In our last post, we studied one physical property (density) that was used by people to judge black powder quality. Today we will study another technique that people used to judge the quality of black powder: the hygroscopic properties of powder.
The term "hygroscopy" refers to the phenomenon of certain substances attracting water molecules from the surrounding air and absorbing it. Examples include table salt, sugar, honey etc. This is why they are usually kept in sealed containers, otherwise they tend to absorb water from the atmosphere and spoil.
In the case of black powder, two of its three components have hygroscopic properties: saltpeter and charcoal. The saltpeter is usually hygroscopic due to the presence of impurities such as calcium salts and sodium chloride. In general, calcium sulfates or calcium oxide can react with the sodium chloride to form calcium chloride, which is very hygroscopic in nature. The calcium chloride on the surface absorbs enough water to become a liquid and dissolves some saltpeter and the solution spreads itself through all the grains by capillary action. This causes the saltpeter to be no longer evenly distributed in the powder grains. Therefore, keeping the saltpeter as pure as possible helps keep the hygroscopic properties of the powder down.
Charcoal also influences the hygroscopic properties of black powder. As a general rule of thumb, the more charcoal that the powder contains, the more water it will tend to absorb. One more interesting factor has to do with the temperature that the charcoal is manufactured at. The lower the temperature at which it was manufactured, the more water it can absorb. As a result of this, red charcoal will generally absorb more water than black charcoal.
If the powder becomes damp, it may be restored by drying in the sun or in a dry, well-ventilated room. As a general rule, if the powder does not show an efflorescence of white crystals of saltpeter on its surface, it may be possible to dry it. Powder of smaller gravimetric density will absorb more moisture than a powder of a greater one, and a glazed powder will absorb less moisture than an unglazed one, all other things being equal. Powder that has become damp can be easily recognized by its unequal distribution of color and by the grains crushing more easily between the fingers. However, if it manages to absorb a large amount of moisture. the powder will form hard black lumps and if is reaches this state, the powder is generally useless and cannot be serviced.
To determine the moisture content of a sample of powder, a standard amount (usually 100 grains or 50 grams, depending on country) would be carefully weighed onto a glass plate. Then the glass plate would be placed in an oven and heated for a few hours at a specified temperature that depended upon the country (160 °F for England, 190 °F for Germany etc.) and then placed in a dessicator to cool for 20-30 minutes, after which they would weigh the sample again. The difference in weight is the amount of water absorbed by the powder sample.
To determine the tendency of a particular powder sample to absorb moisture, the powder sample was put alongside a sample of standard powder over a layer of water in a tub, which was closed air-tight and left for a period of time. The two powder samples would then be removed and the amount of water absorbed by each sample would be compared. This test would let people know how much their particular powder differed from the standard sample powder.
In the next few posts, we will look at some of the chemical properties that people would look at to judge powder quality.
Note: I trust my American readers had a happy Thanksgiving holiday so far. Your humble editor was temporarily hospitalized for a little while, but I recovered just in time to spend the holiday at home with family and friends, just as it should be :-).