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APPLIED exothermia!

5 November 2009 — dan

When I finally got around to making myself some thermite, which like all right-thinking people I've been meaning to do since about the age of 10, the thing that surprised me was how bright it is. The combustion temperature of standard aluminium/iron-oxide thermite is about the same as the operating temperature of a light-bulb filament, and that's how bright the whole burning mass shines.

Here's a nice video of the process of thermite welding, which has for more than a hundred years been used to join train tracks together.

There are lots of other thermite welding videos on GooTube, though not all of them let you see the aftermath, when they remove the crucible, knock the mould sectors away and shape the still-glowing weld.

People who do this trick frequently clearly get rather blasé about it after a while, and hang around close to the crucible, or even do stuff like lighting cigarettes off the top of it. I don't think that is actually a very good idea, unless you are absolutely 100% bet-your-eyes-on-it certain that there's nothing on, or even under, the crucible that may unexpectedly flash to vapour when heated to these extreme temperatures.

Classically it's water, or even damp stone, that causes thermite to "explode", but many other substances will too. As I've mentioned before, many metals will boil at thermite temperatures, and there are all sorts of other usually-considered-inert substances that also don't play well with thermite.

Like, for instance, asbestos. The molten iron from a thermite reaction may have cooled enough to not even melt an asbestos mat, but if you put a chunk of asbestos in with the thermite, it will definitely melt and quite possibly boil.

(This ought, at least, to render the asbestos harmless. Asbestos is basically just silica in an unusual shape, so if you melt it and then allow it to cool, you get a lump of non-toxic glass.)

Posted in Nerdery, Science. 7 Comments »

geobas1 Says:
6 November 2009 at 1:52 am

Here, stateside at least, you can buy a pack of thermite (sort of) over the counter. There are exothermic welding kits for ground wires (cadweld) that are copper oxide and aluminum. It's all prepackaged and available to the public at your local electrical supply distributor. Not sure of the differences in temps and energies between the iron oxide reaction and the copper oxide reaction. There is even a new version that is "safer"; it comes in sealed containers and is set off electrically with an electronic igniter.

geobas1 Says:
6 November 2009 at 3:02 am

Here is the company site on the above, and the welding manual:

http://www.erico.com/products/CadweldPlus.asp

http://www.erico.com/public/library/fep/LT30323.pdf

all available to the public, no license or even identification required.

Here it's actually easier to buy this than it is to buy fireworks.

Stuart Says:
6 November 2009 at 10:51 am

My brother is a amateur (Although it wasn't *me* that said that if he asks) knife maker, and recently he has been experimenting with thermite casting. People forget that thermite doesn't have to use iron, there are many different metals that you can use (and therefore cast with) in the process. The end product is a metal that you cannot easily buy off the shelf (so it isn't entirely about the novelty of the reaction).

Shadowex3 Says:
6 November 2009 at 6:58 pm

I think it's also worth mentioning that just looking at thermite reaction without proper welding protection can be very hazardous to your health.

bmorey Says:
6 November 2009 at 8:10 pm

I made thermite when I was about 15; just a small quantity as I could only 'borrow' pocketable amounts of iron and aluminium powder from the science lab.

I found a better form in a local scrap yard: a box of the thermite heating patches used in portable soldering irons along with the igniters (like a large match coated in sparkler-like material). The circular patches were about 1" x 1/2" and burned for a minute or so. I had great fun with those in the backyard.