Explained very well by Barefootboy, from the Homesteading Today forum. Edited by Tentance.
Alternative Method for Making Lye
Due to the restrictions being placed on lye sold in stores because of the criminal use of it, I'd like to bring up an alternative method of making lye that does not require wood ashes.
Get hydrated lime at a hardware store (as builder's lime) or garden supply store, it is used in gardening and I found it under the Hoffman brand. The next ingredient is a little trickier, you need to find some basic washing soap powder. It should be straight Sodium Carbonate with NO additives or perfumes. Arm and Hammer (Washing Soda) does sell it in the old blue box some of us remember for childhood, but most big box stores don't carry it. Try discount or dollar stores. I found 100% Sodium Carbonate being sold as a pool chemical inexpensively, called "Pool Time pH UP". Or, you could try heating regular baking soda (NOT baking powder) then use it as the soda ash. I'm not sure how long to heat it, as I have not tried that method, but chemically it's correct.
If you do get these two, simply make an equal mix of each in filtered or distilled water and carefully combine. I suggest starting out with a cup of each to get used to the steps. This will create a white solid (Calcium Carbonate = chalk) and a liquid (Sodium Hydroxide = Lye). Using a funnel and a coffee filter, filter out the white solid.
I mixed 1 tsp of the Sodium Carbonate with 3 oz of distilled water, and 1 tsp of the lime with 3 oz of distilled water. I poured them together and waited 5 minutes for the solid and liquid (lye) to separate, then filtered the mix through a coffee filter in a funnel. The result shows all signs of being Lye.
This process does NOT require heating at any stage of it. It is simply dissolving two powders in enough water so they can mix and separate, and then filtering off the liquid. If you use 1 ( oz, cup etc) of each chemical (lime and carbonate) and 1 (oz, cup, etc) of water in theory you'll end up with 1 (oz, cup) of the solid and 2 (oz, cups) of the liquid which will be 50% strength lye (which may need to be further diluted with water to be used with a lye calculator).
BUT as they said in MASH "this is meatball surgery" this is meatball chemistry, so you will not get the exactness you'd get under controlled lab or industrial conditions.
On the other hand, it will work, and has signs of being economical (yield/cost) and IS an alternative to getting hassled whenever you want to make a batch of soap.
I found that I had to use 2 cups of water to each cup of lime/carbonate (4 cups of water). I am filtering the mix now, and it's looking like I'll recover 2 cups of lye solution, so the 50% strength still looks viable. At the max I'll get 2.5 cups of lye solution, so that's still somewhere between 35 to 40%.
The fancy term for this is a "double displacement reaction" [Editor's Note: Double displacement reaction - aqueous metathesis (precipitation)]. The Sodium Carbonate swaps with the Calcium Hydroxide (slaked lime) to create Sodium Hydroxide (lye/liquid) and Calcium Carbonate (chalk/powder).
Read more for a better recipe and breakdown of the method. Check out this post for confirmation of the validity of this method from a chemistry and soaping text.
You can grind limestone or seashells (calcium carbonate) and then heat it in a kiln/bonfire (1200F) until it calcinates and forms quicklime (calcium oxide), then soak the quicklime in water to create slaked lime (calcium hydroxide).
You can also burn kelp/seaweed to create soda ash (sodium carbonate) instead of heating baking soda. Soak your soda ash with water and filter to leach out the carbonates, and mix that solution with our calcium hydroxide water solution to form Sodium Hydroxide (and dry calcium carbonate again).
Or burn wood to create potash (potassium carbonate) and a small amount of soda ash (sodium carbonate). Soak the ashes with water and filter to leach out the carbonates, and mix that solution with your calcium hydroxide water solution to form Potassium Hydroxide (and dry calcium carbonate again).