2009-08-28
Senko Hanabi is an old and relatively little known pyrotechnic device of Japanese origin. The name translates as "Incense Firework" a reference to its appearance when first ignited. Essentially they are a kind of hand-held sparkler but offer a much more delicate effect than the conventional dipped-wire Barium Nitrate/Aluminium/Steel device mass produced and widely available.
In addition to being less retina-blasting than dipped-wire sparklers the Senko Hanabi firework demands more of its user. For best performance the device must be held correctly and can not be jostled around or operated upside down, such as being used to adorn a birthday cake. The device also releases a little smoke and sulfurous fumes which make it suitable to be enjoyed outdoors only.
The device itself is also much more difficult to make, a feature that has limited its commercialisation. Traditional construction is performed completely by hand, and has resisted attempts at mechanisation. A dipped variety has been attempted but the result has been found to be generally inferior to the hand-made design.
To many the Senko Hanabi sparkler is a microcosm of life. The violence of birth, the growth of childhood, exuberance of youth, the productivity of middle age, the slow decline of old age, the fade away (or sudden disaster) of death. In a very Zen way Senko Hanabi represents the impermanence of everything.
It is said that if you make a wish while burning a Senko Hanabi sparkler and manage to prevent the droplet from falling before the effect finishes your wish will come true.
Senko Hanabi is traditionally handed around at the end of a night of Fireworks, as the last act before returning home. This is probably tie in to its Zen-like nature and subtle soothing effect on the nerves.
In operation sparks are emitted to a foot or more in all directions and the device can drop a small amount of very hot fluid. Both should be kept away from clothes, feet and flammable or easily damaged materials. Green grass, concrete or a small bucket of water are ideal to operate over. Clothing should be non-flammable, fairly tight and preferably incapable of holding errant sparks or molten dross against the body. Closed-toe footwear is recommended.
In many ways Senko Hanabi devices are safer than dipped wire sparklers which leave an extremely hot piece of metal and burn at a much higher temperature. The total chemical energy content of Senko Hanabi devices is also much smaller making even deliberate injury less devastating. The dross-ball which Senko Hanabi often drops is its most dangerous feature, and is very capable of burning feet or igniting dry grass.
A small amount of sulfurous smoke is released, most find it quite pleasant - that wonderful black powder smell - but sensitive individuals may be irritated by it or desire not to have the scent linger in their hair and clothes.
Senko Hanabi sparklers generally look like a stick of incense with a bulge at one end where a small quantity of pyrotechnic composition is contained in a twist of paper.
To use the devices one generally heads out-doors, holds the device at the stick end well away from the body, pointing downwards and ignites the end with a lighter or candle. Once ignited the composition inside the twisted paper will flare up and burn swiftly rather like Chinese cracker fuse. Once all the composition is consumed the glowing remains will melt together and shrink into a glowing bead of dross at the end of the shaft. One must hold the sparkler still and keep the end pointed down to prevent the droplet falling off and ending the display.
After a short while the temperature of the dross ball will increase and it will begin to bubble and seethe with gasses, at this point fine branching sparks will start being emitted. The spark emission will build to a climax then taper off and become less branched until almost no branching sparks are emitted. Non-branched spark emission will continue for a short while, then the droplet will cool off and the effect terminate. The droplet may also just fall off at any point ruining the display, keep the device still and protected from drafts to prolong the display.
The composition used in Senko Hanabi is a variant of Black Powder, but contains more Sulfur, less Potassium Nitrate and specially selected Charcoal (and/or Lampblack). The branching sparks that are the prime characteristic of Senko Hanabi are strongly dependant on the kind of Charcoal or Lampblack used. Generally a Charcoal from Pine timber, a Lampblack from Pine Resin or a mixture of both is used to get the largest and most appealing sparks.
Some Senko Hanabi compositions use Arsenic Sulfide to replace most or all of the Sulfur. This practice is much less common of late, due to the cost and toxicity of Arsenic compounds, and because the effect is not markedly improved by it.
The exact mixtures and selection of Charcoals utilised remain a closely guarded secret of the commercial manufactures. Variation from one batch to the next is common, what makes the "perfect" charcoal/lampblack is somewhat mysterious and also open to interpretation. Some consumers prefer lots of fine sparks, other more bushy bursts projected a large distance. The exact details of the branching spark production are not sufficiently well understood at present to offer rigid guidelines, only traditional wisdom is available to guide selection of the raw materials.
The paper in which the composition is twisted is also a critical component of the device, long fibre tissue papers are preferred and additives like alkaline buffers discouraged. Kozo and Gampi papers work well, but Cotton and Pine fibre paper is cheaper and more easily available.
Presentation is very important for traditional devices. The bamboo or paper shafts being brightly coloured and the bundles of devices packed into with boxes with attractive artwork.
While the branching spark production is not well understood the basic chemistry is fairly well studied. The combustion of the Black Powder components leaves Potassium Sulfides as a residue. It is this drossy "Liver of Sulfur", unconsumed Charcoal and paper fibres that forms the droplet mixture. It is believed that atmospheric Oxygen dissolves into the Polysulfide dross and forms Sulfates which accumulate until a threshold amount if achieved and then rapidly oxidise the Charcoal explosively. The dependence on atmospheric oxygen is easily displayed by dipping the sparking device into a jar of Oxygen (the reaction will be accelerated significantly) or an inert blanketing gas (which will extinguish the reaction).
Studying the internals of the dross droplet are challenging because of its size and transience. While some efforts to snap-freeze the droplet or the flying sparks and then study their chemistry has been fruitful there is still much supposition about the exact conditions and mechanism. Similar pyrotechnic reactions like Glitter, Transformation, and Spur-Fire are equally frustrating to study in detail. The detailed organic composition and physical form of the Charcoal or Soot used in Senko Hanabi is especially complicating.
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