Last updated: 22 June 2016      

The information below was provided by Helen Dawson:

  • Pack dry sticks in a metal tin as tightly as possible.
  • Heat wood in the closed tin to approximately 300 °C (572 °F) in a solar cooker.
  • (Somewhere between 200 °C (392 °F) for a couple of hours and 400 °C (752 °F) for half an hour will do). There is a good explanation of the overall benefits of turning woodchips or other biomass such as corn cobs to charcoal on the Solar Fire open source web page:
  • The wood will give off smoke and water vapour below
    Solar charcoal 006

    Pyrex bowls trap heat stop wind loss

    200 °C (392 °F) unless the lid is very tight..
  • Wipe the pyrex bowl lid to remove moisture and soot if its building up on the inside, before it gets too hot to handle.
  • Over 200 °C (392 °F) approx. the wood is carbonizing.
  • The time and temperature needed to turn the entire contents to charcoal depend on the wood used, the size of the sticks and the solar variables like cloud cover.
  • Over 400 °C (752 °F) approx. the wood which is now charcoal ignites and burns to ash.
  • Best to leave one set up for a whole day first time, with enough mirror capacity to get over 200 °C (392 °F) for a couple of hours, but less than what it would take to set it alight. Same as with cooking food, you're cooking the wood.
  • This is the solar pyrolysis process.
  • The wood doesn't create embers, when there is no air flow in the tin, there isn't enough oxygen for it to burn, and under 400 °C (752 °F) it is less likely to ignite.
  • Sit an oven thermometer on top of the tin under the pyrex, so you can see the temp climb.
  • Remove tin from pyrex bowls in the evening or next morning, when cooled.
    Solar charcoal 009

    Solar Cooker two mirrors and aluminium reflector base

  • When you open the cooled tin the heat from the sun has turned the sticks to charcoal.
  • You can do a drawing with them, and/or the following project.
  • Solar cooked charcoal retains the tar in the tin and on the charcoal, making it a bit sticky to handle. As the temperature increases the stickiness decreases. Many Australian seed pods such as gumnuts and banksias, are germinated by fire, so putting them in the solar cooker will result in seedlings in the PET moss garden.
  • Charcoal improves the soils ability to grow plants AND captures carbon dioxide, it is cabon sequestration.
  • You can help cool the planet by putting charcoal in the soil.
  • Terra preta owes its name to its very high charcoal content, and was made by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil. It is very stable and remains in the soil for thousands of years. Its called Biochar in English speaking countries.

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Helen Dawson