19th century charcoal pile before it is covered with sod (left) and a diagram of its internal structure (right).
Charcoal is made by burning vegetation (usually wood) in the absense of oxygen, or rather without an external source of oxygen. The process of making charcoal burns off the water and other impurities and leaves the carbon behind. The result is relatively pure carbon. If you burn the charcoal it attains much higher temperatures than burning wood, in part because you don't have to fight with the natural water content of wood. Burning charcoal can easily reach temperatures exceeding 1,100 degrees Celsius (2,010 degrees Fahrenheit), making it the perfect fuel for working with iron, which has a comparable melting point.
A video on the production of charcoal is here:
Primitive Technology: Charcoal- https://www.youtube.com/watch?v=GzLvqCTvOQY
Charcoal also has a long history in medicinal recipes stretching as far back as there are records. It is still used medicinally, especially for detoxifying certain poisons and direct applications to wounds. The internet if full of uses, from face creams to teeth whiteners to desiccants. It's in your Odor Eatersª.
It is also a great pigment and is found on the walls of caves dating back 40,000 years. Charcoal is also one third of the recipe for black powder, but that's another story. In our story charcoal is an amazing filter.
Charcoal is naturally alkaline... or so it seems. How would you test for this?