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!!!!