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One of the oldest recipes to create lye is this one:

  1. Take well burnt ashes and put them in a barrel
  2. Fill the rest of the barrel with water
  3. Leave the suspension for a week or two
  4. You can now try the lye concentration with egg - if 2/3 remain above the water, the concentration is sufficient for domestic uses
  5. Separate the lye solution from ashes by carefully draining the water. You can also use filtration.

Now I know there are traces of various metals in us, as well in flowers and trees. But how can ashes accumulate such a big amount of sodium? And how is the sodium bound into it. How does it turn into hydroxide?

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There is sodium and potassium in wood. When the wood burns some of the sodium and potassium become bound to oxygen. The oxides become hydroxides when exposed to water.

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  • $\begingroup$ What are such amounts of sodium doing in wood? Are they in humans too? Does the potassium also end up in water as KOH? $\endgroup$ – Tomáš Zato Sep 6 '14 at 14:12
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    $\begingroup$ Sodium chloride (common salt) is present a lot in most common organisms. Some Potassium ions for ionic balance. $\endgroup$ – user2617804 Sep 6 '14 at 14:17
  • $\begingroup$ Wood ashes are usually stated to be a source of potash, porassium oxide. Rarely have I seen wood ashes stated as a source of lye. $\endgroup$ – Brinn Belyea Sep 6 '14 at 14:19
  • $\begingroup$ @user2617804 And salt does burn into sodium oxide? Wouldn't than be more profitable to add salt into fire and then make lye of the remains? $\endgroup$ – Tomáš Zato Sep 6 '14 at 15:12
  • $\begingroup$ I speculate that it would depend upon the heat of the fire. A hot fire could drive off chloride and chloride would be replaced by oxygen as the fire cools. $\endgroup$ – Brinn Belyea Sep 6 '14 at 15:31
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Only a very small percentage of the original mass of wood remains after combustion. The Combustion oxidizes the cellulose and other carbohydrates into CO2, CO, H2O, and other gases.

What's left over are the trace minerals, unburned creosote, tars, and other solids that couldn't be gasified. Out of the first burn, much of the residue can be burned off with a higher temperature.

I have a large fireplace insert wood stove and I have burn through 10 cubic meters of wood to get less than 1 cubic foot of ash residue. These are the minerals that include the sodium and potassium that cannot be vaporized. Sodium and potassium are required by all life in trace amounts, including trees for the chemical processes involved in cells.

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