How to Make Soap from Ashes
Soap making in the woods can be almost automatic. Hardwood ashes are some of the best producers of lye. Add a bucket of rain water and some left-over cooking fat and you can easily brew up enough soap to clean everybody and everything.
If you need a large quantity of lye, you may want to build a lye leaching barrel. 
Soap Making Is Almost Automatic
Now, soap making in the woods can be an almost automatic thing. Anyone who's done much camping knows that — if you throw some white ashes from a hardwood fire into your frying pan after dinner — the lye in the ash will combine with the fat from the cooking to make a crude soap. This works fine for rough-washing tin plates and hunting knives, but there are times when even the most ornery outdoorsman needs bar soap. We were no exception and — thanks to our instructors — soon became adept at making both soft and hard soap, starting at ground zero with lye from our own leaching barrel.
All you really need to turn out the same sort of non-polluting cleanser that our pioneer foremothers scrubbed with, you know, is lye and animal fat. Whatever meat scraps and drippings you have on hand will supply the fat and the lye comes from wood ashes and water.
To make lye in the kitchen, boil the ashes from a hardwood fire (soft woods are too resinous to mix with fat) in a little soft water, rain water is best, for about half an hour. Allow the ashes to settle to the bottom of the pan and then skim the liquid lye off the top. You can do this daily and when you've got enough of the weak solution, start the soap making process by boiling the liquid down until it'll float an egg. One word of caution: DO NOT use aluminum dishes or pots. The lye will eat right through `em!
Now put that meat fat, left-over cooking lard and vegetable oil into a kettle not over half full, and heat the whole mess until all the liquid has been rendered out of the solid scraps. While it's still hot, add this clean grease to the bubbling lye and continue to boil the mixture—stirring all the while—until it reaches the consistency of thick corn meal mush.
You should have a wooden box two inches high, three inches wide and six inches long handy. This is the mold for one bar.  If you're making more soap, use a larger box and cut the hardened finished product into convenient chunks. Cover the bottom of the box with waxed paper or grease to keep the soap from sticking, pour in the mushy mixture and let it cool. You've got yourself some backwoods soft soap!
Hard soap is made the same way, except that you add a little salt to the mushy mixture as you pour it into the mold. The best proportion we found was two and a half pints salt to five gallons of tallow, and we also discovered that a little powdered rosin added to the grease, just before the lye is mixed in, helps the soap to set more firmly.
Make a Lye Leaching Barrel
Since lye is used in the backwoods often and for many purposes besides the making of soap, you may find it desirable — as we did — to build your own leaching barrel. To do this, take any large wooden or steel container, cut holes in its bottom and put in a layer of pebbles. Place two or three inches of straw or dried grass on top of the little rocks and then fill the barrel almost full with hardwood ashes from the fire. Tamp the ashes down as you fill the container and leave a small depression in the top.
Support the barrel about three or four feet off the ground and place a sloping trough under the keg to catch and funnel into a bucket the lye that seeps out. When you have the apparatus set up, fill the depression in the barrel with water.
Slowly, that water will seep down through the ashes and—after six to eight hours—a solution of lye will begin to trickle (not run) down the trough. Don't get anxious and try to speed the process by adding more water up above until the depression in the ashes is empty. When it comes to leaching out lye, patience is a decided virtue.
The first run will be strong enough to cut grease, but succeeding runs of lye will have to be poured through your processer twice. The finished solution is finished, though, since the leaching barrel produces the same results you'd get by boiling the wood ashes.
A bit of trial and error taught us that hickory, sugar maple, ash, beech and buckeye are the best producers of lye. Most hardwood ashes will do, though, and with them—plus a bucket of rain water and some left-over cooking fat—you can easily brew up enough soap to clean everybody and everything that needs it . . . and maybe even some that don't.