July 14 2012 7 14 /07 /July /2012 21:15
Flash powder is a pyrotechnic mixture that, when ignited, burns with a bright flash. The reaction happens so rapidly that the powder seems to disappear after ignition. This mixture is often used in fireworks due to its ability to explode when confined. The most common flash powder is aluminium – potassium perchlorate mixture. However, perchlorates are not readily accessible. A less spectacular but more available composition is magnesium – potassium nitrate.
Course magnesium powder can be easily made from a file, muscles, and a magnesium bar. I obtained my magnesium bar from a camp fire starter, which generally costs about 10 dollars. There may be a cheaper source for large magnesium chunks. The shavings are approximately 100 microns across when ground properly. Potassium nitrate, also known as saltpeter, is also readily available. Some fertilizers use this ingredient to supply nitrogen and potassium, two essential plant materials. It is also an important ingredient in black powder. The best source of pure potassium nitrate that I have found is Spectracide® stump remover. This comes in fine white crystals which may be ground further if desired but still make a good flash when whole. A one pound bottle of the stuff costs about six US dollars.
I mixed the potassium nitrate with the magnesium powder in a 1:2 volume ratio. There is an excess of magnesium in this reaction, but that is fine. (If you want to make a more powerful flash powder, use stoichiometric quantities and fine powders.) The reaction of the two chemicals is probably this one:
5 Mg + 2 KNO3 => K2O + 5 MgO + N2
My first flash powder experiment was a success. I ignited it with a magnesium fuse and it burnt with an extremely bright flame, producing a cloud of white smoke. The nitrogen gas blows the reaction mixture apart, making the chance of embers quite small. The heat is intense, however, and it could easily ignite a flammable base, e.g. grass. Caution must be observed because of the heat.
Then I tried igniting flash powder in a test tube. I was curious whether the containment of a test tube would cause the flash powder to explode. To do this, I created an extremely long-winded and fragile ignition setup. First of all, I made a titanium igniter. Titanium has a higher resistance than quite a few other metals and so heats up easier when current is applied. Even better, a thin titanium strip will ignite when heated. Wires are tied to two ends of a thin piece of titanium foil and the wires are hooked up to two nine-volt batteries in series. The load resistance is about 1 ohm at first. This means a huge current spike as the nine-volt batteries heat the titanium, which decreases as the titanium becomes red-hot and is cut off as the titanium ignites and burns to the non-conductive titanium dioxide.
I used this titanium to ignite a pile of coarse magnesium powder and calcium pieces. The calcium was added in the hope of increasing the strength of the burning magnesium powder. Then a wooden splint, one half of which was soaked in potassium nitrate solution, was placed in the bottom of the powder pile. On top of it laid a magnesium ribbon which ran directly into a test tube filled with the flash powder mixture. The test tube was on its side.
It did not work as I expected. The ignition pile ignited when the current was applied, but I used the file to make the majority of the pile and it blew away before igniting the magnesium ribbon. The calcium did not have any significant effect. The wooden splint ignited however, and the nitrate – soaked section burnt up. The magnesium appears to have oxidized, although the bright white flame of magnesium is not seen. Once the splint passed into the unsoaked portion, the flame extinguished due to the wind and the heat removal by the brick. The reducing fumes produced by the extinguishing put out the magnesium for good, about 2 inches from the flash powder.
I decided to remove everything except the magnesium strip and light it using a propane torch. The test tube was standing upright in a holder. The force of the flame from the propane torch melted and bent the strip over the edge of the test tube, invisible to me. When I applied more heat, it ignited in the middle and instead of burning toward the flash powder, it burnt along the outside edge of the test tube, making a black charred spot. I took video of this but since it is boring the video was deleted.
I then laid the test tube on its side and tried ignition. The magnesium burnt up and hit the flash powder inside the test tube. The test tube was immediately melted and ripped open on the top, leaving just a small chunk of black plastic behind. The flash powder burnt brilliantly, even in the broad daylight, throwing flame and sparks around for an entire second. If the powders were fine it would have been gone almost instantly and the test tube would have exploded. However, due to the large number of fails, I forgot to turn my camera on and my biggest flash powder combustion event went unrecorded.
My brother got interested in flash powder and decided to make some himself. He ignited it with a wooden splint. He said that it was hard to see for a minute after ignition.
I then assembled a flash powder module. A small amount of flash powder is placed on some duct tape and a titanium igniter is placed inside. It is ignited using an 18V nickel – cadmium battery pack. The duct tape melts by the igniter, but the flash powder does not go off. I make a slit in the duct tape and the flash powder ignites. Therefore, the ignition of the titanium is necessary for the flash powder to deflagrate. To help this out, I wrap the flash powder in filter paper. The wires are connected to a long power cable which is hooked up to the 18V battery pack. A water bottle cap is placed on top of the paper construction to determine the effects of the blast. Ignition happens instantly, and the burning water cap is thrown 5 feet. This shows the danger of being too close to the deflagrating mixture. Small amounts of smoldering paper are left behind.
At this stage, my flash powder modules are becoming too close to fireworks, which are illegal to use in NJ. Therefore, I will quit my experimentation with Mg – KNO3 flash powder for now.
After a couple of months, I decided to make Zn - KNO3 flash powder. I ground up a piece of zinc from a carbon-zinc battery on a file and mixed the fine powder with potassium nitrate. The mixture ignited with difficulty. The zinc tended to melt at first, then suddenly combined with the potassium nitrate, making a dim green flash. This is expected because zinc is a much less flammable and much less reactive metal than magnesium.
Then I tried to coat a wooden splint in glue and magnesium - potassium nitrate flash powder. I hoped that when the splint was ignited, the flash powder would behave as a sparkler. Unfortunately, the nitrogen produced during the flash powder reaction tended to immediately extinguish the wooden splint, making reignition necessary. I hope to try using potassium bromate/chlorate as the oxidizer in the future as it seems to produce much less gas.