROILER IN A BOTTLE can safely reach 480 degrees Celsius¿enough to vaporize most carbon compounds. The heating rope, which is held in place by a wire screen, is powered via a dimmer switch, mechanical timer and ground-fault socket. A thermocouple attached to a digital voltmeter measures the temperature.

When set at its maximum temperature of about 260 degrees Celsius (500 degrees Fahrenheit), a kitchen oven is quite capable of rapidly reducing an expensive steak into a sizzling mass of crunchy carbon. This I know from sorry experience. But ordinary ovens are still not hot enough for many research needs. Measuring the organic content of soils is one example. Fertile earth contains all sorts of biochemical and microbial goodies that higher plants cannot live without. To discover how organically rich a soil is, you have to weigh a sample, remove the organics and then weigh it again. The only way I know to eliminate all the organic material is to bake the soil at a high temperature. At around 450 degrees C (840 degrees F), organics break down into their constituent elements, and the carbon bonds to atmospheric oxygen to create carbon monoxide and carbon dioxide gases. The charred residue evaporates, leaving the soil devoid of all the trappings of life.

Illustration: DANIELS & DANIELS WIRE SCREEN is bent into a thermos-size cylinder...

Because the same process that cooks organic material out of soil will also remove it from the surface of glass, a furnace that approaches 500 degrees C can be used to clean the most intricate laboratory glassware. Likewise, baking sorbents at this temperature drives away chemical contaminants and recharges them for reuse in, say, pumps for producing ultraclean vacuums (the topic of the October 1996 column). Such a furnace would have other uses as well, including melting enamels, activating glass beads for use in chemical separators, annealing glass and metals, and making electrical feed-throughs for laboratory glassware.

So you can see why I was thrilled to learn that Roger Daschle, a talented musician and hiking buddy of mine, had developed a small furnace that is safe to operate at these temperatures. It consumes a scant 80 watts, heats up in less than an hour and can be built for as little as $60.

Roger and I are part of an informal ensemble of self-absorbed iconoclasts who hike every Friday in the San Diego County foothills to get away from our offices and talk tech. His ingenious innovation came to him while he was pouring a cup of hot chocolate during a lull in our discussions of chaos theory and homemade infrared detectors. Roger wanted to build a stout furnace to service a small chemical separator he was developing. When he poured a cup of cocoa from his thermos and saw the rising steam in the cold afternoon air, he realized that he had found the perfect container. A thermos is inexpensive and has negligible thermal mass. He knew that if he could secure a high-temperature electric heater inside a suitable thermos and plug the top with an insulator, he would have a fully functional and highly efficient desktop furnace.

Illustration: DANIELS & DANIELS ...around which the heating rope is then wrapped

Roger showed off his invention at our next hike. He had purchased a Stanley-brand wide-mouth thermos (the kind typically used to hold soup) for $25 from a local discount store. But the brand doesn¿t matter. Just make sure the vacuum bottle is made of steel and not glass, which might break, or aluminum, which might soften and implode. Roger got things cooking with a rope heater: a prefabricated bundle of Nichrome wire wrapped around an insulating core and covered with an insulating sheath. These cords run on wall current and are much safer than bare wire. Omega Engineering sells them in three-foot lengths for $22 (www.omega.com, part no. FGR-030). The rope is rated for operation at 480 degrees C (900 degrees F). This sets the safe operating temperature of the furnace. The device will get much hotter if you run too much current through it. You can keep the current at a safe level by wiring in a household dimmer switch and monitoring the temperature. As a precaution, Roger wisely wired in a one-hour mechanical timer to make sure that his unit could not be accidentally left on.

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