19 replies to this topic

[alany]

I'm wondering about particle size too! Molten blobs that sparkle sounds like a poorly milled attempt at BP too!

I find the charcoal has to be very fine for a good effect. I specifically tried coarser charcoal assuming it's microstructure might be important, especially in producing the dividing sparks, but found that actually killed the effect. This may be why some lampblacks work well. Surface area may be important, I've noted that the dross balls shot out from the main dross ball get brighter and pulse in diameter just before they explode. I did this by careful examination of long exposure photography of senko hanabi. I strongly suspect internally generated gasses blow the droplets apart and the subdroplets continue to react In the same way until they run out of chemicals or cool off. It is probably important that there is plenty of fine charcoal in suspension in the melt to continue the reaction down in the daughter droplets to get the most lacey effect.

I wonder what the gas bubbles are? Carbon dioxide from sulfate oxidizing the charcoal? The role of volatile compounds in the charcoal might be worth study. I've tried replacing the charcoal with organic compounds like sugar and benzoates with no luck, it may be that charcoal has just the right reactivity.

[phildunford]

I tried to duplicate your rather nice flowerpot gerb using that composition, but was not able to get many of the nice "flowers". I suspect the lampblack I am using is the problem. Where did you source yours?

Can't be much help here, as it's a tub I've had for donkeys years that was originally obtained from a laboratory supplier called Hogg. I think they have now changed their name to SciChem, but they don't seem to list it any more. It's a very dense and greasy lampblack. Will have to try some obtainable ones and see if the formula still works...

[alany]

Here is a video of a fairly typical example of my Senko Hanabi devices:



I don't seem to get the large, far ejected bursts that some other pyrotechnicians do, especially those that use pine soot. I do however get rather long-lived performance with an extended period of non-branched bright yellowish spark production after the initial branched spark effect. My devices appear to be a little smaller and contain less composition than some others I have seen on the net, but I have problems retaining the dross droplet if I use too much composition, even with large amounts of paper. This particular batch of composition is very reliable, but not the best spark producer.

On the odd occasion the dross droplet falls off, about half the time when it hits the ground it produces a rather loud report and scatters branching spark producing bits everywhere. This does not appear to be a moisture + hot dross steam explosion, but rather some kind of internal reaction in the dross droplet. It only seems to happen when it falls towards the end of the most "active" part of the initial branched spark display. If the droplet falls into water some material dissolves leading to a yellowish green solution.

OT, but vaguely related, here is a video of the experimental flowerpot gerb I spoke about earlier:



It starts out quite well, especially considering it uses no lampblack at all. The effect is not as rich as Phil's and degrades rapidly as the dross builds up inside the device instead of being ejected.

[MDH]

You could probably clean it up with granulated black powder in small amounts (i.e., not integrating it finely into the composition, but just adding granules here and there to eject the dross inside of the tube).

Edited by MDH, 17 July 2010 - 11:45 PM.

[alany]

You could probably clean it up with granulated black powder in small amounts (i.e., not integrating it finely into the composition, but just adding granules here and there to eject the dross inside of the tube).

Indeed, there is already granulated mill dust in there (probably why this one kinda works and doesn't explode like some others did). But I suspect it is getting crushed when I ram the gerb as the grains as no where near as robust as properly corned BP. Using corned BP is something I will try, but it would be nice to be able to use "riced BP" instead because of the labour involved in making "candle lift" quality corned BP. Lancaster talks about loading them loosely, funnel-and-wire, this is perhaps why? Although when I tried that I had the opposite problem, voids (I am assuming) led to explosion of the device. More experimentation required - and some lampblack runs too...

I've considered trying the reverse too, granulating Senko Hanabi composition as microstars and using a gassy but low-spark propellant to eject them. (Whistle comes to mind as an interesting variation if the grains of microstar are hard enough to withstand the pressing.) My understanding is the traditional device had none of this magic construction as was made quite cheaply in large quantities?