Wednesday, February 01, 2006

On the Existence of Wooden Pots, Part 2

Greetings Science Squaddies! Nice to have you back. Before we get started, don't forget that if you have a question that's been burning in your soul like a flaming nugget of ignorance, please drop me an email at suburbanjoe -@- gmail . com and I'll do my best to answer the question. You can also leave questions in the comments if that suits your fancy.

Last week we talked about why you wouldn't use a wooden pan, discounting the obvious reason. Today we look at that obvious reason. When preparing this post, I asked my wife why wood burns but iron melts and here response was that iron is a "melty thing". I'm assuming that this means that wood is a "burny thing" but I didn't take the time to probe further. My curiosity only goes so far.

In truth, it's not that much more complicated than that. Iron is a pretty simple substance. All of the iron atoms arrange themselves in a nice crystalline pattern, metallic bonds keeping the atoms all nestled snug in their iron-y beds. Provide enough energy, from heat in our case, and these crystalline structures break down. The atoms still have enough of an attraction to each other to not go all over the place and iron turns into a liquid, or melts. Thus, iron is a melty thing.

Wood on the other hand, is anything but simple. Yes, it's a solid, as anyone who has ever been whacked with a 2 x 4 can attest to, but it's not a solid in the traditional, chemical sense of the word. Wood is made up of a number of things, but for our discussion, we'll talk about the big 3, water, cellulose and lignin. Water, is well, water. Not much to talk about there. Cellulose is a carbohydrate (carbon, hydrogen and oxygen whipped up into a creamy compound) that makes up the wood's cell walls. Lignin is a chemical compound that acts as a glue for the wood cells. There's also a whole bunch of other stuff in wood that we won't get into, mostly because I don't understand a blasted word of it.

Although not pertinent to our discussion, I'll offer this article up for your enjoyment. It's pretty interesting and talks about how the angle of the cellulose microfibers in the wood cell walls, relative to the orientation of the cells themselves offer various structural characteristics to trees. Cool stuff. You know, or not.

Anyways, back to burny things. So, you carve your pan out of wood, place it on the stove to get it warmed up, and 10 minutes later you're out on the front lawn with a fire department blanket on you while you watch your collection of Neil Sedaka cd's go up in flames. What happened?

When wood comes into contact with a large enough heat source, the first thing that happens is that the water inside the wood starts to evaporate. Once the wood gets to around 300 degrees, the cellulose starts to fall apart. As we mentioned before, cellulose is made up of Carbon, Hyrdogen and Oxygen. As two of these are gasses, we would expect some of the decomposing cellulose to turn into gas. And looky here, it does. That would be smoke. There's carbon in there too, so don't think that it doesn't get to fly around all willy-nilly, letting the other two elements have all the fun. Once the temp gets up to around 500 degrees, the compounds in the gases start to break apart and ignite, and we have fire. What doesn't burn travels up into the atmosphere or sticks to your chimney as creosote and then you have to have Dick Van Dyke come and clean your chimney lest you start a chimney fire and burn your house down. See cd collection, Neil Sedaka.

Now, this explains what happens when iron melts and when wood burns, but doesn't explain why wood burns and not melts and why iron melts and doesn't burn. Iron melts because you can apply enough energy to free up the atoms enough to let them slide around as they would need to, to be a liquid. Not so with wood. The structure of cellulose is such that the melting point is actually higher than the burning point, so you won't ever see it melt, it'll fall apart from the heat first. Blame it all on those pesky hydrogen bonds.
For the record, iron will burn, if in an environment of pure oxygen, but I don't see that being the case in your kitchen, so I think you'll be OK. Similarly unfounded are your fears of your iron skillet melting onto your stove, taking your eggs with it, as the melting point of cast iron is around 1200 degrees Celsius. However, don't let that dissuade you from doing some smelting in your home. It certainly didn't for this guy.

So, there you have it. Fear the stove no longer!

Next week: It's the Bubble Stupid!

Resources:

How Stuff Works - How Fire Works
Ask A Scientist - Wood Does Not Melt
Ask A Scientist - Chemical Makeup of Wood
Wikipedia - Cellulose
Wikipedia - Lignin
Nikon - MicroscopyU: Confocal Image Gallery - Wood Cells

3 comments:

Bones said...

You know, if you had been my science teacher in high school, I may not have ended up such a dumbass. Another enjoyable science read. If I can think of any questions, I'll mail 'em.

suburbanjoe said...

Excellent! I'm glad you like them. I went to school to be a physics teacher, but alas, no jobs were around. Now it'd be too much of a pay cut for me to enter the field. Plus, the lack of support that teachers get from parents and the administration is appalling. And you're not a dumbass. ;)

Capt Crash 31 said...

Thanks for the follow up.... you totally rock.