Saltpeter is the common name for potassium nitrate (KNO3). Using urine is an old school method for manufacturing saltpeter. To make saltpeter from urine you need only a couple of ingredients. Urine and organic matter such as straw, straw being nothing more than cut grass. However, as you can guess it isn't as simple as just mixing urine and grass. No, this is a long long process that takes months.
First you need a place to keep and mix everything. You will need a large container such as a barrel or a tarp, plastic or whatever. The thing to consider is you want a bottom that will not let all the valuable urine escape. Evaporation is OK, but you do not want the urine to ever just leave by flowing into the ground. The saltpeter bed will also need to be absolutely protected from rain. Rain water will wash away all of the salts that you are trying to collect and you want as little sunlight as possible.
Here is one very good method for manufacturing saltpeter bed. Collect a good supply of animal excrement. To make the bed you need a floor as stated earlier. The floor that doesn't ever collect water should be selected. A clay floor was traditionally made, but any impermeable floor material can be use. Next cover the floor of the bed with manure, wood ash and straws.
Now you need a roof to protect it from rain and sun. The roof that doesn't leak can be anything of your choosing. Wood, plastic, tin or a tarp are all acceptable.
Now your bed can be any size, but since this is a long process that doesn't yield a whole lot of saltpeter the bigger the better. 15X10 and a few feet deep is a good goal. Sides or desirable, but not required. Now the fun begins. Continually dump urine on the pile. That is right you need to pee on the saltpeter bed. The goal is to keep the bed always moist, but not dripping wet. So don't make the bed bigger than your piss can keep up with.
If you want to improve on this, you could have drains or even a slope to allow you to collect runoff from the heap which can then be added to the top of the heap again or saved for a later time when the heap needs more liquid to keep it moist.
The saltpeter bed should be turned over to a depth of five or six inches every week, and the whole heap turned over every month. This will facilitate the entire bed producing and not just select parts that have the right conditions.
This entire process is long and is greatly influenced by the temperature of your climate. In the south it will be much quicker than in Michigan! In the south it might take 7 months. Up north it can take a year. It is extremely variable as materials used in the bed also affect the speed of nitrification.
During the last few months of the process do not add any more urine or anything else. ONLY add enough water to keep the bed moist. The reason for this is that un-decomposed organic matter will interfere with the separation of the saltpeter. So the process for the first few months is simply a decomposition phase where you are building up a large amount of decomposed organic matter. Better to go to long on the first phase than to move to the water only phase without a lot of really decomposed material.
As the bed ripens, the saltpeter is brought to the surface by evaporation, and appears as a whitish efflorescence. When this efflorescence appears the top couple of inches of material is separated and placed in its own heap where it is continued to be kept moist just like the original saltpeter bed. This process of removing the top of the bed to be added to the previously removed contents continues until most, but not all of the bed has been removed. Let's say 75% has been removed. The remaining 25% is left and used as a catalyst for the next batch of saltpeter. Just add more hay and excrement and your second batch of saltpeter has already started.
Now back to the 75% that has been removed from the original bed. This removed 75% (nitrified earth) is now ready for leaching.
The process of leaching involves putting the nitrified earth into a vat, or tub, or barrel. Cover with straw. Add water. How much? If you have 2 parts of nitrified earth you need to add about 1 part of water. Stir! Then, let sit for 12 hours. Drain off half of the water. Now add water again. Since you drained off half the water add that same amount back. Let it sit for just a few minutes and again drain off half of the water. The first batch of water will have twice as much saltpeter as the second. Continue this process five or six times. This should have pulled almost all of the salts out of the nitrified earth. The now exhausted nitrified earth can now be thrown back onto the original bed or can be used to start an entirely new bed. The water collected from the nitrified earth we will call saltpeter ley.
The saltpeter ley (liquid) contains saltpeter as well as nitrate of lime and magnesia and chlorides of sodium and potassium. The next step is to convert all other nitrates into nitrate of potash. This is done by taking lots of hardwood ashes and mixing them into a vat of water. Stir, then let settle and then carefully pore or siphon off the liquid. The more ash the stronger the water will be.
Now you will need two containers ... One of wood ash water (lye) the other of saltpeter ley. The next step is to pour the wood ash water a little at a time into the saltpeter ley. As you do this you should see a whitish cloud appear. As you pour more and more wood ash water (lye) into the saltpeter lay, you will eventually notice that the white cloud is no longer produced. At this point you will not be helping by adding more wood ash water (lye).
Now allow the mixture to evaporate. A container that is more like a tray than a barrel will evaporate off the water more quickly. The evaporation portion can be done in batches.
Heat is now added to the solution to bring to a slow boil. So boil the saltpeter ley to begin concentrating the solution. Blood can be added to the mixture to encourage organic matter to rise to the top as scum and dipped out. Scoop out and discard any crystals that form while boiling. After a few hours of boiling the water can be filtered through something like cheese cloth to remove any solids. Hopefully there are few are none as this step is not absolutely required.
The ley still contains common salt and some other impurities in smaller, quantities. When the concentration has reached almost the point of saturation, the ley must be allowed to cool. You will know when this point has been reached by dropping a little bit of the boiling liquid onto a piece of metal. If it quickly crystallizes, it is time to stop the boiling.
Now that the liquid is boiled down but not completely boiled away let the liquid cool down, crystals will begin to appear. The first crystals that form will be various non-saltpeter crystals, rake these out. As the saltpeter is more soluable than the other salts. Eventually the crystals will form in a more needle like form. These are the good saltpeter crystals that we have been looking for. Saltpeter crystals will form and sink to the bottom. Anything that remains floating should be skimmed off and discarded.
These collected saltpeter crystals are not pure saltpeter yet. They still contain maybe ~20% impurities.
Redissolve crystals in small amount of boiling water
Remove any crystals that appear
Pour through improvised filter then heat concentrated solution
Spread out crystals and allow to dry
Uses for Saltpeter:
Ingredient in gun powder!
Can be used in curing meats! For better taste.
Tree Stump Removal.