Charcoal Ash Assaying Method
HighUintas - Thanks for bringing up charcoal ash assay. DoubleBuck assayed some Charmin toilet paper charcoal not too long ago, and his results were off the bottom of the charts at something like 0.06% ash, where black powder charcoal ash usually runs between 1.5% and 4%, with 2% to 2.5% about average for sassafras and some of the willows. Why toilet paper charcoal ash is often so low is because the ash minerals have been chemically dissolved-out during the several wood-pulping processes.
Ash is not deliberately removed from toilet paper, it just happens that the ash is soluble in some of the wood pulp processing chemicals, a happy coincidence for us!
How the charcoal is made has a bearing on its ash assay because of its creosote content, where the higher the creosote content, the lower the apparent ash content will be, the operative word here is "apparent". How the wood is roasted does not vary the actual ash content contained within the wood by "cooking out" ash, ash can't be cooked-out. Ash is composed mostly of calcium and magnesium salts that have extraordinarily high boiling points up in the 5,000º and 6,500º Fahrenheit range respectively, well above the melting point of iron/steel alloys that are in the 2,900º Fahrenheit range.
So a search doesn't have to be done to find my old post on assaying charcoal for ash content, here it is again:
Assay your charcoal for its ash content. If you shoot, you have an accurate powder weighing balance, and that's the only piece of precision equipment you need. How to find ash content is a pretty simple procedure. You will need a steel tin can lid like from a large size soup can, a propane torch, a kitchen stove, and some of your charcoal that has been coarsely powdered. Outdoors, use the propane torch to heat the steel tin can lid red hot to burn off the protective coating, and then scrub and wash the lid to get it clean. Weigh out more or less 50 grains of powdered charcoal, and note its exact weight to a tenth grain. Put the powdered charcoal on the tin can lid and spread it around in an even layer, keeping it away from the edge of the lid. Set the tin can lid and charcoal on a kitchen stove burner at just enough heat so the lid is a dull red heat. The charcoal will ignite and glow red hot, and after a few minutes, carefully sweep the charcoal around with the side of a length of wire, like a straightened large-size paperclip. Keep sweeping the charcoal around every few minutes, being careful not to spill any off the side of the lid. After about twenty minutes, there should be just a light gray ash left. Turn the stove off, and when the lid is cool enough to pick up, brush all of the ash into the powder weighing balance pan and weigh it. As an example, if the charcoal sample originally weighed 47.2 grains, and after burning it off on the stove, the ash weighs 1.8 grains, then 1.8 divided by 47.2 equals 0.0381. Multiplying by 100 gives 3.81% ash. Round-off to 3.8%
The math used to compensate for ash content is straightforward. Using avocado wood charcoal with 3.8% ash as an example, the math steps are as follows:
1. 3.8% ash means there is only 96.2% combustible material, 100% minus 3.8% ash leaves 96.2% combustible material.
2. Convert 96.2% to decimal form that is 0.962
3. Mathematically invert 0.962 by dividing 1 by 0.962 to find out how much more ash-contaminated charcoal to add to the black powder mix so it comes out at the correct amount of combustible material. 1 divided by 0.962 = 1.03950. Round-off to 1.04
4. For Black powder using the standard 75 parts potassium nitrate, 15 parts charcoal, 10 parts sulfur:
The next step is to multiply the 15 parts charcoal by 1.04 to get 15.6 parts of ash-contaminated charcoal. So, the new black powder formula using avocado wood charcoal is 75 parts potassium nitrate, 15.6 parts of ash-contaminated charcoal, and 10 parts sulfur.
Ignoring rounding-off, the math formulas used give exact values, they are not approximations.
Ignore that the parts add up to more than 100, since the entire weight now includes the small amount of ash that the charcoal has within it, keeping in mind that there is now the specified 15 parts charcoal, while before compensating for the ash content, there was less than the required 15 parts charcoal.
Compensating for ash below about 2.5% or 3% is not worth the bother because other things come into play that affect how accurately the ingredients can actually be weighed, such as moisture content and purity. However, there is no harm in compensating for ash content when it is below 3% if you care to, I do. The best you can do is make sure your potassium nitrate, sulfur, and charcoal are all dry before weighing them, as well as pure. I am a chemist, and a good part of what I did was pay attention to details when measuring and weighing things.
But the main thing is to find some kind of wood that has as little ash as possible, since ash interferes with the black powder burning efficiently, plus ash makes hard barrel deposits that are insoluble in water. Depending on the kind of wood, ash usually has calcium salts in it, most often calcium carbonate that is virtually insoluble in water. Limestone is mostly calcium carbonate, as is marble.
Informally, the absolute best woods (the guys here have worked hard at finding the best ones, and they have found several that range from promising to sassafras) will produce high-creosote charcoal with 2.5% or less ash. With that said, acceptable results have been obtained with charcoal that had 6% ash. My own cut-off point is 4% ash.
Assay your charcoal! I found what seemed like the perfect charcoal until I assayed it, it was one-sixth ash – 17 percent! I seem to recall the lowest ash assay so far was 1.5%, so set your sights on that, but 4% will do.