by Binnie Smith
The main ingredient in smokeless propellant, comprising from 55% to 90% of the composition, is nitrocellulose. Manufacturing nitrocellulose leaves remnant acid in the product, acid that immediately starts decomposing. Smokeless powder starts deteriorating the day it leaves the powder mill. Eventually decomposition reaches the stage where the propellant becomes unstable and self-ignites. This nasty characteristic resulted in spontaneous, massive explosions at U.S. Government depots following World War I.
A solution to this malady was found in the form of a stabilizing chemical that significantly increased the shelf-life of smokeless propellant. Said stabilizer reacts with the acid to slow decomposition. Stabilizer reacts with the acid as it is consumed. Double base powders are made with nitroglycerin (NG) and nitrocellulose (NC). NG wicks its way into the NC releasing nitric acid gas in the process. Stabilizers consume the nitric acid but dissipate over time. Once the stabilizer is used up, the propellant is no longer protected from the remaining nitric acid.
The US Navy tests powders at the 10-year benchmark, exposing it to litmus paper. If it changes color, then nitric gas is present. If that happens they further test for how much, if any, stabilizer remains in the powder. Below 20% and the powder is scrapped. Typically, double based powders are scrapped at the 20-year mark and single based after 45-years
Obviously, the stabilizer / decomposition process is a function of time and temperature. The higher the ambient temperature where the powder is stored, the shorter the safe life of the powder. Even moderate temperature over an extended time, decomposes propellant. The U.S. Navy puts a 125-degree limit on acceptable storage temperature, even when of a short duration. As a rule of thumb, any temperature higher than a person feels comfortable in accelerates decomposition of smokeless propellants. At normal storage temperatures, propellants infused with stabilizers should have a minimum safe storage time of 35-years. Though actual service life would be far below that due to the decline of propellant performance (burn rate).
However, lengthy shelf-life does not mean a reloader can ignore how the powder is stored, particularly in an uncontrolled environment such as a garage or storage building. Avoid leaving powder in powder measure hoppers overnight or especially not for several days. Store in original containers, not glass jars, not boxes.
A dry, 55-degree Fahrenheit environment is the optimal condition for storage of a tightly-sealed, original container of smokeless powder. Hodgdon recommends 21 °C (70 °F) 50% Relative Humidity Know that decomposition becomes measurable in temperatures higher than 50 °C/122 °F.
Check powder deterioration by opening the container. Sniff the contents. Deterioration smells acidic, an odor not to be confused with common solvent odors like ether and acetone and alcohol. Any powder that has changed color is bad. In fact, it turned bad long before its color changed. Another telltale sign is finding rust in a metal powder can. Meaning rust from acid gas has corroded the can.