To install the heater in the thermos, Roger fashioned a cylinder out of a wide-mesh steel screen, available at a well-stocked hardware store. He loosely coiled the heating rope around the cylinder and covered the entire assembly with a centimeter-thick blanket of Fiberfrax, a clothlike material made of spun alumina fibers. (Because you can¿t purchase Fiberfrax in small quantities, the Society for Amateur Scientists will provide it for $5.) Muffler packing, available at a motorcycle parts store, would also do. The whole thing snugs into the thermos through its wide mouth. Finally, Roger tightly rolled a strip of Fiberfrax into a plug that just fit into the thermos mouth. A single twist of steel wire wrapped around the plug prevents it from unraveling.

Illustration by Daniels and Daniels ...and encased in insulating Fiberfrax or muffler packing

The most economical way to measure the temperature is a K-type thermocouple, which produces a voltage in proportion to the temperature. The voltage can be read with a high-end digital voltmeter that has internal circuitry to interpret this sensor. Otherwise, you can estimate the temperature by measuring the voltage developed between the leads using a digital voltmeter. The temperature in Celsius is given approximately by multiplying the voltage in millivolts by 27.7; for Fahrenheit, multiply by 50. I tested Roger¿s furnace using a bare-wire thermocouple from Omega Engineering (part no. CHAL-015). I insulated it using a short length of Nextel sleeving (Omega part no. XC4-116) and installed the sensor near the top of the furnace. At 480 degrees C inside, the exterior was uncomfortably warm but not too hot to touch. When, as a safety test, I pushed the device to 600 degrees C using an ultrahigh-temperature heating tape, the outer casing got far too hot to handle.

So make sure to monitor the temperature at all times and keep it at or below 480 degrees C. Keep it well away from curious children and pets. Wire in a timer switch. And connect the heating unit through a ground-fault switch, such as those often seen in bathroom wall outlets these days. These switches contain an internal circuit breaker that blows when a short circuit occurs. That way, if the furnace should overheat and short out, the power will be cut off.

Using the furnace, you can easily measure the organic content of soil. First, carefully weigh about 100 grams of dirt from your garden and dry it in your kitchen oven for one hour at 120 degrees C (about 250 degrees F). Then weigh it again. The soil in my garden turned out to contain 33.2 percent water by weight. Tightly wrap the dry soil in aluminum foil and bake it in your thermos furnace for two hours at 480 degrees C. The charring organics liberate a ghastly waft of smelly smoke, so use a fume hood or keep the device outdoors. A final weigh-in revealed that my garden dirt is 8.6 percent (dry weight) organic material. Sand from a nearby playground weighed in at just 3.2 percent water and contained a scant 0.7 percent organics (dry weight).

It would also be interesting to monitor the weight continuously, in order to look for physical processes that occur at different temperatures.

This article was originally published with the title A Furnace in a Thermos.

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