Monday, December 8, 2008

Damn Hot

Posted by Kirk Mona

We're looking to build a new sugar shack at work and that made me think about making maple syrup. When I teach my programs, I demonstrate the art of rock boiling. For rock boiling I build a fire and let igneous rocks (mostly basalt) sit in the middle for a couple of hours until they glow orange. I then transfer the rocks from the fire to a hollow log full of water to show how people boiled maple sap before they had metal buckets. The heat of the rocks transfers to the water and it goes into a furious boil. It is really cool but one of the questions I always get asked is how hot are the rocks? I knew they were over 212° F since they made the water boil and I figured they were way over 212 since if you put rocks in boiling water they don't start to glow. I tried to find a website with a reference to how hot rocks have to be before they glow but found nothing. Then I tried a new approach and found the following on a website about volcanoes.

(emphasis mine)

"By way of its color, incandescent rock gives a crude estimate of temperature. For example, orange-to-yellow colors are emitted when rocks (or melt) are hotter than about 900 degrees Celsius (1,650 degrees Fahrenheit). Dark-to-bright cherry red is characteristic as material cools to 630 degrees Celsius (1,165 degrees Fahrenheit). Faint red glow persists down to about 480 degrees Celsius (895 degrees Fahrenheit). For comparison, a pizza oven is operated at temperatures ranging from 260 to 315 degrees Celsius (500 to 600 degrees Fahrenheit)."

So there you have it. Glow will persist in rocks down to 480 Celsius which means they will start to glow around there too. I've gotten them hot enough to do more than just faintly glow but a faint glow is pretty typical. 480° C equals 895° F and from what I gather, hardwoods in a fireplace burn around 900° F so that jives with the number above.

I guess what I'm trying to say is, "Hell no I'm not picking that rock up with my bare hands."