My blog entries tend to be pretty random, all over the place, and not always directly related to traditional pyrotechnics. I figure ANY entry is better than NO entry, and it's been a while, so I thought this might interest a few specialized enthusiasts.

Pyrotechnics as a hobby makes extensive use of nitrocellulose lacquer. NC lacquer starts life as dried nitrocellulose fibers dissolved in acetone or some other solvent, in varying percentages, normally 20% to 30%. The vast majority buy the NC lacquer already mixed and ready to use. While definitely flammable, it is not particularly dangerous. Some make their own NC lacquer, using commercially available nitrocellulose or single-based smokeless powder as used in reloading small-arms cartridges. The former probably works better (and is cheaper) than the latter. On the "low end" of the scale is lacquer made from dissolved ping pong balls. While not truly nitrocellulose (they are primarily cellulose acetate) and full of fillers, it does produce a flammable and effective binder.

Commercially-available nitrocellulose is available from Hercules, a major manufacturer of small arms powder. It is shipped in 1 pound, sealed plastic containers, and for safety purposes, it is shipped dampened with water. It is best to store this material sealed, damp, and in a cool location. Nitrocellulose, when properly prepared, is stable and relatively safe, but when stored improperly, it can degrade and become less stable. In the degradation process, NOx gasses are emitted, and the rate of degradation increases with temperature.

In Industry, there are a variety of tests used to measure these NOx emissions, and thus monitor the relative "health" of the nitrocellulose batch. One traditional test is called the Abel Test. If you search the internet for this test, there isn't as much information as one might think.

Being curiosity-driven, I decided to see if it would be possible, at home, to replicate this test... create test papers of sufficient sensitivity to detect NOx emissions from a nitrated compound like nitrocellulose.

The test papers in their basic form are a simple starch-iodide paper. One of the first demonstrations in 8th grade chemistry is the detection of iodine with starch. In the presence of iodine, starch forms a brilliant purple coloration. In a nutshell, and blatantly stolen from another web site:

Yesterday I took myself over to the Chem dept and got an illustrated mini-lecture on the subject: yes, starch in aqueous solution forms helices like "slinky toys". The iodine has to be I2KI or something with both elemental iodine and iodide ion. These latter form a linear structure like a broomstick. The broomsticks find their way into the coils of the "slinky" thus causing a change in light absorption, and the blue-black color. The point for the students, who ask why the "blue-black" stuff doesn't diffuse out of the starch bag, is not that the complex is to big - it just doesn't exist as a compound, and the iodine and starch are still separate entities in the bag!

Too much info - essentially, starch, plus iodine = blue-purple coloration. We can take advantage of this by mixing starch and potassium iodide intimately, soaking this mixture into thick paper, and allowing it to dry. This paper is extraordinarily sensitive to oxidizing agents... ozone, in particular, rapidly oxidizes the iodide, creating iodine. This in turn reacts with the starch, and the paper darkens. The nitric oxides evolved by nitrocellulose (and similar energetics) produce a more brown coloration.

The goal, then, is to produce, rather than buy, a paper that is sensitive to the decay products of nitrocellulose (and other) nitrated esters. Potassium Iodide/Starch paper is ridiculously cheap, but by varying the amount of KI, the sensitivity of the paper is improved.

Work begins with starch. I used defatted starch, but regular corn starch works well. 3.0 grams is weighed, and 250 ml of distilled or deionized water (very important) is heated to a boil.

The starch is best added to the water in the form of a thin paste. I added the starch to a tiny teflon dish, and added water. A bit of manipulation formed a thin paste. The PTFE is amazing stuff. Even so sticky a mixture as starch paste rolls right off, and into the water, with zero loss.

The starch/water mixture must be gently boiled for at least 10 minutes. This is important... starch not boiled, for some reason, produces a less sensitive paper.

While the starch boils, the KI is weighed. I have tried both 1g and 3g in 250 ml of water, with the latter producing a more sensitive paper. The KI is added to a separate 250 ml portion, cold. Stir to dissolve. Cleanliness is important throughout this process. Any contamination by oxidizers will trigger a premature reaction.

The two, 250 ml portions are added together after the starch is allowed to cool. Keep both solutions covered with a clean paper towel as much as possible in all the phases of this project.

The style of paper is not critical, but a good quality filter paper is a fine choice. Find a shallow pan which will hold the filter paper flat, clean it thoroughly, and add the now-cooled starch-KI mixture into the pan. Each piece of paper is dipped, immersed, for 10 seconds, then withdrawn and allowed to drain. Again, cleanliness is crucial. If there are pollutants anywhere, the paper will begin to darken prematurely.

Now comes the challenging part - keeping the disks protected while they dry! The paper must be allowed to dry in a clean environment that is not exposed to airborne pollutants, with ozone being the chief culprit. My first batch turned dusky purple before it was fully dried, as I let it dry in my workshop, exposed to normal atmospheric ozone. Probably the easiest method is to find a dark closet and cover the papers gently with a clean and permeable cloth or paper barrier.

Once the paper is completely dried, cut it into convenient strips while wearing nitrile or latex gloves, removing the often discolored edges, and store the remainder double-bagged) in a cool, dark area. 500 ml of mixture is 20X more than most people will ever need, and the recipe can be easily scaled down to make 50 or 100 ml of solution.

To test the paper, I took three PTFE vessels and added two samples, with the third remaining empty as a control. Sample 1 was 100 mg of dried Hercules nitrocellulose. Sample 2 was 100 mg of nitrostarch, not commercial. Sample 3 was the empty container. All three samples were placed in a computerized oven for 24 hours at 100 degrees Celsius, with the lids inverted and placed loosely on top of them to limit air movement.

While the samples were "cooking," a simple test shows how sensitive the paper is. Two test tubes were used. The test tube on the left has a miniscule smear of 20% nitric acid on one internal side wall, while the test tube on the right remained empty. A few hours later, the test tube with the acid shows the obvious dark coloration created by the oxidative gasses from the nitric acid smear.

Back to the heated samples: After 24 hours at 100 degrees C, the control paper (no substance) was as white as it was when it was added to the PTFE container. Examining the actual samples of nitrocellulose and nitrostarch showed that they had taken on a yellow shade, with the NC showing the greatest change - from snow white to distinctly yellowed.

The test papers in those two vessels both showed a color change. In the case of the commercial nitrocellulose, the coloration was very faint, but perceptible. The test strip from the nitrostarch showed the most coloration, but it was not excesive. 24 hours at 100 degrees C is probably not long enough to correctly test, but it showed that, in principle, the papers worked.

As usual, the camera does not show minute coloration differences as well as the human eye. The difference is a bit more pronounced than these pictures.

If you search the internet for the Abel test (Mr. Abel was an early pioneer in the development of nitrocellulose as a propellant), there is not too much out there. It has probably been supplanted by more modern testing systems, but the simplicity of it, as shown here, makes it an interesting test and worthy of investigation. Further, these papers can be used to monitor a small supply of NC, compare storage strategies, and test other nitrated substances for cleanliness and hopefully lack of residual acid.

Be sure your pyrotechnic materials are properly stored in a correct magazine, away from human habitation. As always...