Of Charcoal and Ashes

Charcoal is formed by the incomplete burn, or combustion, of wood. Made of mostly cellulose wood does not burn immediately, first it releases steam and turns from white to black; it chars, thus becoming charcoal. When charcoal burns in contact with air, its carbon combines with oxygen to form the gas carbon dioxide (and lots of heat). The white ash leftover from burning charcoal is what remains of the small amount of non-flammable minerals which were present in the wood from the start.

The charring of cellulose: Cellulose (CH2O) in combustion forms Carbon (C) and water (H2O)
The combustion of charcoal: Carbon (C) heated with Oxygen (O2) forms Carbon Dioxide (CO2)

As different varieties of wood grown on different soils have different compositions, ash will also have a variable composition, something to keep in mind when using ashes to leach lye. The part of ash that can dissolve in water and be leached out is called potash, or potassium carbonate. But potash is not the only soluble component, and depending on soil conditions, sodium carbonate might also be present. Along the seashore, and especially when burning marine plants there may be more sodium carbonate than potassium carbonate present and this product is called soda ash. Sodium sulfates and potassium sulfates can also be present, and for the proper making of lye for soap, these contaminants should to be removed as they can interfere with the saponification process.

Potash and soda ash can be crystallized by evaporating water from the leached alkaline solution. As potassium carbonate (147g/100ml) is highly soluble compared to sodium sulfate (11g/100ml), potassium sulfate (12g/100ml) and sodium carbonates (22g/100ml) the contaminants will crystallize out of solution first. If the alkaline solution is left alone for a while the less soluble compounds crystallize out and will sink to the bottom while the potassium carbonate would stay in solution as long as possible. Pour off this solution to leave these crystals behind and the lye made will have less contaminants.

When wood is burned without the presence of oxygen (anaerobic) it turns black as the water is evaporated out, leaving charcoal behind. If access to air is limited and heat is removed, the charcoal will become stable and available for future use. Charcoal takes up less space and is able reach a higher temperature with the addition of extra air (bellows) than a pile of wood, which makes it ideal for use in a smithy’s furnace. To make proper charcoal an anaerobic burn is necessary, and in history people have found different ways of doing so, mostly by either digging in wood in hill sides or covering wood with a burn resistant material (like a metal kettle) while building a large fire right over it.

A side note:
The burning of marine plants creates sodium carbonate rich ashes. At the sea shore where barilla is available in abundance making soda ash is not much different from making potash. But it’s a challenge for the small scale experimental soap maker as the goal is pure barilla (or seaweed, or kelp) ashes, without potassium carbonate (land based plants) contamination. As marine plants don’t easily burn on their own, unlike wood, a way around this is to first make for instance seaweed charcoal, which does burn, and then burn it to ashes, with plenty of air for complete combustion to create fine white ashes for the lye leaching vat.

Download the complete document, with step-by-step photo essay, at
https://www.academia.edu/27757474/Of_Charcoal_and_Ashes

Copyright by Susan Verberg, 2016 - For educational use only.