I ran across the musketeer site long ago and it is good.
He even demonstrates he made saltpeter from naturaly occuring saltpeter from a wall. It was just as effective even with some contamination. However contamination can easily be removed (and for general use should be as it can be hard enough to scratch the barrel.)
Some additional highly valuable information:
Saltpeter can in fact be made by anyone just be leeching soil if necessary. Essentialy it can be made from animal waste, (even human waste, but for the same reason they don't use human waste for agricultural purposes you should avoid that.)
A really informative reading would be to look up early salt peter production.
Specificly Nitre beds.
That will teach you how to make black powder from scratch even in a doomsday scenario with no chemicals or commerical products.
Bottom line, you could make gunpowder from your own bodily wastes if you wanted.
Of course you could also make smokeless powder and even high explosives from scratch by making some acids from scratch, nitrating and then purifying various things with everything you could aquire on a deserted island.
Some of the most informative reading when read with the understanding of modern chemistry is very old historical alchemy experiments many done as early as the 800's. They are some of the simpliest, and generaly use materials you could collect outdoors.
Be sure to double check them with modern chemistry, but the correct ones are great.
Knowing such information could allow you to make even some of the most powerful modern explosives and propellants on a deserted island so is quite valuable.
Such information is also very important to spread as with it widely known it is appearant that firearms and firearm technology could never be controlled or eliminated.
Of course currently you can just purchase saltpeter and sulphur, and make your own charcoal (the most important part.)
There is various methods to combine them. JCT gives a great one.
For safety purposes be sure to use all non sparking equipment. For an electricly powered mill that includes making sure no fine powder will reach the motor (and some can become quite fine dust). Many mills also come with or suggest steel media or ball bearings for other purposes. DO NOT USE any material that will or can spark, including iron or steel. Lead works well for this purpose.
