Crackling Microstars; lead-, lead-copper- and bismuth varietyCrackling microstars (also called Dragon's eggs) are small pyrotechnic stars which first burn for a period giving a visual effect and then explode with a sharp crack. This Chinese discovery is extensively used in both consumer and professional fireworks. When several stars are lit at the same time, a crackling effect is achieved as the individual granules each explode with a slightly different delay.
Note that the eggs are notoriously sensitive to friction and, to a lesser amount, to shock.
There are many ways to make these basic compositions into granules, including ricing (granulation using a screen), cutting (by hand or with special tooling), pumping (with special pumps) and crushing a dried mass. Adequate protection (gloves, mask etc.) is necessary in any case.
When dampening dragons eggs, nitrocellulose lacquer is commonly used as the solvent. This is mainly because the granules are intended to be integrated into common star or "matrix" compositions which are consolidated using water/alcohol. If the eggs would rely on a water soluble binder also, they would go out of shape or disintegrate once mixed into any damp comp. The use of NC instead of dextrin drastically shortens the drying time. If we desire to use the eggs as granules, e.g. as an added effect in fountains, using a primer coat may be a good idea to ensure ignition.
Mixing and dampening the comps
The proper amount of each chemical is weighted and the ingredients are hand-mixed well in a bowl or on a sheet of paper (mask and gloves are essential). This is followed by passing the composition through a mixing screen (e.g. 40 mesh) several times. Some compounds, especially red lead, tend to cohere to the wires of the screen and are very messy to handle as well as hard to remove completely. It may be a good idea to reserve a special mixing screen for mixing egg comps only.
Once the mixture has passed the screen for several times the composition is basically complete and well-integrated. It can be packed, labelled and stored for future use or dampened to make it into granules immediately. In the latter case the composition is repeatedly dampened with small portions of nitrocellulose lacquer. Each time the solvent is kneaded into the dough. This step is repeated until the whole mass shows a flexible, rubber-like consistency. In case the mass has been over-wetted (and thus shows a grease-like consistency) more dry composition can be added for compensation. If the mass becomes too dry during the following granulation step, add a portion of cellulose thinner to re-wet it. The same thinner must also be employed to clean the tools, as water doesn´t dissolve any traces of composition once dried (here, too, it may be useful to keep the bowl as well as other tooling for this special purpose only).
Manufacturing eggs by ricing (Example)
Once dampened with NC lacquer, push the mass through a 10 mesh screen quickly to obtain small granules resembling grains of rice. Try to keep the batches to about a 100 gram maximum or else the lacquer will probably dry before you get the stars screened. If the granules are intended to be primed, roll them in dry prime while they are still damp. If they are already dry, spray them with acetone to make them take up the prime.
Manufacturing eggs by crushing a dried mass
Once dampened with NC lacquer, the mass is transferred onto a working surface e.g. a wooden board and rolled out to give a plate several mm thick. A few irregular cuts are made with a spackling tool or similar to give handier pieces for the next step. The rolled out mass is let dry as is. Drying won´t take long (probably several hours on a summer day) as the solvent evaporates very fast.
The dried pieces are then transferred into a stable container e.g. a bowl made of non-sparking (!) metal such aluminum and crushed by hand using a wooden tool (the handle of a rawhide mallet works well). A mask should be worn as the crushing operation produces some dust. Dragon´s eggs are quite sensitive to friction and, to a lesser amount, to shock. However, if we use the right tools the process is quite safe.
The resulting irregular grains are separated by size using screens. The best size for use passes 8 and is retained on 18 mesh and will look like colored 3FA (note that grains too small for use and/or dust are not wasted but instead may be stored and recycled/redissolved in a future batch). The grains produced by this method are very durable as well as easily ignited due to their sharp edges. However, the technique itself seems to be rather unknown among amateurs, although it (or a very similar one) is the one commonly applied commercially.
When crackling microstars are embedded into a surrounding star composition, preferably of a tailed type, these stars are called matrix stars. Some comps were designed specifically for this purpose while many others are expected to work and harmonize equally well. Here is a charcoal streamer-type composition from Hardt (p.172), which is intended for this special purpose.
This is prepared like any other star composition (by preparing materials, weighing, hand-mixing and screening several times), but the 3% flitters are left out from this operation and mixed in by hand after screening (as the coarse flakes will not pass a common mixing screen).
Once the matrix comp is ready, it is heterogeneously mixed by hand with dragon´s eggs (any variety will work) in a ratio 80:20 matrix: granules; e.g. for every 100 parts of the matrix comp 25 parts of eggs are added. Hardt suggests the use of dragon´s eggs showing a particle size between 8 and 18 mesh as this size gives the best cracks as well as sufficient delay to make the matrix stars look nicely.
This blend is then dampened with water and made into common pumped stars (the fast burning rate of the charcoal type comp must be taken into account). It is difficult to cut or roll a composition containing granules, so these two methods are best avoided for one´s own good.
The stars display a charcoal tail with additional silver streams as well as an intense crackling effect. They harmonize well with most other effects and add a popular acoustic dimension to the display.