Deconstructing David

In my previous attempts using smoke devices, I mixed potassium nitrate and sugar powders. This technique is of course pretty straightforward, however it doesn’t allow a very intimate mixture. A good contact between the reagents is necessary for the combustion to be fast and steady. Milling the powders in a ball mill or any other grinder would lead to smaller particles and thus better interparticle contact, but grinding fuel oxidizer mixtures is never 100% safe. As sugar and KNO3 are both water soluble, it appears that the best way here would be to dissolve them in water and make them recrystallize together. This can be easily and safely performed with some water and a gentle heating. The mixing and heating processes are carried on in glass containers.

About 100 g of KNO3 (60%wt) and sucrose (40%wt) mixture is put in a beaker and some water (about 10 ml) is added to help mixing and dissolving the powders. It’s useless to drown the reagents as the water will have to be removed so the combustion takes place. Therefore, more water requires more heating afterwards…

The heating must be as gentle as possible.
I’m generally trying to keep the mixture around 100°C, however, the very low heat transfer coefficient of the liquid makes it almost impossible. At the bottom of the beaker, the temperature locally rises to 150-200°C, so the sugar turns to caramel and the mixture gets brownish. Over heating is a huge issue when heating the mixture as it might lead to fires. Then one has to be careful and keep and eye on the temperature, the viscosity and the foaming. Vigorous stirring is obviously required.

Once enough water has evaporated, the hot liquid mixture (which is some sort of amorphous colloid) is cast in a casing. Some experience is required to determine the appropriate moment for casting, however, one can get satisfactory results by taking some of the mixture out of the beaker and put it on a cold and dry surface. If the mixture gets dry and solid when it’s cold, it means that enough water was removed. If it’s still too wet, the mixture will remain soft and sticky. The amount of water remaining in the smoke mixture influences the ignition temperature, the combustion speed and the amount of smoke.

After a few hours of drying, the mixture becomes compact, dry and solid, very easy and safe to handle. The ignition requires more heat than the dry powders, which lessens the accidental ignition problems and a wick is very easily installed after drilling a little hole in the mixture. My first attempt with these cooked mixtures lead me to pour about 100 g in a paper casing wrapped with some tape. It made a decent little smoke bomb although the tape burnt with a bad smell.

Fuses are required in any pyrotechnic test for obvious safety reasons. However, they’re sometimes expensive or difficult to find. Therefore, it’s sometimes interesting to use homemade fuses.

Theory is easy: one has to mix an oxidizer and a fuel and make the mix stick to a rope.
The most common fuses are called black matches and are made of cotton strings dipped in a black powder and binder mixture. More advanced fuses are visco fuses: they’re made of a water tight thin tube containing black powder. It’s very reliable event on wet surfaces and burns very evenly.

As I have some experience in sugar/potassium nitrate mixtures, I used these powders for these first attempts of homemade fuses.

I first prepared a saturated solution with 10g of sugar and 60 g of KNO3. I dipped hemp fibers ropes in this solution and let them absorb as much liquid as possible. Then I hung them to let them dry. Note that vertical drying leads to fuses with a thicker bottom end, as the solution goes down the fibers because of gravity during drying.

The overall texture of the dried fuse is ok, although sugar/KNO3 crystals tend to crumble as there is no binder. The average combustion rate is very slow (12 s/cm during an upwards vertical test) and unreliable. This prototype is not to be used in any serious application.

During my previous experiments with smoke devices, I had bought potassium nitrate in a pharmacy. Only small quantities of medical grade KNO3 were available and the price was rather high.

Therefore, for making more serious “smoking works” I needed bigger and cheaper sources of KNO3. I was rather lucky to discover a chemicals wholeseller who could provide me with 25 kg bags at an affordable price (approx. 3€/kg).

Its purity is 97 or 99%, the main impurities being K2O and moist, which is suitable for combustion uses.