Tuesday, January 18, 2011

Why Fire is Cool - entry #3 - Ash Ash Baby

When sitting around a campfire I almost always find myself silently staring, mesmerized by the smoldering ashes.  Once I snap out of it, instead of re-joining the conversation with my campfire pals  I often start taking pictures of the ashes.  Though it doesn't live up to the awesomeness of the moment, here is one (notice backlit marshmallow in foreground):

Before we discover the amazing things humans have done with ash, let's figure out what the hell ash is and why red-hot ashes look so awesome.  Once you burn away all of the combustible molecules in wood, the only things left behind (around 1% of the original unburned weight) are the non-combustible nutrients the tree used in order to stay alive.  Ash contains nutrients like calcium (~30% of the ash), potassium (~10% of the ash), and sodium ions (~1% of the ash) along with other metal and non-metal ions (reference).  It is partly these metal ions that make red-hot ashes look so awesome.  As you heat up metal ions in a fire, their electrons will gain energy then lose energy, in a process that results in the emission of light.  Each metal emits light of a specific wavelength, and if you take any substance containing metal ions and put it in a flame you will see this light (this is known as a flame test).

The flame-test emission of metal ions commonly found in wood ash (calcium, potassium, and sodium).

If you compare the flame-test picture above with the red-hot ash picture, you will notice the red-hot ashes look most similar to the red-hot calcium ions, which is in agreement with the fact that calcium is the most-abundant metal ion in wood ash.  Awesome!!  Here are two of my favorite flame-test metals, copper and lithium:

The flame-test emission of metal ions I wish were found in wood ash.

Here is me performing a flame test on a solution of lithium carbonate in methanol:

Flame test of lithium carbonate in methanol. Believe it or not I am a trained professional, do not try this at home.

If I had way more free time and motivation I would figure out a way to impregnate a tree with copper and lithium ions, then (safely) burn it down.  The resulting ashes, glowing lithium pink and copper green, would probably look AMAZING!

Now that we know what ash is, which is the first reason why it is awesome, I can tell you the other reason.  Humans first learned how to make soap using ash!

Back-yard soap making in the American South, ca 1939.  Photo by Carter Poland.

One of the first documented cases of soap making comes from a Sumerian (modern-day Iraq) stone cylinder dating from the third-millennium B.C. (Bronze age).  It says to mix water, "alkali" (extracted from ash), and oil (awesome reference, subscription required).  This chemical transformation of oil into soap is possibly the first chemical reaction ever devised by humans, aside from burning stuff, of course.

Soap-making instructions and other stuff.  Cylinder of Gudea, ca 2100 BC.

Stick with me to the end, because I promise an awesome ending.

Ash can be used to make soap because in addition to the metal ions discussed above, soap contains other ions like carbonate and hydroxide ions.  These are known as bases and are so reactive that they will transform fat and oil molecules into soap molecules.  This reaction, known as saponification, is the same reaction that Tyler Durden performs in the movie Fight Club, and is shown in nerd language below:

The base-catalyzed saponification of olive oil into fatty acid salts and glycerol.

Importantly, fats/oils/grease do not mix with water, but all the products of saponification (below the arrow in the reaction above) can be dissolved in water.  So, soap making was most likely discovered when some ancient Sumerian pot-scrubbing person was trying to clean a bronze cooking pot encrusted with grease, and in a fit of desperation decided to throw some ash in the water.  The base molecules from the ash would then turn the fat molecules into water-soluble soap molecules, which would then be washed away, leaving the cooking pot as good as new.  WOWZA!!!  It was probably only later that someone figured out that the ash+oil mixture could be used to clean other stuff too.  For more on how soap works, go here and here.

Finally, to tie it all together, let's think about how fire gave us charcoal.  We used charcoal to turn rocks/minerals into bronze cooking pots (entry #2 of why fire is cool).  Those pots became encrusted with grease, which we learned how to clean using ashes, which is how we discovered soap!!  All that, because of fire!!!!

Related Posts:
FirePost #1: "What are Flames Made Of?"
FirePost #2: "How Charcoal Changed the World"
FirePost #4: "Ancient Energy Unleasher"


  1. Do they still have those awesome flame test photos in the hallway in Madison? I love chemistry art!
    Great post, Lee! Fire is awesome.

  2. epic fire/beard gif! it doesn't get much more bad ass than that. (unless you had on your three wolf t-shirt).

  3. @Amber Thanks! They def still have the flame test photos up. The hallway science stuff @ madison is awesome!

    @Mike Thanks! I make gifs out of anything I can these days. I need to get a wolf t-shirt that is a magic eye but is also an animated hologram and make a gif out of that then the wolf demon from my dreams will finally come and suck me into the underworld!

  4. Lee what about sparks? Are they technically just tiny flames, just little bits of electrons getting freaked out? Why are static sparks blue while friction sparks are orangey?

  5. @Dave excellent questions! Light is always caused by electrons getting freaked out, though that can still come in many different forms. As for static sparks, those are similar to lightning in that they are caused by the flow of electricity. A difference in charge between two things (the clouds and the ground, or your hand and the doorknob) provides the energy necessary to excite the electrons in atmospheric gas molecules (nitrogen, oxygen, etc), and the light is emitted as those electrons move lower and higher in energy. As for why that is blue, that has to due with how the electrons of nitrogen and oxygen molecules are arranged. For more on that:

    As for friction sparks, the most straightforward example is flint on steel. That works by flint shaving off tiny tiny little iron particles off of the steel. Those iron particles are heated up by the friction, hot enough that they actually burn, and turn into tiny rust particles. The light in that case I'm less certain about right now... it could be caused by blackbody radiation ( from the hot-as-hell-but-not-yet-burnt iron particles, or it could be caused by the electrons in the iron atoms actually flying off then falling back down in energy and emitting light. This is similar to what would happen by holding iron atoms in a flame, as iron in a flame test is supposed to be gold-colored, which seems pretty consistent with what sparks look like.

  6. Awesome - now I want to build a giant copper green campfire! Actually, last year I had a fire that burned a crazy hot pink...different wood maybe?

    1. That's crazy! I would assume there were some lithium ions in there, but a plant biologist might call me crazy. Strontium gives a pink-ish flame test:
      and I know that strontium can substitute for calcium in our body. So, maybe there was some strontium on the soil??? Again, wild hypotheses flying here. Would love to see a pink campfire though!

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  8. Hello,What a post :) Great work. Why are static sparks blue while friction sparks are orangey? Thanks for sharing.

    1. I'm assuming by "friction sparks" you mean sparks that happen when a piece of metal scrapes along the road, for example. Those are orange because they are little pieces of super-hot metal coming off the larger piece of metal. They are orange because of something called Blackbody Radiation, which I am still trying to wrap my mind around. Friction sparks are blue because they come from the air becoming conductive. In order to do this the air becomes a plasma. The electrons moving around in this plasma give off the blue color. Excellent question. Hopefully this answers it!